Ham Radio Tip of the Day
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Today’s tip is from Bob, KG6AF. For submitting this tip, Bob will get one of my e-books. Thanks, Bob!
Low SWR isn’t the “be all and end all” it’s sometimes made out to be. Just because you measure a low SWR, it doesn’t mean that your antenna is radiating efficiently. Conversely, a high SWR doesn’t mean that an antenna won’t radiate. Remember that a 50-ohm dummy load has a 1:1 SWR.
Most modern rigs will reduce or cut off transmit power if the SWR the transmitter sees is higher than 2:1, so you do have to make sure that the impedance the transmitter sees at the antenna connector is close to 50 ohms. To do that, we often use an antenna tuner. An antenna tuner will make the transmitter happy, but that doesn’t mean that the transmitter’s output power is being radiated by the antenna. There are many other factors that come into play.
Take the time to read reliable material on the subject. The ARRL Guide to Antenna Tuners, by Joel Hallas, W1ZR, is a good starting place. You can also find lots of solid information in the QST archives.
73!
Dan KB6NU
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Tom N1YR says
Reply to an old 2013 post found linked to a 2023 Amateur Radio Weekly article:
Amen and Amen.
In 18 years of mobile radio servicing, I found one case (and my Service Manager related another) where a coax with no antenna connected had the SWR perfectly measuring 1:1.
In my case, I had a ‘no transmit’ complaint on a State Patrol car. The VHF transmitter perfectly measured 100 watts forward and zero reflected, measured on a Bird 43 Thruline with the meter’s jumper properly cut to 1/2 wavelength. But the mobile couldn’t be heard more than across the street.
A check of continuity showed no connection between the antenna whip and the coax connector’s center pin. Apparently, a trooper had removed the whip off of the mount for a car wash per department policy, and probably over-tightened the nut when the whip was being replaced. The mount’s projecting metal stud had twisted slightly in it’s plastic insulator, and snapped off the coax center conductor where it was soldered inside the roof. My theory is that the length of the coax was just right, so that the reflected wave was at a null passing through the Bird meter’s sensor tube. The car got a replacement antenna mount that day.
Also remember any resistive loss in an antenna system decreases the amount of reflected power sent to a meter. Some of the ‘miracle antennas’ marketed have been found to be designed with a resistance in series with the radiating elements, burning up some of both the forward and reflected power as heat, not signal.