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

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