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eHam Forums => VHF / UHF => Topic started by: N6KB on September 19, 2010, 01:18:37 PM



Title: Ohm meter testing Larsen antennas
Post by: N6KB on September 19, 2010, 01:18:37 PM
I have a NMO-270 two meter / 70 cm mobile antenna which does not seem to work. I have not done wattmeter foward vs. reflected test on it yet. There is DC continuity through the "loading coil" (or whatever you would call the thing in the middle of the antenna), but there is no DC continuity through the "base coil" (or whatever you would call the thing at the bottom). Should there be DC continuity there?


Title: RE: Ohm meter testing Larsen antennas
Post by: W9CN on September 20, 2010, 09:04:56 PM
No.  The input from the NMO side has a capacitor so testing it with an ohm meter will show that its open.  If you can measure capacitance it runs in the 60 pF or so range on the four I have.

Best regards and '73,

Mike Pappas
W9CN


Title: RE: Ohm meter testing Larsen antennas
Post by: N6KB on September 22, 2010, 10:29:11 PM
Thanks Mike. I think the problem was a cheapo NMO mount, by Maxrad or Antennex, not sure which. The insulating washer between the grounded part of the mount an the hot lead button contact was deformed, sticking upwards above the hot lead contact. This made the hot lead contact spring of the antenna touch only the insulating washer and not make contact with the NMO mount hot contact button. I put a different antenna on, on Larsen 5/8ths wave VHF, and it worked, probably because I screwed it down tighter. Later I used a plain 1/4 wave, which doesn't reall have a spring contact, and it definitely pushed the insulator down and made good contact on the hot lead. The ultimate solution will be to put a quality NMO mount in place of the cheapo one.


Title: RE: Ohm meter testing Larsen antennas
Post by: VE3ZXK on October 01, 2010, 08:05:58 PM
...it would be easier to start your troubleshooting using a wattmeter first.