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

[Ian White GM3SEK]

Paul_group wrote:
>
>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.

Good ready-made chokes can be found from this range by Epcos/Siemens:

<http://uk.farnell.com/jsp/level5/module.jsp?moduleId=en/204081.xml>

There is a tradeoff between inductance and DC current capability. The 
100uH choke (9752102) is good up to 1A,  but if you need higher DC 
current and good performance down to 1.8MHz you might do better with two 
of the 40uH chokes in series.

For capacitors, three Murata 10nF 1kV disc ceramics in parallel will 
carry well over 1kW continuously into 50 ohms:

<http://uk.farnell.com/murata/debf33a103za2b/capacitor-0-01uf-1000v/dp/95
27222?_requestid=134673>

Those components work very well for remote switching via the coax, from 
1.8 to 50MHz.

By the way, it also needs about 100nF for effective RF bypassing at the 
"cold" end of the choke. At 1.8MHz, the typical 1nF feed-through cap has 
nowhere near enough capacitance - it's little more than a wiring 
terminal.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek