Jim,
Congratulations on your excellent placing in the contest.
Somehow I just can't get interested in low band contesting during the
summer.
Once it turns cold (which is about a month away) I will come down out of my
antenna field and get back on the air.
That was indeed interesting about changing the choke.
Ever since reading your "RFI-Ham", I have always broken up my coax with
multiple chokes.
Perhaps that's part of the reason that I haven't fried my mid-sized balun.
I don't run 1500w because we're only allowed half that much power here.
There is probably a significant difference between running 1500w and 700 or
less watts.
I agree that if you are going to run very high power, you should be running
antennas designed to better cope with that. The Windom is usually chosen by
people with limited space, living in congested areas. It is a compromise
antenna. Most of these people do not run such high power. I certainly
would not when using an OCFD.
Cost of Ferrite here in EU is indeed a problem. It's getting better.
The prices are coming down, especially if you buy in quantities.
I order my private ferrite together with Spiderbeam when they place larger
orders.
GM3SEK has suggested another method to save on ferrite cost. It uses the
principle that W2VJN described in the 2010 ARRL Handbook. I don't know if
there is a special name for this kind of choke.
Ian divides the hf spectrum up into 3 segments, low bands, mid-range bands,
and high bands. He uses multiple loops of coax through 2-cores (high bands)
or 3-cores (both other chokes) chunks of ferrite. These are large and ugly
but cheap; about $3 apiece.
I built the mid-range bands and high bands versions and tested the mid-range
version in my CMC test in 2013. In that test I compared it to 3 other
chokes - a fairly simple Maxwell, a Radio-Works T4, and a home brew 1:1
Guanella on a #43 core toroid. I wanted to see how these different
technologies compare under real life field conditions. (EXACT details of
everything is on my web)
In order to do that, I did something insane; I pulled the coax into the air
to the same height as the antenna, then pulled the coax to one side so that
it was almost running parallel to the leg of the antenna. Common Mode
Current shot up like a rocket!
I measured the common mode current produced by running 100w into the antenna
and recorded its value.
The measurement was taken at the transmitter.
Then I inserted each of the 4 chokes just before the transmitter, one after
another and measured CMC again (and recorded).
I was able to see how effective each choke was in reducing the CMC.
In each configuration, besides measuring the CMC, I also scanned each ham
band with my analyzer so that I could visualize the impact the CMC had on
the SWR curve as well as "perceived resonance."
I repeated this test with different antennas and using several different
baluns in the antennas.
Altogether I made about 500 measurements of CMC, along with corresponding
SWR curve.
Under these circumstances, the Maxwell was worthless, the T4 just a tiny
tick better.
As Ian says, these are only good for cases of "soft CMC."
The Guanella completely eliminated the CMC in all cases on all bands.
The GM3SEK completely eliminated the CMC in almost all cases, but in a
couple of cases, for instance where I was running a single-core 4:1 Guanella
balun in the OCFD, Ian's choke had around 10mA of residual CMC. This small
amount of CMC has no negative impact.
So the Guanella won by a nose but it was darn close.
That was 2013. In the meantime the price of ferrite has fallen and #31 has
finally become available.
I find it easier to wind a Guanella than the SEK chokes so everything I have
built since then has been a Guanella.
One of the interesting things I learned is that the CMC on the fundamental
of an OCFD antenna was always a magnitude higher than it was on any of the
even harmonic bands, and the upper bands - under normal circumstances, had
hardly any CMC at all... when running 100w and a decent balun.
Exception: The 3rd harmonic had CMC that rivaled that measured on the
fundamental.
I am clueless as to why, but I did learn to design the choke's peak CMI in
the range between the fundamental and the 3rd harmonic. THIS is why I
switched from #61 to #43 ferrite in my baluns for 80 and 40m OCFD antennas.
BTW, when taking this "worst case" CMC measurement, I also used a half
wavelength of coax which is the length where CMC is usually the highest.
IMPORTANT NOTE: ALL tests were made on 40m dipoles or OCFD antennas.
This summer I have been testing 80m OCFD antennas and am already noticing
significant differences.
Solutions that worked on the 40m OCFD (i.e. resulted in no measurable CMC on
the feedline), are inadequate on an 80m OCFD.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Tuesday, September 22, 2015 7:54 PM
To: tentec@contesting.com
Subject: Re: [TenTec] It's getting cold. Perfect antenna weather for a OCF
dipole
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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