I have the same question with a bit of a twist - the 2.4" cores cost
about $6 each @ Arrow, and make a lot of sense in a QRO transmit
application. But, that is a lot of ferrite for a receive antenna choke
when they get stacked 5 high for top band applications. I'm in favor of
several chokes on the flag/pennant/Beveridge feedline so cost and size
start to add up.
The G3TXQ and K9YC charts cover RG8 and RG6 coax outside diameters, but
not subminiature stuff like RG316, which should be super for receive
only applications or "QRP" to at least hundred watts +/-. With RG316
sub miniature TFE coax it seems reasonable to use a type 31 1" od +/-
core ( e.g.Fair-Rite 2631101902 1.122 od x 0.543id x 1.125 h in stock
at Arrow) as it can handle many 316 turns and neither power handling or
coax loss are concerns. The small core volume is a plus since I can
then put it into a small box with the antenna-feedline matching
transformer. The F-R chart shows a Z @ 1 Mhz of 32 ohms vs 12 ohms for
the 2.4" size, and the Arrow prices are $2.28 vs $5.32 so the $/ohm
ratio is about 1:6 for one 2.4" core and 1:30 for a stack of 5.
I haven't seen any evaluations of such a design and was wondering if
anybody has built and measured one. For those using RG6 feedlines, then
some RG316 like 70 ohm coax is needed (or maybe just live with the
mismatch). Any suggestions?
Grant KZ1W
On 4/22/2012 12:37 PM, N2TK, Tony wrote:
> I am looking for a balun (common mode current isolator) recommendation for
> my receive loops, if I need a balun.
> Because of my lot size I only have pennant and flag type antennas. I feed
> them with a binocular core transformer per W8JI's and other's web sites
> where the primary and secondary are two separate overlapping windings. For
> my receive loops (3 fed from both directions in order to get two directions
> out of each) I bring the RG6 feedlines away at right angles for 15', then
> drop to the ground then they are underground till they get to a aluminum
> panel on my shed. The receive lines from the antennas to the shed are
> 50-130' long. They terminate at a coax switch (I feed 12V through the coax
> to switch directions on each pennant/flag) with a preamp on the aluminum
> panel. The aluminum panel is tied into my common ground with the tower,
> aluminum panel just inside the basement, AC mains box, etc. with lots of
> ground rods (27), solid #4 copper wire and Cadwelds. The RG6 feedline runs
> from the coax switch underground to the basement where it has a lightning
> arrestor and a limiter. Then it runs up to the shack.
>
> Should I add balun(s)? Where - at the antenna feedpoints, at the feedline
> to the coax switch? Multiple places?
> For receive lines what is the recommended balun? RG6 (how many turns)
> through 2.4" (or smaller) #31 type material (how many cores?).
>
> Tnx for feedback
> N2TK, Tony
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Joe Subich, W4TV
> Sent: Saturday, April 21, 2012 4:51 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Balun Recommendation
>
>
> On 4/21/2012 4:26 PM, Gary Schafer wrote:
>> Yes Ian, I agree with you that "common mode current" on a coax outer
>> shield is a complete misnomer. It doesn't describe what it is "common
>> to". If it were really common mode current it would have the same
>> current on the inner conductors as well as on the outer conductor and
>> be in phase.
> While common mode current may be a bit of a misnomer, it is accurate in that
> the effect of "outer shield current" is identical to that of common mode
> current in a balanced system because of the "pin 1 problem"
> that K9YC has written so extensively about. Since the signal return in most
> amateur equipment is not tied to a complete and effective shielded
> enclosure, the outer shield current is *indistinguishable* from a true
> common mode current as it passes through the unbalanced input/output
> circuits of modern equipment.
>
> Again, if this "outer shield current" is sufficient it can appear in in
> microphone inputs, be fed back into audio amplifiers or even be impressed
> across control and power supply circuits. The results can be anything from
> mildly annoying to fatal to the equipment depending on the level of the
> common mode signal and the circuit involved.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 4/21/2012 4:26 PM, Gary Schafer wrote:
>> Yes Ian, I agree with you that "common mode current" on a coax outer
>> shield is a complete misnomer. It doesn't describe what it is "common to".
>> If it were really common mode current it would have the same current
>> on the inner conductors as well as on the outer conductor and be in phase.
>>
>> A better name for what is commonly called "common mode current" on the
>> coax outer shield would probably be "outer shield current".
>>
>> 73
>> Gary K4FMX
>>
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