Interspersed.
On Sun, Oct 21, 2012 at 9:01 AM, Tom W8JI <w8ji@w8ji.com> wrote:
> I find after some interesting conversations that readers of posts
>> often read as if written to them. So I write my posts as to the
>> general audience. If an expressed situation is so specific as to be
>> exceptional to general use, I reply in private.
>>
>
> K4XS had such a situation, as does **any case** where the feedline has a
> long distance to earth. The general application is a choke with reactance
> pretty much always aids the system when the feedine to ground path is short
> in terms of wavelength.
>
I see K4XS as still being in the general case. That is because a
wavelength on 160 is 539 feet. He wound up going up 55 feet (a separate
TopBand thread) or 0.10 (a tenth) of a wavelength. That's short,
wavelength-wise, even if we throw in the reasonable if arbitrary 1/8 wave
point as the onset of growing "significant" issues with solenoids that have
gone completely nuts by 1/4 wavelength.
> A reactance caused by spooling cable into a "choke" might not be nearly as
> effective, and can actually hurt the system, if the lead is long.
>
I completely agree, IF the lead is "long". So far we have said 1/4 wave is
long gone nuts, and that you are starting to smell it at 1/8 wave. But I
don't want to be seen approving coax solenoids on 160 at all.
A 1/4 wave to ground on the cable forms a very effective isolation, and
> even 1/8th wave is not bad, so in this case (or any other case without a
> cable shield ground close to the antenna) we have to be careful.
>
> This applies to verticals and dipoles EQUALLY, because the interaction we
> worry about is identical. There is no distinction between the two.
Other than the fact that for dipoles the second pole at the feed point is
connected to a radiator instead of a counter-field specifically
non-radiating device like radials? Or that both poles are in the same axis
for dipoles, and the poles are intentionally orthogonal for a vertical with
radials? To me personally at least, the differences seem rather in the
extreme. I could go on with the list.
*Of course* there will be a few situations where this general purpose
>
>> rule won't add any value. The question is whether a specific owner
>> has the equipment and technical knowledge to identify those
>> situations, which are highly specific and subject to all kinds of
>> local influences, and whether one can post a textbook to explain the
>> choices.
>>
>
> I agree. Simple answers are both easy and popular, but air wound chokes
> are not good baluns or CM chokes.
> They are cheap and easy, but do not fit into "always do this" applications.
>
> Here is a general rule that always works:
>
> When the path along the braid from CM source to ground point is very short
> in terms of wavelength, a coil of coax can work fine. It might add
> significant impedance to the path, because the path is probably a low
> impedance and inductive.
>
> When the path is longer, adding a coil of coax often will not do anything,
> or can even make matters worse. With a long distance to shield grounding,
> we are better off to plan the system.
>
I completely agree, and I just don't recommend use of coax solenoids on
160. For all your reasons plus many people skimp on them based on how much
coax they happened to have on the spool at the time and the choking goes
down with the square of the turns. With the cheap availability of huge #31
ferrite clamp-ons that can take many tight turns of RG400, or any number of
other practical configurations with #31 ferrite devices, WHY does anyone
bother to use coax solenoids on 160??? With a foot per turn on a 4 inch
form, it is likely CHEAPER to do a more effective something with #31.
Despite simplicity and almost universal popular acceptance, coils of coax
> are NOT good universal CM chokes or baluns. They can even make things
> worse. All of us should be worried about their use when we see the system
> has 1/8th wave or longer to ground along the feedline path. Sometimes a
> simple additional ground can be much more effective.
>
Agree, agree, agree. And for K4XS' 55 feet, 1/10 wavelength is less than
that 67 foot inception point at 1/8 wave.
Long cable lengths to ground are tricky, and it doesn't do us any good to
> pretend they are not tricky. Air core chokes or added reactive impedances
> should be used carefully, because they don't always increase common mode
> impedance.
>
> 73 Tom
>
Likewise, Guy.
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