Topband: FT-240-77 Toroid Impedance R + jX Components

[Jim Brown]

On Wed, 29 Dec 2004 10:32:47 -0500, Tom Rauch wrote:

>Chokes are only useful if ou have a low shunting impedance
>to ground, and they are most useful if the antenna or
>feedline common mode impedance is inductive (or the same
>reactance sign as the CM choke) at the point of insertion.

I would revise that to say that chokes are most effective and most 
easily used if there is a low shunting impedance across the load. 
Yes, I would love to have a low shunt Z across the load, but I'm not 
going to throw up my hands if I can't -- I'll simply bring a bigger 
elephant gun. 

Example: I'm working on a problem whereby 850 MHz Nextel phones get 
into the impedance converter stages of condenser mics via their 
output wiring, mostly due to pin 1 problems inside the mics. A 
single big #61 clamp-on (Fair-Rite 0461164281) looks like roughly 
300 ohms at that frequency, and doesn't make much of a dent, but 3 
or 4 of them does. That may sound awkward and expensive -- until you 
happen to be working on a film shoot with some $3,000 mics that you 
can't readily take apart to re-wire, and with crew and talent 
costing upwards of $250K/hour. 

I generally agree with your point re: the sign of the reactive 
component, except that it matters a lot less if R is sufficiently 
large. 


Jim Brown  K9YC