[TowerTalk] Fw: book Tribander comparison test

[Steve Hunt]

Jim,

Just one correction: the length of boom in series with the capacitors 
represents an Inductive reactance provided it's less than a quarterwave 
long; that actually has the undesired effect of cancelling some of the 
"coupling-C" Capacitive reactance and making the problem worse!

73,
Steve G3TXQ

On 07/03/2011 09:04, Jim Brown wrote:
> 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
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