Hi David,
Thanks for the synopsis below. I found it very useful being unfamiliar
with amateur practice over the years with OCF antennas. My antennas are
all center fed.
I replied to this message because it was the most informative. Other,
later messages on this thread were also interesting. But generally, the
discussion lacks quantitative data and so it is difficult to objectively
evaluate the efficacy and drawbacks of different realizations. It is
also difficult to project which realization or changes would fit a given
set of constraints.
The positive aspect is there has been years and years of experimentation
and different approaches to this multiband dipole problem. I would like
to hear more on these.
Could you describe G3TXQ's 2 core Guanella balun solution more
completely? I checked the Spiderbeam website and it is not clear to me.
I think this is relevant to Elecraft since many applications are
portable and QRP. We want that antenna to really work and understanding
it better, helps.
73,
Chuck K0MV
On 11/4/21 9:59 AM, CUTTER DAVID via Elecraft wrote:
Hi Dan
Good question. I'll answer as a non-expert as best I can.
This is essentially a quest for a multi-band dipole that is easy to make and
use. If you can put up mono band dipoles and beams for all your bands, you
probably will not bother going this route.
The popular description of the ocf dipole (going back at least to the 50's) has
a feed point of one third/two thirds on the dipole. (BTW this is otherwise a
normal half wavelength dipole.
This provides approximately 200 ohm feed point impedance for most popular HF
bands, eg 80, 40, 20 and 10m, not 15m.
Back in the 60's we could use coax into a valve PA, which I preferred to the
G5RV which needed an outboard matching unit. In transistor days we used a
modest matching unit with an swr meter and were ignorant of common mode current.
In the 90s I used ladder line to ground level and an auto-tuning unit into a tent for field day, believing it to be more efficient. With grounded coax I didn't have live chassis syndrome and out in the wilds there was no noise pickup. Later I used ladder line through a balun then into the radio with an on-board matching unit and that was, for me a great step forward with auto-tuning.
It was discovered by some users that the common mode current performance using
off-the-shelf baluns and chokes was inadequate and could result in live chassis
syndrome and noise pickup on receive from home locations. Poor matching on 15m
was still a problem as were the WARC bands.
Now in the 21st Century, just a few years ago, Rick DJ0IP tackled the problem
starting with a 40m ocf dipole using a new balun/choke combination. He read
that W8JI and others recommended a 20% feedpoint to bring 15m into the 200 ohm
region. He then used a Guanella 2-core balun solution from Steve G3TXQ and
others to provide the 50 ohm output with low common mode current. It had to be
the dual core version, the single core version simply didn't suppress the cmc.
This is now marketed by Spiderbeam. Moving to an 80m version was difficult but
with a hybrid balun/choke combination he devised a combination that provided at
least as good cmc and this version is also marketed by Spiderbeam. They both
work on 15m and some WARC bands. For some layouts a modest on-board matching
unit is required for complete coverage which is a far cry from bulky outboard
units; good news for portable operation.
I would call these *modern* off centre-fed dipoles, ie well into this century.
I have no financial connection with anyone in that business and I will say that
I have never seen anyone else produce such a large amount of compelling
evidence as Rick and I recommend his web site for all things balun and choke
related to this task. He takes the practical, non-laboratory approach. This
in no way conflicts with any work done by Jim, K9YC and I only wish that he
would make his own measurements on these modern devices and let go the old
prejudice.
73 David G3UNA
On 04 November 2021 at 05:24 Dan Presley <[email protected]> wrote:
Perhaps you could clarify what you meant by a modern OCF. What’s changed from
the traditional model? Thanks.
Dan Presley 503-701-3871
danpresley@me. com
[email protected]
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