开云体育

Gary,

IF the alternative RF path is down a pair of antenna wires near a feed
point where the equivalent impedance is ~75 Ohms, then 3K ohms looks very
good. On the other hand if end-feeding and using the choke to hold back
the RF on the feedline, then RF impedance at the point of the choke might
be >2k Ohms and 3K ohms is hopeless.

This is why my original post of my results attempted to measure the
series impedance (mostly inductive) at various frequencies within the
amateur bands of my CMCs (using the HP-9753C
VNA). I use a 450-foot long doublet fed with (mostly) 400-ohm
parallel conductor transmission line (about 65-feet total of that Xline).
As such, the impedance presented here in the shack
is all over the Smith Chart. I could evaluate the effectiveness of my
chokes only by knowing the series Z they presented and the impedance of the
antenna/feedline presented in the
shack. Others wanted to see the attenuation of CM energy as measured
in a 50-ohm system. That's why I presented that measurement in the second
set of measurements I sent out
yesterday. Used as I do, every installation in the world will be
different.

The impedance of my antenna does not hit the real axis of the chart
anywhere in the amateur bands. The lowest frequency 1/2-wavelength
resonance (with a bit of capacitive end loading)
is roughly 950 kHz in the bottom third of the AM BCB. My w/c
impedance is on 40-meters: 1000 - j 1100 ohms. In my installations, I
believe a better metric with which to measure the
performance of my CMCs is the balance between current going up and
back on the parallel line transmission line. I have built a small piece of
equipemt to measure just that. I've also
taken each CMC and using an o'scope and a signal generator measured
the balance of amplitude and phase reversal from the two DM ports with CM
drive from the signal generator. That
has not appeared in my measurements as I'm trying to stick with the
NANOVNAs.

The antenna where I was trying to use this was in between those extremes,
but the NEC model run suggested that I was losing about 40% of my power
to/through the choke on 160.

The final test I put my CMCs through is in place between the output of
the matching network (L-network) and the parallel conductor transmission
line to the set of wires *at power*. I start
with around 400-watts. That caught one CMC wound with house wiring
insulated with PVC - bad heating of the insulation. It was eventually
unwound. Those wound with solid AWG #12
solid enamelled copper conductors exhibited bad coronal discharge
between winding pairs of the line on the cores. If you are, indeed, losing
40% of your power on 160-meters at 100-
watts, I'd expect some major amount of heating would be evident. If
not, something in the model is missing.

Several decades ago, I started with CMC wound on a large core of 43
material using RG-142 coax. That the 'high power' teflon insulated, silver
plated conductor 50-ohm coax. I finally
came to realize its only function was to present a large inductive
reactance to the CM energy on my transmission line. A choke wound in
bifilar manner accomplishes that in addition to
ensuring the currents cancel within the core which forces CM energy
passing unattenuated through the core - the DM energy. Therefore, all my
chokes of resent build have been bifilar
windings on the cores.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*