Jim, (and all others),
Before reading below, let me say that I deem Jim to be one of, if not "the"
world's top authorities on baluns and CMC. He has been instrumental in my
learning before doing my own research on this topic.
But I also have my own experience, which is extensive, and I will share it
here.
As I said, I have made a lot of real life measurements on real antennas.
THOUSANDS of measurements, in hundreds of hours in the field!
Jim, you are my guru and I use your RFI-Ham as a resource and recommend it
to everyone.
I also have built and used OCFD antennas for several years.
I know you and others (even Dean Straw) speak of frying baluns all the time.
I run these antennas at 600 to 700w and have never fried a balun in doing
so.
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.
In fact I have made over 500 CMC measurements on these antennas.
Yes, CMC is there. But I'm not frying my antennas.
In fact, running 600w into the OCFD, I am consistently scoring over 1
million points in CQWW DX contests, and I'm only running mid-sized torids,
so let's not be too down on this without proof.
NO, I AM NOT FRYING THE BALUN. HAVE YOU DONE IT?
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.
I contend that the chances of frying a balun are higher with center-bed
antennas than with OCFD antennas.
DISCLAIMER: Use the antenna on the bands that the model says it works on;
don't try to match it on to bands it doesn't work on, using an antenna
tuner. If you try to do that, you will indeed fry the balun.
I AM SPEAKING FROM EXPERIENCE.
Do you actually have experience frying the balun, or are you speaking about
a model prediction?
I haven't referred to any coil of coax in my recent postings. I have been
very specific about how to build the baluns. I am recommending specific
balun solutions with specific cores, sometimes multiple (as you suggest in
RFI Ham).
So with all respect, I think you are misleading people on this point.
It is possible to build an OCFD antenna that works 6 or 7 bands and run up
to 1KW into it without frying the balun if you follow a few basic
principles. If you deviate from these principles, YES, you may indeed fry
the balun.
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.
In my recent post I recommedned running FT-240-43 for higher power and
doubling the number of cores for 1 kw. I am certain that these are
sufficient.
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.
Jim, we need people like you to get involved in this topic and help us
determine the strengths and weaknesses of this type of antenna. I'm doing
my best, working hard at it. Spent 4 hours in the field TODAY measuring
OCFD antennas. Mel, K6KBE is assisting me. He was a co-founder of KLM
antennas. Basically we are duplicating each other's work, me here in
Germany and he in California. Each of us is the sanity check for the other.
We are reporting measured data, not modeled data.
What is your justification for the claim you just made?
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Sunday, September 20, 2015 6:28 PM
To: tentec@contesting.com
Subject: Re: [TenTec] It's getting cold. Perfect antenna weather for a OCF
dipole
On Sat,9/19/2015 11:54 PM, Rick - DJ0IP / NJ0IP wrote:
> If you want to run high power (more than 800w), you will need to glue two
cores together.
> You won't find the coax I used, so use RG-142.
Rick,
I hate to be a wet blanket, but off-center fed antennas produce VERY high
common mode voltage/current, enough to fry a very good ferrite choke with as
little as 100W. Running a power amp is really asking for trouble.
I now recommend using TWO 5,000 ohm ferrite chokes on well balanced dipoles
running more than 1,000W.
The "coil of coax" chokes are simple inductors. If the length of the
feedline makes the feedline capacitive, the choke will cancel part or all of
the reactance of the feedline and the common mode current will INCREASE.
Obviously, the extent to which this occurs will be different on every
frequency. For this reason, a coil of coax makes a lousy common mode choke.
73, Jim K9YC
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