Radio Antennas and DC Shorts


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Gerowen

I recorded this video kind of in a hurry in response to an ongoing debate on Facebook.  You'll have to excuse the poor video quality, I replaced my old tablet and the camera on this one is less than impressive to say the least.  I need to invest on a dedicated camera.

 

Anyway, I'm posting this as its own topic because it doesn't just apply to CB antennas, it can apply to ham radios or anything that uses an antenna.  Many antennas come with built-in tuning coils so that they can be adjusted to operate more efficiently on different frequencies.  The Solarcon A-99 is a popular entry level CB radio base station, but you can use the tuning rings on it to tune it for use on the 10 and 12 meter ham radio bands.  Wilson, Sirio and other companies also make magnet mount mobile antennas for use on cars that have a tuning coil built into their base.  With these antennas, if you check it with an ohm-meter on the coax connector or at the feed point, you will find that there's a DC short between the center pin and the shield/ground.  This is completely normal, and does not indicate a problem with the antenna.

 

Antennas that attach via a stud mount like a 102 inch steel whip, fiberglass trucker antennas (like Firestiks) and such will not show a DC short between the hot and ground because the ground stops at the stud and the antenna uses the body of the vehicle as counterpoise.  If you are using one of these antennas that does not have a tuning coil in the base, and there's a DC short between the center pin or radiating element and shield, then you do indeed have a problem.  Also, when coax is disconnected from the antenna there should not be any continuity between the center pin and shield.  If there is, then your coax needs to be fixed or replaced.  This is a common issue with smaller coax cables like RG-58 that have a single center conductor, especially when they're kinked or pinched repeatedly because that single conductor can break and poke through the dielectric insulator and short against the shield.  This is one reason I recommend running your coax through drain plugs in the floorboard or through the firewall or some place where they can remain stationary and not get pinched, kinked or cut during normal operation of the vehicle.

 

So to summarize, the Solarcon/Antron A-99 and other antennas that have a built in-tuning coil will show a DC short on an ohm-meter and this is totally normal.  If you want to verify whether or not an antenna is good, make sure it has continuity from end to end.  There should be continuity between the center pin at the feed point (where the coax connects) and any exposed metal along the radiating element, such as where the sections of an A-99 or iMax 2000 twist together.  Put an antenna analyzer on it and check the SWR at your desired operating frequencies, or use a spare radio you don't care about with an in-line meter to measure the SWR.  If possible at the time, hit the antenna with as much power as you plan on using, because sometimes the radiating elements can be burned up and will show low SWR readings at low power output levels, but then skyrocket when you actually connect your radio and hit them with higher power levels.  This requires you to actually mount the antenna, even if it's only temporary, because checking SWR with the antenna on the ground will not give you accurate readings.  Check for obvious physical damage, make sure the drain holes in the bottom of an A-99 and iMax 2000 are clear so that moisture can drain out, and make sure the vinyl cap for the tip of the antenna is still present, etc., but don't be alarmed if your A-99 shows a DC short from hot to ground, :-)

 

 

 

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