[TowerTalk] Traps

[Tom Rauch]

> I am most interested in seeing your trap measurement results. Apparently I
> missed an earlier E-mail telling of this venture.  I am particularly
> interested in some comparative data between: W2AU, RG58 coax traps, RG59
> coax traps, etc.

Send me a coax trap quickly, and I'll measure it.
 
> What parameters will you be looking at on each trap?

Only ESR on frequencies where the trap is passing signal and 
EPR on frequencies where the trap is "trapping".

The concern was loss.
 
> I find it particularly interesting that you have built a LARGE shielded
> box in which to do your measurements. I would tend to think  that in order
> to minimize the capacitive effects, etc., on the trap the "box" would
> become so large as to become unwieldily!  

I am not worried about five or ten percent shift in resonance, but I 
am worried about the coupling to everything on my bench including 
the test cables and lights. As I said, if I even get close to the trap I 
can see the EPR of the trap drop drastically. That's because the 
traps mostly have one foot or more of conductor exposed to the 
environment around the trap. At the parallel resonant frequency, 
and dissipative load coupled to the trap through stay capacitance 
will de-Q the trap drastically.

I have to go through a similar exercise to measure physically large 
inductors.

>Being an EMC engineer, I would
> tend to think that an RF anechoic chamber would be just the ticket for
> doing these measurements.

An RF anechoic chamber would do nothing for this problem, 
because the problem isn't the radiated field. The last thing you'd 
want to do when measuring Q is add energy absorbing structures 
around the system under test that are not there in the normal 
environment.

An outdoor test over a "lossless" groundplane would work for 
measuring trap resonant frequency in the actual system, and the 
exact value of reactance added, but I can't thing of any way to do 
that accurately with test equipment near the trap.

What I would do, if I had the time, is put the trap in a vertical 
mounted over a groundscreen and determine the resonant 
frequency and inductance (at pass frequencies) of the trap by 
measuring it's effects on various height radiators.

The problem is the physical size of the trap and the extra tubing 
sticking out each end. That doesn't make for something that can 
be measured using normal "hook it up" techniques. I can get a 
reasonable idea of the parallel resonant frequency in my test setup, 
but not enough to be useful in a model because of how critical that 
effect is on resonant frequency of the elements.

If the trap was physically small, I wouldn't have had to go through 
this mess. I could have laid it on my wooden bench. 
 
73, Tom W8JI
w8ji@contesting.com

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