Saturday, December 31, 2011

SWR – General Information

First, let’s clear up a common misconception.  SWR does not tell you anything about how well your antenna “gets out”, i.e. radiates.  It is a measure of the quality of your antenna system1 or its ability to transfer RF energy from your radio onto your antenna.   It is a very important measurement to make with any new antenna or coax.  It needs to be made periodically on any antenna system, especially with mobiles2.

SWR tells you if your radio will be damaged by the antenna system.  There is another prevalent misconception one hears from time to time.  “The radios these days have protection circuits that cut back the power SO THE RADIO WON’T GET DAMAGED.”  This is half true.  The radios do have protection circuitry.  However, that circuitry will not protect the radio from long term abuse such as transmitting into a seriously mismatched antenna system.  Continuing to operate into a mismatch will “slow cook” your radio.  Just how long that will take is hard to judge.  If there is an emergency situation use the radio, but keep the transmissions as short as possible.  It is even more important that the radio not fail in that case.  The receiver will not be damaged by high SWR.  The receiver’s performance may drop slightly but your ears won’t notice.  However, if you do notice a performance change in your receiver, you can be pretty sure that there is something seriously wrong with the antenna system.  Do not transmit in this case.  There is something so seriously wrong that your signal will likely not to get out anyway.

If you are wondering if an SWR meter is a worthwhile investment let me put it this way.    Do you change the oil in your car regularly so the car will keep running?  Checking the SWR on your antenna system is the same kind of thing and you need the tools to do the job.  You need to invest in a SWR meter.

One thing about buying a SWR meter you need to know.  Be sure it covers the bands where you intend to use it.  Most of the moderately priced ones cover 160 meters through 6 or 2 meters and others cover 1.25 meters through something above 70 cm.  Be wary of ones that cover 160 or 80 meter through 70 cm.  These are either very expensive or are not very accurate especially on the 70 cm.   

Future posts will cover SWR measuring of coax and antennas.

1 - The term antenna system includes the antenna, the feed line (generally coax), and the connectors.
2 - Most FM mobile radios indicate power out and not SWR on their displays.  Mobile antennas take a beating just from wind, bird strikes, trees, garage doors, etc.  I check the SWR around each federal holiday.  That is an easy reminder to make well space tests, ten times a year.

Friday, December 30, 2011

Quick Antenna: Wire Ground Plane

The quick and easy first antenna is a wire ground plane.  This antenna is probably the most affordable and rugged antenna you could have.  It will serve as a base station antenna and will also come in hand for portable use when you get another for your base station. 

You will need a SO-239 connector 8 ½ feet of #12 or 14 wire and four ½ inch by 6-32 bolts with 8 washers and 4 nuts to fix.

Cut 5 pieces to 20 ½ inches long.

Solder one piece of wire to the center conductor where it sticks out of the insulation on the bottom of the connector.   

Bend a loop in one end of each of the remaining wires to fit on the bolts. Put a bolt from the bottom up into one hole on the connector and slide on one washer. Slide on one of the wire followed by another washer then put a nut on top and tighten. Repeat for the other 3 holes.

Once the wire are attached as shown, bend the 4 wires downward about 45 degrees.

Now you’re ready to build the mast.  This will be a piece of ¾ PVC of any convenient length.  Mine is about 5 feet long.

About 6 inches from one end of the PVC, cut an oval shaped hole just large enough to slip the coax connector on the end of your coax cable into the PVC.  Slide the cable up and out the closest end and connect antenna.

Slide the cable up and out the closest end and connect antenna.

Slide the coax back down until the antenna sit down onto the end of the PVC.  You are ready to tune the antenna.  Note that winding the coax around the mast helps hold the antenna in place.

The antenna in the pictures has served me for many years now.  It has fall over while on a 30 foot mast.  The mast was damaged by the antenna was not.  It spends much of its life behind the seat in my truck where it gets bent and banged.  I did originally solder the 4 radial wires on the connector but that proved to be a weak point.  The radials keep breaking off so I went to bolts and have not had one break since.  Maybe it was just bad soldering but bolts work for me.

Antennas: What’s Important?

This question will garner a wide range of responses.  I am kind of a simplistic guy so my answer will sound that way especially to the crowd with degrees in engineering.  Here’s the simple of it in two parts.

One – An antenna should not damage your radio.  Keep the SWR below 2:1.  Even lower is better.

Two – You are able to talk to other hams.

Once these two have been met you are on the air.

It does not mean that you have done all you can do to improve you signal, but unless you can more than double your signal’s effective strength*, it is unlikely anyone will notice a difference.

So what are some things to keep in mind when you are ready to put up an antenna?

SAFETY FIRST!   Where are the power lines?  Too many hams have been killed by a mast or wire coming in contact with overhead power lines.

Generally the higher an antenna further your signal will reach.  This is especially true on 2 meters and up. 

SWR adjustment will be the subject of future posts.

* - This refers to Effective Radiated Power (ERP).  ERP is effected by a number of thing.  This will be the subject of a later post.

Tuesday, December 27, 2011

New Ham?

So you’re new ham.   Maybe you’ve even bought a radio and you are wondering what to do next. 

First, you need to find a club and go to a few meetings.  Check you class hand outs and maybe some handouts you got when you took your test.  There is usually something in there about local clubs.  If not then try to contact someone who taught your class or Google search “ham radio (your town name, state)”.  This search will usually yield a list of hams in your town.  Look them up in the phone book and give one of them a call or email them.  Identify yourself as a new ham.  Most will be willing to help you.  Hams generally have some kind of organization going.  It may be as informal as getting together at a local coffee shop to eyeball QSO (ham speak for talking) or a formal club with officers and such.  Generally these are the best places to find an elmer, (ham speak for mentor) and get tips on all sorts of thing related to the hobby.

Another good source of information is ham radio sites on the web.  I find that most sights have a “links page”.  This will often be helpful in answering specific questions.  However, it will probably raise more questions.  And like most information on the web, it needs filtering.  This where an elmer can help.

Second, once you have bought a radio get on the air.  Sure it can be intimidating.  Mic fright (ham speak being reluctance to make that first contact) is something most all of us had in the beginning.  Not knowing what to say is pretty normal in any new situation.  My advice is to listen for a while and get used to how it goes.   Nets are a pretty good place to get some listening and your first on the air experience.  Most nets will ask for newcomers to check in usually after the formal roll call.  Some nets, however, do not have formal roll call.  If that is the case just wait for a break in the conversation and say, “break (your call sign)” and listen for someone to say, “go ahead the break” or something like that.  This is you turn.  If this is still too intimidating get with another ham and ask to watch him operate for a while.  This will let you ask some questions and “learn the ropes”.

Hope this proves helpful and welcome to the hobby.