Speaking of RF on coax, yesterday I detected a slight sting from the
microphone during TX. Could this be from RF on the coax inside the shack?
>Nothing nicer than ghoti, lightly grilled with butter - so long as it's
ghreti.
Yes, must be ghreti ghoti to avoid the ghotiy taste. Steamed in foil with
butter and lemon is another alternative that is good and quite healthy.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Ian White GM3SEK
Sent: Monday, October 09, 2006 12:17 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Balun on a vertical
Jim Lux wrote:
>At 07:06 AM 10/9/2006, Ian White GM3SEK wrote:
>>Duncan Lindsay -MSC Valencia- wrote:
>> >Once again, one does not use a balun on a vertical antenna because
>> >it is an un-balanced antenna !! You use a UN-UN or an RF Choke at
>> >the feed point. Balun's are for balanced-to-unbalanced, antennas &
>> >coax !! Or did I forget something in Antennas 101 in college ?
>>
>>A coax-fed
>>vertical always needs some kind of device to prevent the outer surface
>>of the coax behaving as an unwanted part of the antenna.
>
>
>I can think of cases where you WANT the outside of the coax to be
>part of the antenna. Consider a portable vertical set up with a few
>radials. The coax running from the feed point on the ground back
>towards the Tx is just another radial, and would improve the
>performance of the antenna. In this case, I'd put the choke some
>distance away from the antenna. (that is, it's just there to keep RF
>from propagating back to the outside of the Tx and you).
>
A vertical with radials on the ground is a valid exception, though not
for the reason you give.
With no choke at all, the total ground return current at the feedpoint
will divide itself among all the radials, plus the outside of the coax.
Most GP antennas have enough radials to reduce the unwanted shield
current to only a few percent of the total.
As it happens, I spent last winter using a temporary 40m GP very much
like the one you describe, Jim. It had only eight quarter-wave radials
plus the shield of the coax, all laid along the ground - and I measured
the shield current with a clip-on meter to see whether a choke would be
needed. In that particular case, hardly any current was detectable on
the coax outside the quarter-wave circle. A further quarter-wave along
the ground, and the shield was stone dead to RF. YMMV, of course,
because it depends very much on ground conductivity as well.
>
>>But the same device can be called by more than one name, depending on
>>what it's being used for.
>>
>>The device we're looking for can be called a "feedline choke",
>>"common-mode choke" or "line isolator" - those names are almost always
>>correct. When used with a symmetrical antenna such as a horizontal
>>dipole, exactly same device can be called a "1:1 current balun"... but
>>not when it's connected to a vertical.
>
>
>There might be a few design and performance differences, depending on
>construction, regardless of what the beast gets called by its
>manufacturer. Some isolators/transformers/etc might have more
>parasitic C around the device than others or less series R/L.
>
Those would be differences between different people's or companies'
designs, rather than between the intended applications. The main
difference between a product designed as a "line isolator" or as a
"balun" would be in the terminals or connectors at each end.
>
>>It's a silly situation, though no more so than many other things in the
>>English language.
>Surely those collapsible fishing poles favored by pedestrial mobile
>people are used to catch ghotis. (apologies to Shaw)
Nothing nicer than ghoti, lightly grilled with butter - so long as it's
ghreti.
Not to be confused with its cousin the gotcha, which can be small but
deadly... especially when it's overdone :-)
--
73 from Ian GM3SEK
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