[UK-CONTEST] 12v down a long run of cable

[Paul_group]

Clive Whelan wrote:
> Hi James
> 
> The principle outlined will handle 400w with ease when properly 
> engineered, and is used in the USA at the 1500w level. Mind you string 
> and bacofoil are no good at these power levels ;-)

Its very easy to design a bias T to work over a small frequency range, 
for example at 390MHz I have one in a system that can insert 20A of DC 
into the feeder to power an ODU, return loss is ca 20db insertion loss 
0.1db - if you measured it at 1.8MHz it would be abysmal..

If you want to make a system for 1.5Kw with a bit of safety margin and 
that covers say 1.8 to 52MHz, its a little more difficult. We ideally 
want to keep insertion loss and vswr under control. Too small a 
capacitor and 1.8MHz vswr suffers. Even the excellent commercial 
Polyphaser bias T's which are specified to 1.5MHz are running out of "c" 
at 160m but conversely too large of a capacitor value can setup some 
interesting current transients in the receiver when switching.

Anyway a 50R system running at 400W isn't qro so a homebrew design is 
fairly straightforward, With a bit of safety margin select components to 
deal with say 1Kv and 5A of RF. Suitable capacitors can be obtained from 
ebay at low cost and the inductor is easy enough to wind, if you want it 
to deal with a few amps of DC at 12v, lots of turns 18/22swg on a ptfe 
12mm core would do.

The inductor will have self resonant frequencies, much like an 
amplifiers anode choke, its worth measuring the inductor in situ to make 
sure the resonances are not at frequencies you want to use.

You could also provision in the design for lightning protection.

-- 
73 de Paul GW8IZR IO73TI
http://www.gw8izr.com