FEED LINE CHOKE FROM COAX

Over the years coiled coax has been used to choke off RF from the outside of coax feed lines.  Sometimes these are called 1:1 baluns, choke balun, but technically a balun is an impedance transformer while these coils are chokes.  (NOTE:  This discussion is limited to air core chokes only.)  The reason for a choke is to stop RF from coming down the coax, causing distortion of the antenna’s radiation pattern and RF problems in the shack. 

Coax line chokes work well if they are used correctly.  However, there are some things one needs to know about using this type of coax line choke.

1      1.       They are most effective on one band only below 40 meters.

2      2.       If carefully designed, they will perform on more than one band 40 through 10 meters (see chart below).

3      3.       Always “flat” wind these chokes.  Scramble winding adversely impacts their performance.

The most important performance limiting factor is inner turn capacitance, called parasitic capacitance. 

Remember that any time two conductors are next to each other, even if there is a DC electrical connection between them, there still is some capacitance between them at radio frequencies (RF).  

NOTE:  Do not confuse parasitic capacitance with the capacitance between the shield and the inner conductor of the coax.   The capacitance between the shield and the inner conductor does not contribute anything of significance to this discussion.

As parasitic capacitance gets larger the choke starts to look more and more like a capacitor to RF.  Remember that a capacitor's or an inductor's resistance to alternating current (AC) varies with frequency.  

Experience over the years has shown that 10 times the feed line resistance for a well match antenna is a good number to use for a coaxial line choke's reactance.  I used 500 Ohms for creating the chart below.  In the real world up to 25% either way is acceptable.  That is 375 to 625 Ohms.  A choke will remain effective so long as both its coil and parasitic capacitance remain in the acceptable range for the expected operating frequency.

The following chart gives a list of coaxial line chokes for various ham bands. 

      Turns         Diameter         Height     Coax type         Band Coverage

      13             8                   3.25           RG-174           160 meters only

                                                              RG-58

                                                              RG-8x

      9              9                    3                RG-174           80 meters only

                                                              RG-58

                                                              RG-8x

      8              6                    2                RG-174           40 meters only

                                                              RG-58

                                                              RG-8x

      9             4                    2.25            RG-174          40 – 30 meters

                                                              RG-58

                                                              RG-8x

      8            4                    2                  RG-174         30 – 20 meters

                                                              RG-58

                                                              RG-8x

      5            5                    2                  RG-174         20 - 15 meters

                                                              RG-58

                                                              RG-8x

     6             4                    2                  RG-174         17 - 10 meters

                                                              RG-58

                                                              RG-8x

     4             3                    1                  RG-174         6 meters only

                                                              RG-58

                                                              RG-8x

     Height and Diameter are in inches.

Be sure to match the length of the coil keeping the spacing even between turns.  This may require using a spacer between turns while winding the coil.  Once the coil is wound the spacer should be removed and the turns need to be glued in place.  Deviating very far from either length or diameter will chance the frequency at which the coil will work.

Here are three graphs that will help you see why a coaxial line choke is limited.  Note how rapidly the inductance and capacitance very with frequency.  Notice that on each graph inductive reactance and capacitance reactance cross on 500 Ohms.  Remember from above that this is the design criteria.

Notice the higher the band, larger the range of frequencies the choke covers.

Other coil sizes and coax types will work.  However, they are too numerous to post.  

73

Dave, K7PDW

SMALL BACK YARDS - THE BANE OF HAMS

It seems that there is an assumption that if you have a small backyard you are doomed to some sort of compromise antenna.  My response to that dire prediction is, “It isn’t necessarily so.”

If you have 70 feet in a straight line that you can put up a wire even at 15 feet above the ground, you have what it takes to make a simple (no traps, no ground radials, no compromise) antenna that will work well from 40 meters through 10 meters and even 6 meters.  (Please remember to be cautious of power line when installing any antenna system.  They’re real killers.)

It turns out that the simplest antenna to build is one of the best radiators of RF.  Sixty seven feet of #14 copper wire with a convenient length of either 300 Ohm TV ribbon or better 450 Ohm ladder line for feed line will do the job very nicely.  Bring the feed line down as perpendicular to the antenna as possible.  It does not have to be perpendicular to the ground as long as it stays 8 feet or so above the ground.  When you get to where you want to enter your house install a balun.  Here a 4:1 balun will work with most tuners and 450 Ohm ladder line, but I prefer a 9:1.  In my experience more tuners, both built in and stand alone, perform better with 450 Ohm ladder line and a 9:1 balun.  If you use 300 Ohm TV ribbon then a 4:1 is a better choice unless you can find a 6:1 balun which would be ideal.  You should be aware that TV twin lead has more loss than ladder line, but both beat an all coax feed line in this application by a country mile.

This antenna performs as a common ½ wave dipole on 40 meters.  On the higher bands it behaves as a collinear.  That is why you want to feed it with twin lead or ladder line.  Ladder line is preferable because it has significantly less loss than TV lead.  You may be tempted to put the balun right at the feed point of the antenna and then run coax all the way back to the transceiver.  That will work but the loss in the coax will be very high especially 17 meters and up. 

Here’s why.  There is a common misconception that by putting a balun in the feed line even when there is a high SWR that somehow the balun will correct the mismatch.  This is just not true.  The balun only transforms the mismatch to another value that may or may not match the feed line impedance. The feed point on this antenna and most collinear runs from 20 Ohms to 800 Ohms depending on the frequency you are running.  The following table compares the loss between a balun at the antenna feed point and 70 feet of RG58 and 60 feet of 450 ladder line between the feed point and a 9:1 balun then 10 feet of RG58 to run through the wall to the tuner and transceiver in the ham shack.

BAND                                    60 ft ladder line and        70 ft RG58 with a balun                  70 ft RG58

                                                10 ft coax w/balun           at the ant feed point                      no balun

---------------                       ------------------------        -----------------------------      --------------

6 meters                              8.52 dBi ERP                        5.84 dBi ERP                                3.93 dBi ERP

10 meters                            7.84 dBi ERP                        4.89 dBi ERP                               -2.66 dBi ERP

12 meters                            8.24 dBi ERP                        6.10 dBi ERP                                0.21 dBi ERP

15 meters                            6.96 dBi ERP                        4.65 dBi ERP                                4.50 dBi ERP

17 meters                            8.34 dBi ERP                        5.79 dBi ERP                               -1.58 dBi ERP

20 meters                            6.83 dBi ERP                        4.02 dBi ERP                               -3.14 dBi ERP

40 meters                            5.71 dBi ERP                        4.37 dBi ERP                                5.67 dBi ERP

ERP – Effective Radiated Power for the antenna system

Note – I did not have the necessary loss specs for TV twin lead for this comparison, but it will fall somewhere between 60 ft ladder, a balun with 10 ft of coax arrangement and the all coax with a balun arrangement.

Another misconception is that an antenna must be resonant to radiate.  This is also not true.  The loss of signal strength in a non-resonant antenna system is almost always due to high loss in the feed line caused by mismatch between the feed line and the antenna’s feed point.  A low loss feed line such as ladder line, TV ribbon, or open wire feed line carries the transmitter’s power to the antenna where it is radiated efficiently despite high SWR cause by non-resonance.  The performance of a non-resonant antenna will be comparable to a resonant antenna when feed line losses are kept to a minimum.  However, you must still do something to prevent the high SWR from damaging the transmitter.  This is the job of a tuner.  For multi band antennas that do not have provisions for achieving 50 Ohm feed point impedance at all the expected operating frequencies, a low loss feed line and tuner arrangement is a reasonably inexpensive solution.  Another solution is a remote auto-tuner at the feed point of the antenna.  This is effective, but considerably more expensive.  And it is often difficult to manage the weight of the auto-tuner at the antenna’s feed point unless it also happens to be at one of the antenna’s support points.  If an auto-tuner is available, it will work just as well at the end of the ladder line where it enters the house.  If you use an auto-tuner be sure it has provisions for feeding a balanced line.  If not, then a balun will be necessary between the balanced line and the auto-tuner.  It would be best to check with auto-tuner manufacture about how to implement that.

Some will note that this antenna looks suspiciously like a G5RV and that is a valid observation.  The major difference is that the balanced line can be any convenient length and the coax can thus be kept to a minimum length.  In this type of antenna one should always try to achieve the shortest run of coax possible and thus reduce the feed line losses to a minimum.  It is true that the balanced line that is cut to a specific length on the G5RV does act as an impedance matching device.  But again it will provide a good match at some frequencies and not so good at others just like a balun.  The loss that occurs in the coax will still be present. So always keep the coax as short as possible and the balance line as long as is sensible so as to reduce the loss in the antenna feed system to a minimum.

Happy HF’n and 73

K7PDW

PS - By the way, this antenna can be bent or sloped but its efficiency will drop some and often the bandwidth will get narrower making it difficult or impossible to tune some bands.

Back To Back Mobile Whips For Portable Stations

Back to back mobile whips have long been used to create rotatable portable dipoles.  You may have heard varying reports on how well they work.  Some folks even claim that they are nearly unusable because of high SWR.  My experience working contests while portable QRP with them has been good.  My setup has good SWR and I don’t seem to have problems making contacts with my FT-817 on SSB.  I was actually please with the ability to turn the dipole in null out strong signals.  The one disadvantage I found was that my setup only covered about 100k on the 20 meter band at less than 2:1 SWR.   I expected that because it is the nature of wound antennas to have a narrow bandwidth.

I began to wonder why others had such bad experience with this type of antenna.  I believe I discovered the reason when I began to consider buying a commercial mount design for back to back mobile whips.  I noticed that the commercial version simply grounded one whip to the mount while insolating the other one as in a standard mobile installation.  My home brew setup uses insolated mounts on both whips.  I connect the feed line, coax, as any other dipole.  Shield to one side and center conductor to the other side.  I use a coaxial wound choke to control RF on the outside of the coax.

 That got me to thinking.  I decided to experiment with grounding one whip, the one connected to the coax shield as the commercial mounts are designed.  I used a short jumper for the connection.  I usually use about 18 inches of PVC as an interface between top of my mast and the antenna mount.  I went ahead and set up the antenna as usual with the PVC as usual.  I didn’t see any difference. 

I next decided to try doing away with the PVC and connect directly to the mast still leaving the antenna and coax shield connected to the mount and thus now connected to the entire mast.  The mast I was using is 4 foot sections of the military mast I bought off the surplus market.  I have both aluminum and fiberglass sections.  I had use the aluminum in the previous tests and did so this time.

This time the SWR exceeded 5:1 which is well out of the range of many automatic tuners.  I grabbed my MFJ Travel Tuner Model MFJ-902 and was able to tune the antenna to an acceptable SWR.  However, tuning the FT-817 more than a few kHz required retuning the tuner.  I decided I was onto something but just to satisfy my curiosity I replaced the top two section of the mast with fiberglass sections.  The SWR dropped down to where it was when I still had the PVC between the mount and the mast.  I decided to confirm my result.  I put the PVC back in place and went back to the aluminum mast section.  The SWR remained low as expected.

 So if you are considering back to back mobile whip, I would avoid commercial mounts that simply ground one whip to the mount unless you plan to use non-conducting mast material or at least 18 inches of PVC between the metal mast and the mount.  I also recommend that you make provisions for keeping the RF off the outside of the coax.  A 1:1 balun, ferrite beads, coax wound on a ferrite core or a coiled coaxial choke are all solutions for this.  The coiled coax is the least expensive but if it is not constructed correctly it will not be as effective as one would like.  This will be the topic of a future post.

73

K7PDW

SWR Changes When I Increase My Transmit Power

Have you ever tuned up your rig running low power only to have it change when you increased the power?  (It is recommended that you reduce you rigs power to its lowest setting when you tune up your antenna.  This is so you will not cause accidental interference to ongoing QSOs or overheat your transmitter.)  This becomes more of a problem as you begin to work the HF bands, but it may also happen at VHF/UHF particularly if you run a hundred watts or more.

What is the cause of this problem?

The cause is usually the RF from your antenna is coupling into some surrounding conductor that is not part of the antenna.  This can be almost any piece of metal from a nearby wire to a metal building.  It may not be noticeable on all bands but the cause is still the same.  You may think the wire clothes line below your antenna is not a problem.  But if it is within 15% of some part of the wave length you’re running, it may become resonant and start to radiate thus effecting you antenna.  Remember this is how a yagi works.  Wire or long metal objects such are metal rain gutter running in parallel to the antenna are most often the cause for most hams.

Should you be concerned?

Probably yes.  1) It will impact the way your signal leaves you antenna thus resulting in your antenna “not working as advertised”.  2) It can cause all the problems associated with high SWR.  3) Depending on what it has coupled to, there is a potential for someone getting RF burns, could cause RF on the feed line, and/or RF interference with other electronic equipment.

What is one to do? 

The easiest solution is move the antenna.  Try to determine what the antenna is coupling to.  Metal objects are almost always the offender.  Hidden wiring in building walls/ceilings or metal siding/roof are some of the more common problems.  However, sometimes trees can cause this especially if they are beside the antenna.  Trees at the ends of dipoles aren’t usually a problem nor are metal masts/towers.  It is always good practice to use several feet of rope at the end of an antenna to isolate it from whatever you use to support it.  (Don’t forget to use an insulator.  Wet rope will conduct RF.) 

However, things are not as bad as they might seem though.  Generally moving one end of the antenna a few feet away from the offending conductor is sufficient to resolve the problem.

RF Gain Control

So my HF receiver has a RF gain control.  That's nice.  What does it do?

The answer you will often get goes something like this.  "Just turn it all the way up and leave it alone."  Or, "It is just a leftover from the old days.  It's kind of a traditional thing to put it in.  Just leave it wide open."  Or, "It is just another kind of volume control."  Or, "Don't worry about it.  Just leave it turned up and let the AGC do the work."  All of these answers are true to a point.  But modern HF receivers have RF gain for much the same reason as the old time receivers.  Their main task is to reduce overload in the following stages of the receiver.

                                                                   Copyright (c) 123RF Stock Photos

DON'T WORRY!  A technical discussion is not about to follow.

Let's see.  The question was, "What does it do?"  In simple terms it can make a signal easier to understand and listen to.  I find there are three situations where adjusting the RF gain helps me out.

SITUATION 1 - I am listening to a very very strong station and it sounds bad.  I often will turn the RF gain down to the point that I have to turn the AF gain up.  That will many times make the signal more "listenable".

SITUATION 2 - A ham neighbor near by is on the air.  His signal is not splattering me but other signals seem to be weak or just don't sound as clear as usual.  Turning down the RF gain some will often clear up the others signals that I am interested in hearing.  This is most often helpful on the lower HF bands.

SITUATION 3 - There is a lot of static noise and the noise limiter just does not seem to help very much.  Turn off the noise limiter and turn down the RF gain slowly.  I often find that there is a point where the noise goes down more than the signal.  I call this the "sweet spot".  I may even turn the noise limiter back on.  See note below.

If you don't encounter any of the above situations, then yes, leave the RF gain all the way up.

I most often reach for the RF gain when listening to weak signals in noisy conditions.  By finding the RF gain's "sweet spot" on a signal it reduces my "listener's fatigue" and I enjoy my time on the air much more.

Hope you find this helpful and happy hamming.


NOTE:  You may find that adjusting the RF gain with the noise limiter on works fine for you.  It's easier for me to find the "sweet spot" with the noise limiter off on my radios.

What Is Ham Radio?

The easiest answer “almost anything you want it to be”. That probably sounds trivial to someone not familiar with all the activities that hams find to include in their hobby.  But take a step back and consider all the ways you communicate in today’s world that are not face to face.  Cell phone is a two way radio.  Email is available through packet radio.  Skype is not unlike amateur TV.  The list goes on.  Ham radio uses GPS technology to provide positioning systems over radio link just like smart phones.  Remote weather reporting techniques over packet radio links not unlike weather bug.  Satellite communications and even radio communications bounced off the moon and meteor trails.  Hams are converting WIFI systems to ham bands to set up communications networks for ham radio operations.  Recently a group of hams launched and tracked a balloon across the Atlantic Ocean clear into the central Mediterranean.  And what does that all mean.  It means that ham radio is one of the most unlimited and diverse hobbies around.  You can specialize in one or two areas or many. 

Now if you’re thinking why would I go to the trouble and expense?  I have a smart phone.  To that I would say this.  Ham radio still works when all other forms of communications fail.  Hams are not dependent on internet, cell towers, phone lines, or even satellites.  This is because most ham gear is designed to run from 12 volts.  With almost any car battery and a length of wire for an antenna and a ham will get up and running.

Amateur Radio also has a public service aspect that many people don’t even realize.  If you have been to a parade, a bike race, foot race, a community fair or celebration, or a public emergency that involved many people, there were probably several hams with their radios, providing communications services.  These may have included traffic and parking coordination with the public safety folks, communications at first aid stations, sag wagon communications at bike races, lost children assistance, search party communications, communications for logistics at wild fires and other disasters, shelter communications in disaster areas, health and welfare messages in/out of disaster areas.  The list is nearly endless.  It might be said that ham radio operators are the silent communicators because we go unnoticed most of the time.  We use our own equipment, provide our own transportation, bring our own food, water and shelter in many cases, and sometime even risk our lives so that messages, emergency and otherwise, get passed along.  We do it all for love of the hobby and community service.  We don’t get paid.

If you are reading this and you are not a ham, consider looking into the hobby.  There is a niche for nearly every interest.  If ham radio is not for you, that’s ok too.  But next time you’re out and about at an event and you see that geeky looking gale or guy with a two way radio, ask “Are you a ham?” and a say thanks for helping out.  He or she will probably be a little embarrassed and not know what to say, but it will be appreciated.

SWR vs Signal Strength Demonstration

I first got started in amateur radio in the late 50’s.  I lived in a small town in south central Idaho.  That town had less than 2500 people and over a dozen hams.  This was due mostly to the efforts of one dedicated ham, Ted Goers, W7ORB, now silent key (ham speak for deceased).  Ted along with the owner of the local electrical shop, E.I. Shaw also a ham whose call now escapes me, were responsible for  elmering most of the ham in town.  Ted would have these sessions when he would demonstrate a ham radio principle, usually at the electrical shop.  One of these really stuck with me.  IT was an antenna with an almost perfect (1:1) SWR but almost no signal radiated.  He was demonstrating that a good SWR does not necessarily mean that the antenna is any good at all.

He took an untuned mobile whip antenna that was mounted on an insulator with a coax connector.  He had connected a coaxial T to the antenna.  On the second leg of the T he connected a 50 Ohm dummy load.  On the third leg he connected about 100 feet of coax.  He connected a 125 Watt multiband transmitter.  He first set up the transmitter for 80 meters.  We check the SWR and it was 1 : 1.  We tuned around and found a QSO going on and called of a break.  After several tries one of the people in the QSO said, “I think I hear a station trying to break.”  After a couple of tries the guy on the other end said, “I can tell there is someone in there but you’re signal is too weak to copy.”  Ted switched to an 80 meter dipole he had up about 40 feet and tried again.  The guy came right back with a 599 signal report 100% copy.  We thanked him and checked the SWR on the dipole.  It was 1.6 : 1 at the same frequency.

This not only points out that a good SWR doesn’t tell you much about the capability of an antenna system but points out the need for having someone check your signal strength. 

Just to prove the point Ted then removed the T and put a tuner at the base of the whip.  He tuned it up and broke back into the same QSO*.  This time the signal report was 569 but armchair copy*.  The whip was 9 feet tall standing on the ground with no ground radials for RF ground.  In spite of the fact that this arrangement is an extremely poor antenna, it still was better than the perfect SWR “antenna”.  And by the way the SWR on the whip with the tuner’s help was 1.9 : 1.  That is a marginally acceptable SWR.  As I remember the station we contacted was in California.  Not a great DX* contact, but certainly not line of sight/local.

* - See Glossary

WEAK SIGNAL AND LOW SWR

There are a number of things that can cause this problem.  Here is a list of common ones.

                Placement of the antenna

                Very long feed line
                High feed line loss
                Signal Ground Issue
                Local terrain

These apply differently at different frequencies.  I’ll note this as I go.

Placement of the antenna - above 50 MHz

Higher is always better.  Always place an antenna for VHF/UHF as high as your budget, safety, and local ordinances will allow.  Keep it clear, as much as possible, of metal sheeting, metalized shingles, wires and cables, trees, even some types of glass in windows.  Running a handy talky from inside a house, vehicle, or camp trailer is not going to work well at all.  If you are lucky enough to be close to a repeater, you may get away with it.  But simplex operation will be very marginal to non-existent. 

Very long feed line - especially above 30 MHz

Feed line loss can seriously reduce the amount of power that arrives at the antenna.  Use as low loss feed line as you can afford.  Feed line loss is discussed in another post, “Feed Line - Introduction to coax”.

High feed line loss - gets worse as frequencies get higher

A feed line with high loss can mask a high SWR.  Any signal on a high loss feed line loose power.  It does not matter whether the signal going toward the antenna or being reflected back from it.  (Remember SWR is a ratio of the RF leaving a source and that being reflected back toward the source.)  When you measure your SWR at the transmitter end of the feed line, the reflected RF is lowered by the feed line loss too.  So the SWR may be at an acceptable level but there still may be a mismatch at the antenna that is seriously impacting the signal strength.  For this reason it is best to check SWR of a new antenna with a short piece of coax that is known to be good before you connect a long feed line or a feed line with considerable loss.

Local terrain - all frequencies but especially above 30 MHz

Generally radio signals are absorbed by soil.  However, radio signals also reflect off soil and rock that is highly mineralized.  So as a general guideline the best I can say is give it a try.  You just never know.   HF antennas placed in deep valleys or close to mountain ranges may suffer from poor performance. 

  

Signal Ground Issue - especially below 30 MHz

I find an analogy first told to me by my Elmer (ham speak for mentor) helpful.  “Radio signals want something to push against to get off the antenna.”  A poor signal ground will reduce a signal’s strength just as effectively as high loss coax.  Verticals at all frequencies are most susceptible to this problem.  Long wire antennas also need good signal ground, especially random wire (length) antennas.  Most VHF/UHF antennas have signal ground built in as radials that do not require tuning.  (This doesn’t mean VHF/UHF antennas don’t need tuning.)  Signal ground is the subject of another post.

VHF/UHF Dual Band Antenna "Feature" You Should Know

Most dual band antennas available for VHF/UHF have a little discuss “feature”. 

Maybe you have noticed that very few antenna manufactures ever give their angle of elevation[1] information for each band.  They are very happy to share the maximum gain in dB for each band.  However, there is a characteristic of most antennas that causes the higher frequency signal to leave the antenna at a high angle.  It is not unusual for the major lobe on 2 meters to be less than 10 degrees while the 70 cm major lobe to be higher than 45 degrees.  The truth of the matter is that there will usually be a secondary lobe on 70 cm that is lower than 10 degrees.  But that lobe is not the maximum gain lobe.  Now as far as operational performance, this is seldom a problem.  The only time this may be noticeable is if you find that 2 meter signals are getting difficult to copy and change to 70 cm expecting to get an improvement in signal strength.  You may still find that the signal is easier to copy, but this will most like be due to better signal to noise ratio[2] rather than a stronger signal.

So is this a problem to be concerned with.  Generally speaking I’d say no.  However, you should be aware of it and understand that the gain numbers you see in advertising are necessarily representative of an antennas performance in the real world.

There is one antenna, in my experience, that does have low angle at both VHF and UHF.  It is the ARROW OSJ 146/440.  See http://www.arrowantennas.com/osj/j-pole.html.  It is one of the best antennas for the money.  It is rugged build from aluminum rod not tube.  It has good SWR on both bands.  It is heavier than antennas made of tubing but still not unreasonable.  I have one that I use for portable operation with my RV and just “out in the field”.  It has been blown over while up on a 20 foot mast and didn’t even bend when it landed on a parking lot.  I know a number of ham locally that own one of these and I have not heard one complaint.

---

[1] - Angle of elevation is the angle above the horizon at which the strongest lobe leaves the antenna.  The smaller the angle number is the better for reaching maximum distance.

[2] - Signal to noise ratio is a measure of signal strength to the background noise, i.e. noise floor.

Radio Frequency Ground

A good Radio Frequency, RF, ground is as important to having a strong good quality signal as a good Safety Ground is to a safe operating environment. 

RF grounding is a very broad subject.  It includes but not limited to topics such as coaxial shield, antenna elements, ground planes above, on and below the ground, in-shack grounding, baluns, coaxial chokes, bead chokes, sometimes called bead baluns, wave guides for the microwave folks, and the list goes on.  However, for purpose of brevity this post will be limited to some approaches for preventing or solving RF in the shack.

First, how might you know if you have RF in your shack?  Here is a list of common problems that indicate that you might need to work on improving RF grounding.

                Getting RF burns from metal surfaces of the equipment

                SWR fluctuations when you get near or touch metal surfaces

                Reports of poor quality audio, usually a buzz or distortion[1]

                Poor receiver reception or noticeable drop in receiver reception

                A change, usually for the worse, in SWR[2]

                High SWR on one band but not others

So what to do? 

First, if this is a new problem, check all your ground connections to be sure that they are all secure.  This means check that all coax connectors are screwed down snugly[3] and the same for all ground lugs on station equipment, ground rods, and all safety ground connection points. 

There is one thing that I’ve observed many new hams doing that often is at the root of RF grounding problems.  They fail to provide any type of RF ground at all in the mistaken idea that their safety ground will serve as an RF ground.  This is not completely in error.  The truth of the matter is that safety ground does contribute in some part.  The problem is that safety ground is seldom sufficient for radio frequencies especially on HF.  On higher frequencies this is not as often a problem in some part because most antennas for VHF/UHF provide built-in RF grounds. 

RF grounding problems often show up at a particular band while other bands are fine.  This situation is because something in the station installation is providing the necessary ground for all the frequencies except the one that is causing problems.  This is particularly true for tuned random wire antennas that often used on HF.  Almost all end feed wire antenna must have a tuned length counter poise[4] for each frequency of operation.  I will have more on this later when I discuss random wire antennas.

In cases of balanced antennas fed with coax, a balun, be it wire wound, coiled coax or ferrite beads, will usually resolve RF in the shack problems.  This point is often neglected when balanced antennas, such as dipoles, loops, and yagis are discussed.  For best results with these antennas something should be done to accommodate the interface between an unbalance feed and a balanced antenna.

If you have installed a balun and still have RF in the shack, you might measure the feed line to see if it is a ¼ wave length[5] on the frequency that is giving you problems.  Sometime a feed line can act as a “receive” antenna and pick RF.  And it does not have to be the feed line that is in use.  This is especially true if the feed line runs parallel or even close to parallel to the antenna that is in use.  (Running feed lines in parallel to antennas should always be avoided) 

The suggestions in the posting are by no means exhaustive.  However, if you are having problems with an existing installation or are building a new installation the things discussed here should be followed as a minimum.

---

[1] - On VHF/UHF this might be multi-path.  If the problem come and goes it may be multi-path.  Multi-path will be the topic of a future posting.

[2] - This may take place slowly over time.

[3] - There was a recent article in QST where the author promoted using pliers for tightening PL-259 coax connectors.  I would recommend using caution.  This may be appropriate if you expect a lot of vibration such as a mobile installation.  However, I cannot remember ever having a connector back off on its own.  I recommend you use fingers to tighten connectors.  If you expect vibration or encounter this then a turn or two of electricians tape will go a long way toward keeping things in place.

[4] - A counter poise is a length of wire cut for a specific frequency that provides a RF ground.

[5] - A quarter wave or multiple of quarter wave (¾ - 1 ¼ - 1 ¾ etc) can cause this to occur. 

Ham Speak: A Glossary For New Hams

A collection of definitions and links to amateur terms.  It is by no means complete since ham radio is such diverse hobby.  First some links then a alphatical list on terms I have use on this blog.  Hope these help.  I you find a broken link send me an email.  My address is a t the bottom of the blog.

ARRL - http://www.arrl.org/ham-radio-glossary

armchair copy - easily copied,

antenna system - any equipment following transmitter's output up to and including the antenna.  In some cases this may include RF ground equipment depending on the antenna type in use.

CW abbreviations - http://www.radiotelegraphy.net/prosigns.htm


DX - a long distance contact usually to another continant or some folks say over a 1000 miles
elmer - mentor

ERP - Effective Radiated Power, the sum of the gains and losses of the transmitter's power output that is transferred to the air by an antenna system

FB - Fine Business (very good, good idea, excellent)

home brew - home made

Mic fright - being reluctance to make that first contact

QSO - talking or making a contact.  For more see this List of Amateur Qsigns

QSL - I understand or do you understand, also a cards ham exchange to confirm contacts

silent key (sk) - deceased, sometimes with a call sign such as "W7ORB, sk"


FOR LOOK AT HAM SPEAK FROM A HISTORICAL PERSPECTIVE SEE http://www.ac6v.com/73.htm

Safety Grounding

Grounding can be a very complex and confusing topic.  Therefore, many hams ignore it and hope for the best. 

I’ve heard folks say, “My power supply is grounded back through the wall and my radio is grounded to that.  That’s good enough, isn’t it?  The radio works fine.” 

This is a damaged radio waiting to happen.  Nearby lightning, let alone a nearby strike, has the potential to cause damage in this type of an installation.  If you are stuck with this arrangement, like in an apartment or hotel room, then you must disconnect your power supply from the wall and disconnect you radio from the antenna whenever they’re not in use. 

It is also important to make sure that the antenna connection is at least a couple feet from any conductors even the house wiring.  If at all possible connect your antenna to a ground connection that is not part of the house wiring.  I know of a case where an arc jumped about eight inches from a dangling antenna connector to a wire running from a power supply to a radio and destroyed both the radio and the power supply.  The antenna had not even been hit directly.  The hit was on a fence several feet from the antenna mast.  The mast’s base was buried in the ground.  This can be thought of in simple terms as the antenna and its feed line act as receiving antenna very close to a one terawatt[1] transmitter, i.e. the lightning strike.  There are inducted voltages like go on in a transformer too.  The point is, it does not take a direct hit to cause a lot of damage.

So what to do?  Here is a link[2] to a very good discussion on some practical things you can do.  My recommendations are as follows

                First, read “House Ground Layouts” carefully

                Second, see what others have done

                Third, read “House Ground Layouts” carefully again

                Last, put in the best system you possibly can

One thing I should mention here.  Most modern ham radio equipment has a lug of some sort to attach a ground wire.  One should not fail to connect this to ground.  The owner’s manual for the equipment will have recommendations for this connection.  This becomes very important for radio frequency, RF, ground and needs to be done properly.[3] 

I have not gone into equipment safety grounding aspect of safety grounding primarily because if one follows the advice in the references, it will be covered adequately for commercial equipment build after the 1980’s.  However, if you are using home brew (ham speak for home made) equipment or equipment built before 1980 you should assure that metal cases are grounded properly[4].  Failure to do so could result in fatal electrical shock.

---

[1] - Lightning and Atmospheric Electricity Research at GHCC  This is a very informative but somewhat technical.

[2] - House ground layouts  This reference is recommended by ARRL as a practical approach to the problem.

[3] - See posting “Radio Frequency Ground”

[4] - Grounding for this type of equipment must be handled on a case by case basis so I will make not recommendations along this line.  It is up to the user to become informed on the hazards and how to mitigate such hazards.

Station Grounding: An Introduction

Properly grounding a radio station is far more important than is usually discussed in most ham radio license classes.  A number of problems may arise from a lack of, improper, or poor grounding.  First, there are two kinds of grounding to be managed in a radio station, signal ground and safety ground.  These are not one and the same, but they are integral parts essential parts of any station installation.  This discussion will not go into detail of grounding techniques.  It will discuss, however briefly, some things that may result from grounding problems. 

First, Safety Ground

There are two aspects to safety ground.  Most importantly it protects you, your house, and others from electrical shock or possible fire due to faulty equipment.  It provides the path by which fuses are blown should a dangerous condition occur in any piece of electrical equipment.  Secondly, it provides a path to dissipate high voltage that may happen due to a lightning event.  Notice I did not say lightning strike.  In truth there is almost nothing one can do to prevent damage from a direct lightning strike.  The best we can hope to do is mitigate the circumstances that lead to a lightning strike.  In other words we make high objects less attractive to lightning by providing pathways to dissipate static electricity to ground before it can build to critical levels. 

Second, Radio Frequency Ground

Turns out RF ground can be much more complicated to manage than safety ground.  However, the lack of or a poor safety ground can almost assure that a station will have RF ground problems.  This is particularly but not exclusively true to frequencies below 30 MHz.  So what are some of the problems you may encounter? 

You may have or develop RF in the shack.  That is a condition where the metal surfaces of some or all of your equipment may exhibit radio frequency radiation.  This is most often noticed when you touch the metal parts of your station equipment and get a RF burn or at least it feels hot as in temperature not an electrical shock.  In less severe cases you may notice that your SWR changes when you touch the station equipment.  In fact you may not even have to come in direct contact to see the SWR fluctuate some.  Another indication is you may have someone tell you that your audio sounds bad or is poor quality[1].  It may show up as a buzz or distortion.  RF in the shack can manifest in a number of ways, but the ones listed are the most common.  In any case RF in the shack is to be avoided.

Details and references to help with grounding issues will come in other postings found under the Grounding label on this blog.

ARRL has a number of references that can be reach by starting with this link. http://www.arrl.org/grounding-q-a

---

[1] - On VHF/UHF a condition called multipath may also cause very poor or unreadable audio.  However, this condition will cause both your transmitted and received signals to sound bad.  And it usually only happens in a single direction.  Other stations in different directions will be just fine.

Feed Line: Introduction to coax

Two rules to keep in mind.  One, keep it as short as practical.  And two, buy the best quality your budget will afford. 

I’ve heard a number of new hams ask, “What kind of coax should I buy?”

First, for almost any ham antenna, 50 Ohm coax is about the only requirement.  The next consideration is loss.  The very lowest loss type of coax, usually called “hard line”, is extremely expensive while higher loss small diameter coax is less expensive.  Generally speaking, larger the diameter coax is the lower loss.  The amount of loss also changes with the type of insulating material between the outer conductor (shield) and the center conductor.  This is taken into account on coax loss specification tables.

However, the greatest contributor to coaxial line loss is frequency of operation. 

Here are a couple of loss tables for some common 50 Ohm coax.

This one is for Belden 9258 (RG-8x) which is .242 inch diameter.  Belden is a cable manufacture.

Frequency (MHz)                  Nom. Attenuation (dB/100 ft.)

1                                              0.3

10                                            0 .9

50                                            2.1

100                                          3.1

200                                          4.5

400                                          6.6

700                                          9.1

900                                          10.7

1000                                        11.2

This one is for Belden 8214 (RG-8/U) which is .403 inch diameter.

Frequency (MHz)                  Nom. Attenuation (dB/100 ft.)

1                                              0 .1

10                                            0.5

50                                            1.2

100                                          1.7

200                                          2.6

400                                          3.9

700                                          5.6

900                                          6.5

1000                                        7.0

Note that RG-8x is about half as big around as RG-8.  But RG-8 has about half the loss at all frequencies.  Because of the weight and flexibility differences, RG-8 is used mostly for home installations while RG-8x is a good choice for portable operations. 

Follow this link to an abbreviated table for most coax types.  http://rfcafe.com/references/electrical/coax-chart.htm

For RG-8x see. http://www.dxengineering.com/pdf/Belden%20RG8X%20Date%209258.pdf  This is a complete specification.

If you already have the coax, check it carefully for the manufacture and numbers printed on the outer cover.  Try a Google search with the manufacture name and numbers, at least type in the RG type followed by the word spec.  You may have to look at a number of “hits” to find what you’re looking for.  Don’t give up easily.