[TenTec] It's getting cold. Perfect antenna weather for a OCF dipole
k9yc at audiosystemsgroup.com
Mon Sep 21 02:42:19 EDT 2015
On Sun,9/20/2015 1:23 PM, Rick - DJ0IP / NJ0IP wrote:
> I have measured the common mode current on OCFD antennas with an RF Ammeter. I'm not guessing or modeling, I'm speaking of real life situations.
But Rick, the common mode current varies along the line, just as it does
on any antenna. I suspect that you measured the current at or near the
shack. That is VERY different from what it is up in the air where the
choke is (or at least where it SHOULD be to be effectively kill common
If you haven't done so, I suggest that you add the choke, coax, and any
matching section to the model. Then compute the currents. Also, turn on
the display of currents in the View Antenna window. W7EL's instructions
talk about how to do that. In the common mode circuit, coax and window
line has a velocity factor around 0.98, depending on the thickness of
the jacket, and much closer to 1 for open wire line.
> NO, I AM NOT FRYING THE BALUN. HAVE YOU DONE IT?
> I HAVE BEEN DOING THIS FOR 4 YEARS NOW and I am running a balun with mid-sized cores.
> My balun uses 2x FT-140-43 cores and I run 600w CW/SSB.
You have been lucky. The values of common mode current and voltage at
the choke vary as noted, depending on frequency and line length.
> I can show you my scores and country totals for the past 4 years in CQWW DX contest.
> No fried balun, but lots of countries, like 70 countries/band average.
That means NOTHING. What is meaningful is the transmit duty cycle. How
much do you RUN? For how long? How much do you run with no response to
your CQs? How much time listening between CQs? How much do you S&P?
> We are reporting measured data, not modeled data.
But no details of HOW you measured, or WHERE you measured. Are you
suggesting that NEC does not accurately predict how antennas work? MANY
would disagree with you on this issue.
> What is your justification for the claim you just made?
Why the word "claim?" I'm not selling anything. The basis of all of my
statements is a combination of what I've learned while getting my BSEE,
60 years in ham radio, the last ten of which have included extensive
studies of how antennas, transmission lines, ferrites, and common mode
chokes work. I taught math and transmission lines at DeVry in the '60s.
> I have fried several baluns in the past running symmetrical center fed antennas as you are recommending.
> That's because the impedance in these antennas can be super high on even harmonic bands.
What fool would run a dipole on its second harmonic? That's an
un-natural act! I have NEVER recommended this. Further, I do NOT
recommend the practice of loading a non-resonant dipole anywhere other
than its fundamental resonant band and third harmonic, where it is a
reasonable match to coax. I do NOT recommend the use of open wire line
as a band-aid for badly matched antennas.
Further, badly mis-matched feedlines cannot be choked with any of the
chokes I've designed or recommended because of the DIFFERENTIAL loss in
the coax as it goes through the choke. N6BV (retired editor of ARRL
Antenna Book, ON4UN Book, author of HFTA and TLW) clearly showed this in
a talk at Pacificon a few years ago (which he ran past me for peer
review), and which was subsequently published in QST (within the last
year). In my review of Dean's work, I noted that the differential
dissipation he was describing was IN ADDITION TO the common mode
> I contend that the chances of frying a balun are higher with center-bed antennas than with OCFD antennas.
On what basis do you say that? Are you talking about a center-fed wire
that is mis-matched at the operating frequency because it is far from
resonance? If so, I agree -- see the paragraph above. But again, this is
an un-natural act. It is stupidity.
My choke designs are specifically limited for use on symmetrical,
resonant antennas that are matched to the feedline at the operating
frequency. In a slide presentation (on my website) on ferrite chokes, I
showed how dissipation in the choke increases as the feedpoint moves off
73, Jim K9YC
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