Batboard

A couple of years ago I purchased a NMO to SO239 adapter to include with my test kit. I just used it the other day when I did some testing on my UHF Maxtrac in the truck. When I connected my Bird wattmeter dummy load to the radio I had full power but when I connected it to the NMO on the roof I lost about 25% of my output. I thought I had a bad cable so I replaced it with a new cable assembly but with the same outcome. I then took it to the bench and had the same thing happen. has anyone else had the same problem, is it a bad adapter or are these adapters not suited for UHF or should I buy one the ones made by AS or Larson rather than another inexpensive one?

Thanks
Steve W

Someone correct me if I'm wrong here.. my math skills aren't what they used to be..

I believe 25% would be about 1.25dB
Besides the loss through the NMO mount and adapter are you also taking into account cable loss? RG-58 has what, about 12dB/100ft of loss at 450MHz, so if you've got 10-15 feet of it that would lose you around 25% right there.

Jack also forgot to mention that RG58 cable is NOT 50 ohm, more like 53.5 ohm which was the original Mil specification.

The NMO to SO239 adapter's SO239 connector has a mismatch at frequencies above 80 mHz, adding that with the mismatch in the PL259 connector at UHF going to the dummy load/ wattmeter, the losses can add up really fast.
Remember that the "UHF", PL259 and the SO239 connectors, are NOT 50 ohm charictoristic impeadance, more like 54 to 90 ohms at frequencies below 80 mhz.
UHF in the days these connectors were developed was 30 to 100 MHZ.

Beware of the cheep low cost junk "RG58 TYPE" cables that are poor even at VHF (150 mHz).

Changing to a true 50 ohm cable like RG58/CU can help at 450mhz, the Cu cable IS 50 ohm specification.


There is a NMO to Type N fe connector adaptor that is much better up in the 400- 900 mhz frequency range.