The benefits on receive are that they can reduce the noise picked up by
the outside of the coax. That noise can come from the house, or the
neighborhood, or a vertically polarized storm many miles away, presuming
your antenna is horizontal and discriminating against those noise
sources. But if the DX signal was being heard by the vertical pickup on
the feed line, it may get weaker.
The benefits on transmit are slight, unless that coax radiation has been
putting more RF into nonham equipment like the neighbor's stereo. There
is a chance of reducing the vertically polarized component of your
transmitted signal improving the spacial "purity" of your signal, but if
the choke is more dissipative than reactive the increase in the
horizontally polarized component will be too little to measure at the
distant station's location.
73, Jerry, K0CQ
On 8/8/2010 2:19 PM, Richards wrote:
Hi Jim.... I am working my way through your piece, and getting the
drift that clamp-on ferrite beads do not typically hurt me, but unless
I have A LOT of them, they won't help much (if at all).
I did like this quote (taken slightly out of context, of course...) :
Receiving Antennas benefit significantly from the addition
of a common mode choke to isolate the antenna from its
feed line. Both ends of the RG6 feed line to each of my
Beverages takes 8 turns through one of the Big Clamp-Ons.
And yet, if I read your Figure 35 on page 26 correctly (unlikely, but
I will try, anyway...) it seems to say one needs A LOT of ferrite beads
to make a substantial difference - on, say 40-10 meters, even more on
lower bands - but the chart indicates numbers tested were like 20 and 40
beads... and if those are the typical 1/2 inch long beads (or the .19"
beads mentioned int he text), then it seems one needs just 20-25 inches
of beads to have some meaningful
impact / effect. THAT seems tolerable if one was going to use clamp-on
beads - as did W0IYH using a string of clamp-on beads on his 10M
antennas - with good effect (acknowledging he preferred them over other
types for convenience and specific reasons associated to a phased
cabling already in place...)
So, they seem to have some positive effect - BUT AM I CORRECT THAT THEY
HAVE NO SIGNIFICANT DELETERIOUS OR HARMFUL EFFECTS if placed on the coax
near the feed point?
I am especially interested in whether or not they might have more or
better impact on a receiving antenna than on one I transmit with. I
use several receive only antennas, and anything to lower noise thereon
is a bonus. I would anticipate they would be more benign in a receive
only setting than on a transmit - receive setting.
Any traction? Or just wasting my time.
============================= JHR =============================
On 8/8/2010 1:05 PM, Jim Brown K9YC wrote:
benefit from reading my tutorial on coaxial chokes. Dissipation can be
made quite small if you make R large enough, and, as it turns out, it's
easy (and inexpensive) to do that if you understand the materials.
http://audiosystemsgroup.com/RFI-Ham.pdf
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