[TenTec] It's getting cold. Perfect antenna weather for a OCF dipole
k9yc at audiosystemsgroup.com
Tue Sep 22 13:54:08 EDT 2015
On Tue,9/22/2015 12:42 AM, Rick - DJ0IP / NJ0IP wrote:
> When Jim said you fry the balun, my interpretation was, it gets so hot that it self-destructs.
> This includes things like wire insulation burning up, wires short
> circuiting, and toroids cracking or even shattering.
That is a correct interpretation.
At least 5 years ago, after seeing a piece by GM3SEK noting that Farnell
didn't sell #31 cores in the UK, and parts that they did sell were quite
expensive, I began building and measuring some chokes formed by winding
a pair of #12 copper on a single #31 core. I used both enameled copper
and insulated "house wire," designated in North America as THHN. The
enameled copper pairs are close to 50 ohms Zo, and the THHN pairs are in
the range of 85 ohms. These Zo values are in agreement with what W2FMI
cited in some of his later work.
The THHN chokes are QUITE broadband -- 12 turns on a single 2.4-in o.d.
#31 toroid yields 5K from about 80M to 10M; 16 turns from 160M to 15M. I
published descriptions, measured data, and applications recommendations
for these chokes in 2010, and my neighbor, W6GJB came up with a very
nice mounting for these chokes as part of a center insulator for a wire
dipole. W6GJB and I have been using those center insulator chokes on 80
and 40M wire dipoles quite successfully. The antennas are a good match
to the RG11 coax with which I feed them. I have a second choke on each
feedline near the ground to function as an "egg insulator" to prevent
interaction between the feedline and a nearby 160M vertical.
I run 1.5 kW in major contests and had no failures until a few months
ago. The failure occurred when I was "beacon CQing" on 80M near the end
of an opening to JA during All Asia CW, and I had worked almost everyone
I could near. The choke fried then whereas chokes had not fried before
because it was the only choke on that antenna -- I had recently replaced
the feedline and had not yet installed the second choke. When we pulled
the antenna down to replace the choke, we observed that the core was
cracked at a single point, and the insulation on the THHN wires had
fried adjacent to the crack.
After the failure, I modeled the antenna with the choke, and the antenna
with the two chokes that I usually use. Keydown dissipation with a
single choke modeled in the range of 60W; with a second choke near the
ground, it was in the range of 25W in each choke. The model was for the
actual antenna, mounting height, soil quality, and feedline length, and
for a parallel RLC circuit that approximated measured data for chokes I
had measured years earlier.
BTW -- according to 3830 scores, I posted the highest 80M score for the
world with 125 Qs. Official results have not been posted. To put that in
perspective, this was mid-summer, JA is 5,000 miles from me. After the
choke fried, I switched to the other 80M dipole (broadside VK/EU) to
finish the contest. I still made a bunch of Qs -- the limiting factor
was what I could hear on my JA Beverage.
The dissipation in any choke will depend on the actual common mode Z of
the choke at the operating frequency, the power level, the duty cycle,
the electrical length of the line, the imbalance in the antenna, other
impedances in the common mode circuit (including the antenna tuner), and
the differential loss in the choke (mostly due to mismatch).
73, Jim K9YC
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