Hi Tom,
What is your recommendation (mix type and quantity) for ferrite beads on
each of the 1/4 wave feed lines (RG11/U) for a 160M 4 square? I have
Amidon FB-77-1024 in my inventory.
Rich K7ZV
-----Original Message-----
From: topband-bounces@contesting.com [mailto:topband-bounces@contesting.com]
On Behalf Of Tom W8JI
Sent: Tuesday, August 28, 2012 12:28 PM
To: Topband@contesting.com
Subject: Re: Topband: Wideband Balun...?
Hi Eddy,
> I am thinking of incorporating a homebrewed 4:1 balun as part of a
> transmatch that I have here, which was designed to feed coaxial
> cable-fed antennas only...
Why did you decide on a 4:1? Generally a tuner matches a much higher
impedance than a 4:1 balun will safely permit, and has the most difficulty
with low impedance loads. If you use a 4:1, it makes every load lower
impedance.
> Is there really & truly such a thing as an air core 4:1 balun that
> will cover the entire spectrum from 1.8- to 29.7-MHz...? I've looked &
> I've repeatedly searched on the internet, but can not quite come up
> with such an animal. Maybe the frequency range of such a beast is far
> too great for any practical design...
>
> Anyone have any leads in this regard...?
Generally a 1:1 ferrite core balun has the least problem with grossly
mismatched impedances, the widest useable frequency range, and the best
balance for a given size and cost.
Air core would be the most restricted in impedance and bandwidth, and have
the poorest balance over a wide range of load conditions. It also would have
very high Q, being mostly reactive in any "choking" impedance, which means
it could actually make balance worse under many load conditions.
Let me give an example. If I had an aircore balun with a common mode
impedance of 5 +j500 ohms (Q=100) in series with a feedline having a common
mode impedance of any reasonable R with a reactance of around -j250 or
higher, the balun would make unbalance increase.
Those big air core coax baluns you see will actually make many systems have
worse balance, because they are largely a pure reactance. They only help
balance under certain conditions, contrary to what is often assumed.
As a general rule we do not want a high-Q balun, but something with very
high resistance. We have to compromise that at times because of heat and
impedance limitations, but almost always one of the very last things we want
is an air core.
I use air core baluns on some of my antennas, but I control the common mode
impedance to ensure the baluns actually do what I want. I never use them in
random situations.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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