[TowerTalk] Fw: book Tribander comparison test

[Jim Brown]

On 3/5/2011 3:06 PM, Steve Hunt wrote:
> Let's say there was 5pF from each side of the choke to the boom - that's
> 2.5pF net. At 30MHz that's a reactance of 2000 Ohms shunting the choke.
>
> Not sure if that makes a difference in your application, but if I'd
> carefully designed a choke to have several thousand Ohms Resistive
> impedance I wouldn't want to be shunting it with 2K reactance.

My response on this was trimmed.  What I said was that the capacitance 
via the boom is in parallel with the choke, WHICH IS A VERy LOW Q  
PARALLEL RESONANT CIRCUIT, so that the effect of the capacitance will be 
to lower the resonant frequency a bit. I also noted that  the boom 
itself is in series with that capacitance, and that if it's shorter than 
a quarter wave, will look capacitive, so that's also in series with 
those small capacitances.  Typical values for the capacitance of a choke 
that might be used on a tri-bander are on the order of 4-5 pF, so 2 pF 
stray WILL move the resonance down a bit.  As my old prof would have 
said, doing the sums is left to the student. :)

So the capacitance is NOT negligible, it does need to be considered, it 
should be minimized, but it is not a train wreck if reasonable care to 
do so.

73, Jim K9YC