On 1/20/2011 12:54 AM, Steve Hunt wrote:
> For me, distortion of the radiation pattern of a simple dipole or loop
> is the very least problem that CM currents can cause.
> Much more significant is the potential for RFI, and increased noise
> levels on Rx, because the feedline has become an active element of the
> antenna system. It's usually easy to spot if you have the RFI problem;
> but the Rx noise problem is more difficult to identify, and many folk
> are unaware that they could potentially lower their noise levels by
> addressing the CM issue.
I completely agree. The first observation about this that I saw (about
2006) was from W1HIS, in an app note on using ferrite chokes for this
purpose. It's a very good piece, except that he didn't know how to
measure the impedance or performance of the chokes he was building, so
his winding instructions were very wrong.
On 1/19/2011 5:00 PM, Steve, W3AHL wrote:
> The current flowing in each element of a dipole is a result of its impedance.
> If the impedance is not balanced, neither will the current be.
> To demonstrate this use EZ-NEC to create a simple dipole and make it
> unbalanced by moving the feed point off center or by tilting one side closer
> to the ground. Using an ideal point source to feed it, look at the currents.
> They will not be balanced. It is NOT dependent on the feed line, although
> that can also cause additional imbalance.
YES! A year or so ago I did exactly this, and summarized the result in
a presentation to the Antenna Forum at Pacificon this fall. What I was
specifically looking at was the dissipation in the choke as the result
of imbalance. I also looked at the noise reduction. I used a 40M dipole
with the feedpoint offset by an amount that varied from 5 ft to about 20
ft, and with a feedline having a common mode length of a quarter wave or
a half wave. The Power Point is on my website. Select the one on Coaxial
That presentation also shows several bifilar chokes for the HF bands
wound with AWG #14 THHN (house wire) that should perform very well in a
balanced line. The model (cited above) showed that these chokes will
safely handle relatively small imbalances (5-10 ft offset) at maximum
legal power, but that greater imbalances are likely to fry them for
feedline lengths that place the greatest common mode voltage across the
choke (near a half wave, or multiples of a half wave).
Note that while these chokes are a short section of parallel wire line
wound a round a toroid, with a characteristic impedance on the order of
100 ohms. They're a relatively small fraction of a wavelength at HF, so
the additional loss they introduce is negligible, and the small mismatch
is easily compensated by the antenna tuner.
GM3SEK -- Ian, I built and measured these chokes in response to your
need for low cost chokes!
73, Jim Brown K9YC
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