Wednesday, February 8, 2012

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

Tuesday, February 7, 2012


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.