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Base Antenna resistance (Ohms)

J

jimbowilly

Member
Mar 25, 2011
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Hi,
When I check the resistance on my dummy load, I get 50 Ohms.
Should I see the same resistance when testing for resistance at the radio end of my coax coming in from my antenna?
Thanks,
jimbowilly
 

I am also reading 51 Ohms when I test each end of the antenna coax with dummy load attach to opposite end. This is RG-58/U coax, which has worked just fine for the last 7 years.
 
The problem is suddenly my swr's have jumped from 1.3 to 2.5 - 3. And I'm seeing the antenna light come on when the radio is connected to the antenna. Radio checks out just fine w/dummy load. Coax shield and center wire each have continuity. Disassembled antenna set up, inspected and cleaned all components. Installed new antenna mount. But my swr's continued to be 2.5 - 3. I'll figure this out. Just another project to work on while hanging around the house. Keeps me busy so my wife doesn't start with the "honey do list".
 
I tried a 2nd radio, nothing changed, same result.
Installed new antenna radials. Testing with a Astatic PDC7 swr meter I am able to adjust the swr meter's needle to the set line. The Dosy TC3001P swr meter still will not adjust to the set line. I have two TC3001Ps. Both have similar issues with the swr meter. Thank you for your comments.
 
Yeah, the multimeter checking your antenna's resistance at DC isn't doing anything for you beyond confirming that the center conductor and shield aren't shorted together.

Have you moved the antenna?

What is it mounted to?
 
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jimbowilly said:
The problem is suddenly my swr's have jumped from 1.3 to 2.5 - 3. And I'm seeing the antenna light come on when the radio is connected to the antenna. Radio checks out just fine w/dummy load. Coax shield and center wire each have continuity. Disassembled antenna set up, inspected and cleaned all components. Installed new antenna mount. But my swr's continued to be 2.5 - 3. I'll figure this out. Just another project to work on while hanging around the house. Keeps me busy so my wife doesn't start with the "honey do list".
Click to expand...

The problem you have is you're checking your coax with DC at 0Hz. The impedance of coax is determined by the ratio of the diameter of the outer surface of the inner conductor to the diameter of the inner surface of the braid. If that alters, such as if the coax is pinched or crushed or you bend it tighter than the minimum bend radius, it affects the impedance and therefore your SWR. You can't test the impedance with a DC multimeter, you need an antenna analyser. There's been plenty of people with mobile installs with SWR into the red seemingly unable to find a fault, measuring the coax and finding no dead short but unaware where it's been coming into the door and getting crushed that the inner conductor and braid are almost touching and to the RF appearing as almost a dead short.

How do you bring the coax into the house from the antenna to the radio? It could be you've got the problem I've described especially if you've brought it in through a door or window.
 
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2NC995 said:
Yeah, the multimeter checking your antenna's resistance at DC isn't doing anything for you beyond confirming that the center conductor and shield aren't shorted together.

Have you moved the antenna?

What is it mounted to?
Click to expand...
Thank you 2NC995. The problem was due to oxidation where the radials connect to the antenna mount. SWRs are now minimal.
 
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Might be water intrusion in the coax. Water runs down on the antenna and then over and down the PL-239 connector, wicks in thru the threads, runs down into the coax's braid/shield/center core and corrodes the inside of the coax. Corrosion is resistance.

Never seen it happen. But that is why guys use 'goop tape' (?) or 'coax seal' on the antenna/coax connector to keep water from getting in and creating corrosion. Kind of a semi soft rubber goo, comes in a 1/2" wide roll. Should be used any time an antenna is put up and installed for a semi-permanent seal. I use the stuff; never had a corrosion problem.
 
Last edited:
Robb said:
Might be water intrusion in the coax. Water runs down on the antenna and then over and down the PL-239 connector, wicks in thru the threads, runs down into the coax's braid/shield/center core and corrodes the inside of the coax. Corrosion is resistance.

Never seen it happen. But that is why guys use 'goop tape' (?) or 'coax seal' on the antenna/coax connector to keep water from getting in and creating corrosion. Kind of a semi soft rubber goo, comes in a 1/2" wide roll. Should be used any time an antenna is put up and installed for a semi-permanent seal. I use the stuff; never had a corrosion problem.
Click to expand...
I was just looking at coax seal products on rf parts website, several good options there. Does anyone use dielectric grease on PL259/SO239 mating threads ? I use it on all of my 12v "plugs" and light receptacles, makes removal easier when servicing.
 
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