This is my take on this highly controversial antenna, the Off-Center-Fed
Dipole.
THIS IS NOT MY RECOMMENDED CHOICE OF ANTENNA, NOT AT ALL.
However there are cases where it is the best possible solution on the
planet, and unfortunately I happen to live at such a QTH right now. This is
my only hf antenna.
MY PREFERENCE LIST (Mono-Band Antenna):
- Coax fed, center fed dipole
- Full Wavelength center fed dipole (w. Openwire) whenever its radiation
pattern is advantageous (see previous email on this).
MY PREFERENCE LIST FOR MULTI-BAND ANTENNA:
- Double-Dipole (80m and 40m sharing the same feedpoint) - when I only need
those two bands
- 80m Full Wavelength Lazy Loop (Sky Hook - call it what you like). Feed
this one with openwire or coax, both work quite well on ALL bands.
- 80m Half Wavelength dipole, center-fed with Openwire (has significant
matchbox requirements)
- Shortened Center-Fed dipole, w. Openwire. (has even stronger matchbox
requirements)
- OCFD antenna, either as 80m version or 40m version, depending on how many
bands you want to cover.
As you see, the OCFD is at the bottom of my list. But it is my antenna of
choice here at this QTH.
I live in a rental house with enough space for a full size 80m dipole.
I cannot put up a tower or beam (no permission and other problems with the
topology here).
My landlord graciously allowed me to put up one single wire antenna and
asked that I keep it as low profile as possible. He lives in the upstairs
apartment in this house and the antenna is in perfect view from his picture
window. It also runs to the property edge in the front yard, where it is
view of the neighbors. It must be low profile.
ONE ANTENNA, ALL BANDS, LOW PROFILE / LOW VISIBILITY.
Low visibility prohibits running openwire through the air. End of story.
It has to be coax fed with coax running down the pole and along the ground
to the house.
An Off-Center-Fed Dipole, but much different from those sold by Buckmaster,
RadioWorks, W8AMZ, etc., was the best compromise I could come up with.
My 80m OCFD runs with my Eagle on 40/30/20/15/12/10m without a matchbox as
well as in the CW portion of the 80m band. The Eagle's ATU is required to
work the SSB portion of 80m.
The SWR is not perfect but under 2:1 on most frequencies.
My contest scores with this antenna are pretty decent, usually around the 1
Million point mark in CQWW DX (CW) and about half that in the SSB contest.
Instead of pooh-poohing this antenna, perhaps someone here has a better
suggestion of what other antenna I might be able to use. I know of no
alternative that meets the criteria stated.
There is no single best antenna for all of us.
But there is usually a best antenna for any specific QTH and sometimes the
OCFD is the best.
The SOTA crowd are converting to an ultra-light 40m OCFD because it enables
them to work 6 bands without dropping the pole to change a jumper.
In the recent 2015 IOTA Contest, one of the most popular contests, the class
of "Island Expedition, 100w CW (12hr)" was won, first place overall,
'world-wide' using an ultra-light 80m OCFD which I designed and sponsored to
OZ/DL5CW for that contest. He had all his gear in a backpack and one
suitcase and traveled on foot to the island QTH. (well with a boat as well,
of course).
OPENWIRE OCFD:
Many people feed their OCFD with openwire.
This solution was used in the states and in Germany quite a lot back in the
1940's, 50's, and 60's.
In 1959, Buck Rogers, K4ABT, met with Lew McCoy, W1ICP (SK). Lew told Buck
about baluns. Buck built an ugly balun of 75 Ohm coax and fed an 80m OCFD
with it. He used it on the fundamental and even harmonic bands. I believe
this to be the first coax-fed OCFD ever, at least it is the first case which
I have found documented. A few years later, commercial versions appeared
but they ALL had very bad baluns in them. They became the most popular hf
multiband antenna here in Europe.
40 years later when I was forced to resort to this type antenna, the first
thing I did was change the balun design (thanks to lessons learned from
Jim), then I changed the location of the feedpoint and finally I added an RF
choke to the feedpoint, attached directly to the balun. I also break up the
coax run 2 more times with RF chokes (as Jim suggests). Now it works as I
need it.
Am I losing power in the balun? Of course I am. But I don't care.
Until someone can suggest something better that works ALL BANDS, I have no
other choice.
This one gets the job done.
Thus, IMO it is wrong to categorically state that the OCFD antenna is a bad
antenna.
There are cases where it is possibly the only option.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Tuesday, October 20, 2015 3:12 AM
To: tentec@contesting.com
Subject: Re: [TenTec] Open Wire Line
On Mon,10/19/2015 2:51 PM, Rob Atkinson wrote:
> I suppose it gets down to the meaning of "badly matched." If by that
> you mean for example a 1/4 wave dipole fed in the center then I'd say
> nothing should feed such a load because it's pretty close to hopeless.
Perfect example. Another would be a full wave dipole. :)
But more generally, I'm talking about putting up a single dipole, whether
center-fed or off-center fed, feeding it with open wire or window line, and
loading it on all bands.
73, Jim K9YC
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|