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.

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[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.

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[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.

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[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

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[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.

SWR: Coax Check

Even a new coaxial cable should be checked.  The coax and connectors may look flawless, check fine with an Ohm meter and still have a serious mismatch, i.e. high SWR. 

You will need a 50 Ohm dummy load, a SWR meter, a known good coax jumper, Ohm meter, and transmitter.

Ohm meter tests:

Check the cable from the center pin on one connector to the center pin of the connector on the other end.  These should be a short circuit, less than one ohm[1].  Next check the shield from the outside of one connector to the other.  This should also be a short circuit.  Now check from the center pin to the shield using either connector.  This should be an open circuit, no reading, on the highest Ohms scale[2].  Be sure that you are not measuring you fingers by coming in contact with the meter probe tips or any part of the connectors when you make this measurement.  Stay off the metal.  When these checks have been made and the connection are all correct you may proceed to the SWR check.

SWR meter tests:

Connect the dummy load to one end of the coax you want to test.  Connect the other end of the coax to the Antenna connection on the SWR meter.  Connect the jumper to the Transmitter connection on the SWR meter.  Connect the other end of the jumper to the transmitter’s output connector.

Turn the transmitter power to minimum and set the mode to CW, AM or FM.  SSB will not work.  Now transmit and read the SWR.  It should be no more than 1.1 to 1 at the frequency that you intend to use the coax.   

If the SWR is higher, carefully do a visual check of the connectors and run the Ohm meter test while wiggling the connectors.  This is especially true for a home build coaxial cable.  It is very easy to short coax when installing connectors of any type.  A cable may not be shorted or open but still have SWR problems.

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[1] - 75 feet of RB-8x measured 0.6 Ohms on both the shield and the center conductor.  When the leads are shorted, they measure 0.3 Ohms.  That means the coax is 0.3 Ohms.  This is acceptable

[2] - The meter I use has a 20 MOhm scale.  It has no reading on the same piece of coax.  This is fine.  You can check the meter by wetting a finger on each hand and measuring your resistance to assure that the meter is working.  This reading will vary widely and will even change depending on how tightly you hold the meter lead tips.

SWR – Antenna Adjustment

This procedure is for adjusting the SWR of an antenna using a cross-needle SWR meter.

Connect the antenna to one end of a coaxial line.  (It is best if it is the same line you intend to use during normal operations.)  Connect the other end of the coax to the connector on the SWR meter marked Antenna.  Now connect a short jumper (keep this coax jumper as short as practical) to the connector marked Transmitter on the SWR meter.   Connect the other end of the jumper to the transmitter output connector.  Set the transmitter to FM or CW.  Do not use the SSB mode.

Reduce the power out on the transmitter as low as it will go.  Now tune the transmitter to the low end of the band for which you are tuning the antenna.  Transmit and record the reading on the meter[1].  Next tune the transmitter to the middle of the band.  Transmit and record the reading.  Now tune the transmitter to the high end of the band.  Transmit and record the reading.

If the reading gets progressively higher from the first reading to the last, the antenna is too long and will need to be shortened.  If the reading gets progressively lower from the first reading to the last, the antenna is too short and will need to be lengthened.  If the second reading is lower than the first and last, the antenna is tuned in the band. 

If the SWR is still high where you intend to use the antenna the most, you may adjust the antenna to lower the SWR there, but it will change elsewhere in the band. 

If the SWR remains unacceptably high (greater than 2:1) at the lowest point, it means the antenna is resonant in the band of interest but the impedance of the antenna is not near 50 Ohms.  All the shortening or lengthening will not bring the SWR down any lower.  Other steps will need to be taken to adjust the antenna’s impedance.

Impedance adjustment will be discussed in future posts.

                                                                                 1.9:1 SWR

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[1]  - Read the SWR where the needles cross. 

Some Antenna Do’s

There are some things that one should avoid when planning or working an antenna installation.

SAFETY CONSIDERATIONS

-  Keep away from power lines.  They are deadly.  There is any number of ways to come into contact with power lines and your chances of survival are slim to none.  This also means masts or towers that can fall over onto power lines once erected or during erection.
-  Always wear gloves when handling rope, cable, coax, antenna wire, etc, while putting up antenna installations.  A small slip of any of these can remove quickly remove everything to the bone and beyond.
-  Always wear a qualified safety harness every time you set foot on a tower and hard hats all around.
-  RF exposure must be taken into account.  It is a FCC rule.  The Amateur Radio RF Safety Calculator on DX-Zone’s website will help you perform the RF exposure study required by the FCC. 
                                http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=11352  

LOCATION CONSIDERATIONS (To make it easier on me, this posting will not consider HF antennas.)

-  Generally speaking the higher you can put an antenna the better.  This is especially true for VHF/UHF antennas. 
-  Keep your antenna away from trees.  Trees will absorb RF signals both inbound and outbound again especially at VHF/UHF frequencies.
-  Whenever possible get above buildings.  Metal buildings will act like a reflector to inbound and out bound signals.  Metal siding and roofs, electrical wiring, metal window screens, metal Venetian blinds, E glass windows, etc, can cause problems with your signal.  You may experience a signal that is coming in strong, but the voice is so garbled you cannot understand it.  This is usually caused by something called multipath.  The inbound signal is bouncing off a metal surface somewhere between you and the transmitted signal.  It can often be solved by moving the antenna on one end or the other by as little as a quarter wave length.
- Orient your antenna with the radiating element in a vertical plane for FM work and in the horizontal plane for SSB and CW.  These are conventions use by ham radio and other radio services.  Tilting the antenna or wrong orientation will cause your signal to drop off in strength.  This is easy to forget when using a hand-i-talky.

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NOTES:  All of these considerations apply to mobile antennas and portable operations.

                       See Antennas in the topic list and read “Using the Amateur Radio RF Safety Calculator”

Using the RF Amateur Radio RF Safety Calculator

DX-Zone has posted an Amateur Radio RF Safety Calculator for hams to use to make it easy to comply with the RF safety requirements of the FCC.  See link at the bottom of this posting.

Calculate Radio Frequency Power Density

Top of Form

The average power at the antenna: In watts
The antenna gain in dBi: Enter 2.2 for dipoles; add 2.2 for antennas rated in dBd
The distance to the area of interest: From the centre of the antenna, in feet
The frequency of operation: In MHz

My user notes:Bottom of Form

A – Enter the maximum RF output power your radio can run.  This will not take into account feed line loss or loss due to possible mismatch but it will not get you into trouble either.

B – This information is usually available from the antenna manufacture for commercial antennas.  For home brew (ham speak for home made) antennas, you may have to do some research to get an estimate of what a similar antenna’s gain might be.

C – This is the closest point that a person might come to the antenna during transmitter operation.  That means if someone 6 feet tall is standing under an antenna whose center is 18 feet above the ground, that person is 12 feet from the antenna.  This includes people inside a building where the antenna is mounted.

D – If the antenna is a multiband you must run the calculation for each band.  Use a frequency near the center of each band.

This calculator is the easiest one I have found to use.  There is a lot of good information on RF Safety on the DX-Zone website.  Once you are on DX-Zone website run a search for RF Safety using their site search.

Links –

DX-Zone   http://www.dxzone.com/

RF Amateur Radio RF Safety Calculator   http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=11352