At 01:44 PM 8/20/2003 -0400, Chuck Counselman wrote:
>At 6:17 AM -0700 8/20/03, Jim Lux wrote:
>>Hmmm.. is a quarter wave actually how far apart you want them? I don't
>>know that putting two chokes at 1/4 wave apart guarantees that one is at
>>a high current place in the line....
>
>I never said that it did. However, when you insert a choke at one point
>along the line, the common-mode-wave reflection from this choke causes a
>current minimum to appear there, so a current maximum appears
>one-quarter-wavelength away.
OK.. good point.. the choke acts like an open on the TL formed by the Coax
shield /environment
>>...I suspect that empiricism will rule the day here. One probably needs
>>to come up with a way to measure the common mode isolation....
>
>Yes. I use clamp-around RF current probes to measure common-mode current,
>and a simple bridge to measure the complex impedance of a common-mode
>choke. It sure beats working in the dark.
Excellent idea... What does your probe look like, and is it broadband or tuned?
>>Let's consider why you're doing all this choking in the first place:
>>a) to reduce the perturbations from feedlines on the antenna pattern....
>
>OK, although it's not _my_ primary concern. Maybe for others.
>
>>b) to reduce imbalance....
>
>Yes, although that's a tautology.
>
>>c) so that you don't get common mode induced currents
>>coming in from the near field.
>
>OK.
>
>I would add
>
>(d) to keep transmitting RF current from flowing into my house and
>bothering my telephones, alarm system, audio and video entertainment
>systems, computers, etc.; and
>
>(e) to keep QRM and QRN from the electronics etc. in my house from being
>conducted out to my antenna.
I agree... d and e are basically manifestations of c (reciprocal in the
case of e). In the case of those of us with physically compact
installations (i.e. we don't have all the antennas out on the back 40), I
suspect that D and E might be primarily due to the induction into the
victim wires directly, without the coax entering into it. The "RF hot rig"
on the other hand...
Gosh, wouldn't it be nice if someone manufactured coax with an RF absorbing
outer sheath.. maybe there's a use yet for that carbon steel braid... Of
course, that's a bit overkill..
I wonder how well a length of iron pipe would work as a choke?
There's also an issue for multiband operations (one of the previous posters
had mentioned building customized chokes for each frequency)... If you
have, say, 3 antennas, each for a different band(s), then you might need
separate chokes for each band for each TL, which is starting to get
complex. The coil o' coax choke is fairly high Q... (judging from the
measured data posted yesterday, and from the fact that it's basically a
self resonant inductor wound from the coax shield, which is pretty low
loss). The coil o' coax might better be termed a "trap" than a
choke. Maybe the EMI/EMC lossy ferrite (or, the iron pipe) is a better
broad band solution.
If one has appropriate measuring tools and a lot of time, maybe the
solution is strategic grounding of the transmission line (perhaps through a
resonant length of wire) at places where there is high voltage on the
outside. But then, we'll get into a discussion of what "ground" really is....
>-Chuck, W1HIS
>
>
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