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Wednesday 21 November 2018

SWR concerns

Running a mobile station the biggest issue is the environment in which it is set up, that is a moving vehicle which is subject to all manner of rattles and bumps as you're driving along, and also anything overhead that may catch on the antenna and damage it, as well as feeding the coax from a magmount through a vehicle door or boot lid or tailgate (the latter of the three in my case), where the cable may get squashed and this can potentially change its characteristic impedance (which for 2-way radio purposes is 50 Ohms), so it is wise, especially if you have a "big three" radio, to check the SWR from time to time, this was the first time I'd checked it since installing the rig and antenna into the current car.

The SWR on 2-meters was acceptable, even at the full 50 watts the radio could do, the SWR on 70cm simplex however was unacceptable, though when I checked the SWR at the frequency used for the Jumbospot (which I occasionally have in the car) it was acceptable at all three power levels, I since adjusted the antenna and got the SWR on 70cm down to 1.7:1, still some loss present, and as the radio is being fed with RG58 anyway I expect loss on the line anyway at those frequencies, the only cable I have similar in size to RG58 is RG8 mini which has a foam dielectric and would be crushed in the tailgate and would create a new problem so I'd need to find coax that is less lossy at 70cm and wouldn't simply be squashed by the tailgate. a tall order but worth it to get the message across.

I plan to do a second SWR check in a week as I do a lot of driving and things may change, though they shouldn't as long as I avoid any really low branches or multi-storey car parks.

Adjustment of the antenna was difficult as the adjustment points had all seized, however a squirt of WD-40 soon sorted that, the grub screws holding the antenna together were easy to get out with an allen key though, all good in that regard, and with a better SWR at 70cm I feel confident that I won't destroy the finals in a £500+ radio that would certainly be out of warranty by now and the finals would not be covered by that regardless as no manufacturer honours the replacement of the finals, however if the final power transistors are of a common type and are through hole I see no reason why I could not replace these myself however the FTM-400XDE is a modern rig and has a lot of surface mount components so chances are I'd have to send it to the pros to be fixed as I cannot solder surface mount, hence why it is critical I don't blow the finals.

73 de 2E0EIJ

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