Off-Center Fed (OCF) Dipole??
Eric Rosenberg
I'm looking for information on hte off-center fed dipole, as mentioned by
Bill Orr in CQ Magazine last year and briefly written up in the ARRL
Antenna Book.
I'd use the antenna for 80 and 40 meters.
Thanks --
Eric
--
Eric Rosenberg WD3Q, EI4VPS, YJ0AER, J20BY, etc.
Washington, DC
er...@access.digex.net wd...@amsat.org
Steve Ellington
antenna". It's asking for trouble. I tried it many times. Why not just
put an 80 and 40 meter dipole in parallel? Now you have a balanced
antenna with a nice, predictable radiation pattern. Use 50 ohm coax to
feed it. No balun is necessary. Put it up as an inverted vee and seperate
the ends as much as possible. 73
: I'm looking for information on hte off-center fed dipole, as mentioned by
: Thanks --
: Eric
--
Steve Ellington N4...@IGLOU.COM Louisville, Ky
Cecil Moore
>
>I'm looking for information on hte off-center fed dipole, as mentioned by
>Bill Orr in CQ Magazine last year and briefly written up in the ARRL
>Antenna Book.
Hi Eric, I ran one of these in college. The antenna is 180 degrees (half-wave)
on 80m and 360 degrees (full-wave) on 40m. Since sin(60deg) = sin(120deg), a
point 1/3 from the end will give approximately the same impedance for 80m and
40m. IMO, it still needs an antenna tuner but is not a bad match for 300 ohm
ladder-line. Radiation pattern favors the long section.
73, Cecil, KG7BK, OOTC (not speaking for my employer)
R. Bruce Winchell
Provo, UT 84605. I just ordered their TNT 160. Also check with Radio
Works Box 6159 Portsmouth, VA 23703. They make them as well. You can also
buy the plans from Antennas West. I personally chose the TNT over the
Radio Works Carolina Windom because I liked the insulated wire that they
use vs the open copper-clad wire. The open copper-clad wire will corrode
eventually along it's entire length. The corrosion acts as millions of
little capacitors that gain minute static charges and make the antenna
increasingly noisy as time passes. 73 KC8ARO Bruce
R. Bruce Winchell
The antenna is called a WINDOM. Check the ARRL Antenna Handbook and write
to Antennas West, PO Box 50062, Provo, UT 84605 and Radio Works, Box
6159, Portsmouth, VA 23703 for catalogs of pre-built antennas,
pieces/parts, and plans. Don't try to run an unbalanced line without
using a feedline isolator/balun to stop the RFI. Without a line isolator
the RFI will reset your alarm clock at 100 yards. 73 KC8ARO Bruce
R. Bruce Winchell
WINDOM. There is a danger of in-shack RFI if you attempt this OCF. The
feedline becomes a radiator when you have an unbalanced line. You have to
control it. You MUST use an in-line Isolation Balun (Line Isolator). This
will turn your feedline into a Vertical Radiator and add another
dimension to your antenna!!! It will also virtually eliminate RFI
problems. Write for catalogs from Radio Works, Box 6159, Portsmouth,VA
23703 and Antennas West, PO Box 50062, Provo, UT 84605 for antennas,
pieces/parts and plans. 73 KC8ARO Bruce
Steve Ellington
Austonding, Amazing, Unbleivable etc. Yes even the buck of string with a
weight on it for only $$$. The amazing but little know DOUBLE BAZOOKA
that bost signals to strangely heafty levels that even buffalos the
engineers. bla bla bla etc. Balderdash!
The Windom in it's original form was fed with a single wire. I tried it
once and put a nice burn on my lip from the microphone. Now, we have this
so -called Windom using balanced line, 300 ohm or 450 or whatever,
feeding a very unbalanced antenna. YES, the feedline will radiate like
crazy! Is this what we desire?
On 20 meters and above we end up with a very directional antenna with
some gain in certain directions and deep nulls in others. Does anyone
care?
Line Isolators..... Well depending on feeder length and freq. of
operation you may get a really hot choke with lots of power loss. I know
this because I tried it. You will end up cranking on an antenna tuner
before it's all over anyway so why bother with feeding an antenna
off-center when we know it's asking for trouble?
He just wants 80 and 40 meters anyway. With parallel dipoles, there is
little concern for RF on the rig, no tuner needed and no baluns required.
This Windom stuff reminds me of the G5RV hype. Same old thing. A crummy
compromise antenna that can be made to work on all bands using a tuner
and enough lossy coax with a high swr to make tuning easy. Package it as
an all-band antenna kit, charge $49.95 and they sell like hotcakes.
Time does not prove anything except that...Given enough time, people will
try anything over and over even when it doesn't make good sense.
1. Use a balanced antenna
2. Feed it with a matched feeder
3. Enjoy operating
1. Use a balanced antenna
2. Feed it with balanced open wire line
3. Use a tuner
4. Enjoy operating but cranking the tuner
1. Use an unbalanced antenna
2. Feed it with anything
3. Use a tuner, line isolator etc.
4. Anything might happen. Have fun
Steve Ellington N4...@IGLOU.COM Louisville, Ky
W8JI Tom
Ellington <n4...@iglou.com> writes:
>
>The Windom in it's original form was fed with a single wire. I tried it
>once and put a nice burn on my lip from the microphone. Now, we have this
>so -called Windom using balanced line, 300 ohm or 450 or whatever,
>feeding a very unbalanced antenna. YES, the feedline will radiate like
>crazy! Is this what we desire?
>
The two wire line doesn't have to radiate. If you install a choke balun at
the antenna terminals it won't radiate if the source feeding the line is
balanced. If the line is coax and the balun is used the source has to be
ground independent or a typical unbalanced output.
Lot's of variables apply when things are mixed and matched!
73 Tom
Cecil Moore
>The antenna is called a WINDOM.
Hi Bruce, as I said over email, the original "Windom" did not use twin-lead. The writeup in my
1957 ARRL Handbook shows "Windom" to be a modern misnomer for twin-lead-fed OCFs. But trying to
keep the language pure seems to be a losing proposition. Like G5RVs without any coax.
Michael Haydon
put up as long a "dipole" as you have space for.
use open wire line.
don't worry about the SWR if your radio will load into it.
judge how well your antenna works by how many people you can talk to.
note that i said "TALK TO!!!", not "you are 59 please qsl goodbye.
On Thu, 1 Feb 1996, Steve Ellington wrote:
> I have Radio Works silly catalog with all the antennas that are
> Austonding, Amazing, Unbleivable etc. Yes even the buck of string with a
> weight on it for only $$$. The amazing but little know DOUBLE BAZOOKA
> that bost signals to strangely heafty levels that even buffalos the
> engineers. bla bla bla etc. Balderdash!
>
> The Windom in it's original form was fed with a single wire. I tried it
> once and put a nice burn on my lip from the microphone. Now, we have this
> so -called Windom using balanced line, 300 ohm or 450 or whatever,
> feeding a very unbalanced antenna. YES, the feedline will radiate like
> crazy! Is this what we desire?
>
Daniel Senie
I'll agree with Cecil that this antenna design can work just fine.
The one I've used was made by Antennas West, but the design is quite simple
so if I put up another one, I may just build it myself, instead.
The design of the one I use is a 1/2 wave on 40 meters, fed 1/3 of the way
from one end, rather than in the center. The feedline is coax, with ferrite
bead balun about a foot below the feedpoint, and I add another bead balun
where the antenna connects to my remote switching network. Some RF does
couple to the shield of the feedline, even with the balun at the feedpoint,
so the other choke balun helps keep from having RF come back to the shack on
the shield.
The antenna tunes within a 2:1 SWR on 40, 20 and 10. This is a GREAT antenna
for someone starting out with HF, as in combination with an auto-tuner
found in most new rigs, it will perform quite well. I do highly recommend
this approach over the "put up as long a dipole as you can" approach,
especially for newcomers. It allows for quicker and easier setup.
Dan N1JEB
--
---------------------------------------------------------------
Daniel Senie Internet: d...@senie.com,
Daniel Senie Consulting n1...@senie.com
http://www.senie.com Packet Radio: N1...@KA1SRD.MA
W8JI Tom
Tom B E-mailed me this.
I said....
: The two wire line doesn't have to radiate.
He said....
Yes, I agree with this, but ...
: If you install a choke balun at
: the antenna terminals it won't radiate if the source feeding the line is
: balanced. If the line is coax and the balun is used the source has to be
: ground independent or a typical unbalanced output.
I'm afraid I don't agree with this. I just did a little EZNEC simulation
to prove the point. I modeled a simple horizontal .49 wavelength center
fed dipole at 1 meter wavelength. I included a second wire not connected
to the first. It was also horizontal, in the same plane, same length, but
perpendicular to the driven wire. One end was spaced .01 meters from the
center of the driven wire. As expected, the model showed an approximately
sinusoidal current distribution on the driven element, and zero current
everywhere on the parasitic element. But then I moved the parasitic
element so it was 1/3 of the way from one end of the driven element, still
with the end spaced away from the driven element by 0.01 wavelenths. I
claim that's like a feedline, be it coax or balanced, that's isolated from
the driven antenna by a perfect balun, and 1/2 wave long. OK, the current
induced in the parasitic element was nearly half as large as in the driven
element!
If you really don't want antenna currents on your feedline in this
situation, break the feedline up with two baluns, one at the antenna, and
one a quarter wave or so away.
Anything that's resonant and not perfectly balanced in the field of the
driven element will have a significant current (= antenna current) induced
in it. If you make it non-resonant, it very significantly reduces the
problem, and if you space it well away from the radiating element that
helps too.
--
Cheers,
Tom
to...@lsid.hp.com
That was a great clairification. Of course as Tom pointed out...ANY line
will radiate (even a balanced antenna) if the feeder is not a proper angle
to the antenna and properly decoupled.
An OCF antenna is NO WORSE than any other dipole antenna for putting out a
signal. They all require proper installation, most of them are never
installed that way...even the dipoles.
73 Tom
Kenny Anaskevich SLIP/PPP
COAX as the preferred method of using the antenna for multiple bands. The
coax was intended to be used ONLY on 20 meters where there is a
reasonable match between the twin lead and the coax.
The reason we get low swrs with the magic minimum of 70ft of coax is
because of the tremendous attenuation (loss) of coax under high swr
conditions. The swr just appears lower at the transmitter because of the
excessive loss. Coax loss can make a 10:1 swr read 1.5:1 by the time it
travels through 70ft of small coax. Try using some heavy coax like
RG-9913 and although your losses will decrease, the swr will appear much
higher.
G5RV knew this when he wrote the article but he has been ignored because
of the HYPE and the MONEY made on G5RV KITS. Oh sure. We get good signal
reports etc. Shuck, I get good 599 reports on my mobile rig using an 8ft
Hamstick antenna! That doesn't mean a thing folks. If I run inside and
fire up the 560ft loop I also get 599 but now I'm 20db stronger!
--
Dave Skarbowski
>
> In article <Pine.SOL.3.91.960201150610.19455A-100000@iglou>, Steve
> Ellington <n4...@iglou.com> writes:
>
> >
> >once and put a nice burn on my lip from the microphone. Now, we have this
>
> >so -called Windom using balanced line, 300 ohm or 450 or whatever,
> >feeding a very unbalanced antenna. YES, the feedline will radiate like
> >crazy! Is this what we desire?
> >
>
> the antenna terminals it won't radiate if the source feeding the line is
> balanced. If the line is coax and the balun is used the source has to be
> ground independent or a typical unbalanced output.
>
>
> 73 Tom
Where did the notion come from that openwire line radiates? If the SOURCE is balanced (as you
indicate) it will not radiate. Where you attach it on a dipole should not matter as far as
balance is concerned. The center of a dipole as a feed point is no different than the end as
far as balance in the feed is concerned.
73's Dave, n2fam
Cecil A. Moore~
Michael Haydon <mha...@olivet.edu> wrote:
> put up as long a "dipole" as you have space for.
> don't worry about the SWR if your radio will load into it.
> judge how well your antenna works by how many people you can talk to.
Hi Michael, IMO your advice sounds like, "do something even if it's
wrong". IMO a more rational approach to antenna design would be to
use a program like EZNEC to achieve the desired radiation pattern
and antenna impedance and then design a transmission line system
to achieve maximum power transfer. A person desiring east/west
coverage will ordinarily orient his "dipole" north/south. With
"as long a dipole as *I* have space for" the radiation pattern
will not be east/west at all and I would be disappointed.
W8JI Tom
writes:
>Where did the notion come from that openwire line radiates? If the
SOURCE is
>balanced (as you
>indicate) it will not radiate. Where you attach it on a dipole should
not
>matter as far as
>balance is concerned. The center of a dipole as a feed point is no
different
>than the end as
>far as balance in the feed is concerned.
>
>73's Dave, n2fam
>
>
Hi Dave,
Proper balance or isolation at BOTH ends is required. If the source is
balanced and the load isn't, the parallel wire line will radiate. If the
load is balanced and the source isn't it will still radiate.
All lines, even coax, require equal and opposite currents on each
conductor to prevent radiation. And as Tom B pointed out, even field
coupling can make a line radiate even when we take other precautions.
Any end fed antenna *without a counterpoise at the antenna and feedline
terminal* will have feedline radiation, or you wouldn't even be able to
feed the antenna!
73 Tom
Cecil A. Moore~
Kenny Anaskevich SLIP/PPP <n4...@iglou.com> wrote:
>The reason we get low swrs with the magic minimum of 70ft of coax is
>because of the tremendous attenuation (loss) of coax under high swr
>conditions. The swr just appears lower at the transmitter because of the
>excessive loss. Coax loss can make a 10:1 swr read 1.5:1 by the time it
>travels through 70ft of small coax.
I felt exactly this way until I ran the loss numbers for a G5RV.
For 70 ft of coax to reduce the SWR from 10:1 to 1.5:1, it would
have to have a matched-line loss of over 8dB. Not even RG-174
has that much loss at HF. RG58 has less than 2dB matched-line
loss per 100 ft on 20m.
If you take a closer look at the G5RV you will find that the
300 ohm tuning section rotates the 3.8 MHz 20-j200 ohm antenna
impedance around the Smith chart to very close to a current
node. It does the same thing on 40m starting with an antenna
impedance around 500+j1000.
W8JI has actually tested the G5RV radiation efficiency with
reasonable results. Maybe he will repost them.
The G5RV was discussed by Bill Orr in the Nov 92 CQ. He compared the
original design to the W6SAI version. Here are the reported results.
Freq. SWR-original SWR-W6SAI version
102'/26.75' no balun 92.6'/37.25' with balun
3.5-4.0 6.3-5.67 7.68-4.60
7.0-7.3 2.65-4.50 1.72-3.00
10.1 8.50 8.11
14.00-14.35 1.83-3.28 2.50-1.42
18.11 1.84 1.11
21.00-21.45 5.90-5.69 4.96-4.70
24.95 4.52 2.75
28.0-29.7 4.83-1.88 3.38-1.48-2.55
Take the measured SWRs and use the charts at the end of Chapter 16
in the ARRL Handbook to find the SWR at the coax/ladder-line
junction and the additional loss caused by standing waves. For
instance, a transmitter end SWR of 3:1 on 40m means an SWR of
about 4:1 at the other end of the coax with a total loss of
about 2dB in RG58, less than half an 'S' unit (Sterbie unit?).
G5RV radiation efficiency can definitely be improved but not by
as much as one might think. Commercial versions are way over-
priced. But it is not bad as an all-band HF antenna.
Ignacy Misztal
>The Windom in it's original form was fed with a single wire. I tried it
>once and put a nice burn on my lip from the microphone. Now, we have this
>so -called Windom using balanced line, 300 ohm or 450 or whatever,
>feeding a very unbalanced antenna. YES, the feedline will radiate like
>crazy! Is this what we desire?
There were claims that the single wire Windom worked wonders if
mounted really high and if there was some kind of a ground system.
It is unclear whether it acted as a vertical with a loading hat or an
off-center fed half-wave. Of course it would give you burns, so your
burnt lip are not due to that antenna working badly but due to you
being unaware of its properties.
A version of Windom with a 6:1 balun and fed with a coaxial cable has
an efficiency of the dipole but works on several bands. My one,
which had a 6:1 transformer rather than a balun, had a wider bandwidth
than the dipole on 80m, and almost a perfect 1:1 SWR on 40m. On 20m
and up it tuned well but was outperformed by a vertical. As expected.
I suggest to develop expectations on antennas from antenna books
rather than catalogs.
Ignacy Misztal Ham radio: NO9E, SP8FWB
E-mail: ign...@uga.cc.uga.edu
University Of Georgia, 203 L-P Bldg., Athens, GA 30602
tel. (706) 542-0951
Steve Ellington
anyway. A recent article in QST proved that many tuners have considerable
losses. Add that to the 2 to 4 db lost from the coax and you have even
more loss. Why is the G5RV considered a mutliband antenna? Is there some
magic swr the qualifies it? Put up any old loop in any configeration that
is at least one wavelength around and it will have better matches to coax
then this.
: Freq. SWR-original SWR-W6SAI version
: 102'/26.75' no balun 92.6'/37.25' with balun
: 3.5-4.0 6.3-5.67 7.68-4.60
: 7.0-7.3 2.65-4.50 1.72-3.00
: 10.1 8.50 8.11
: 14.00-14.35 1.83-3.28 2.50-1.42
: 18.11 1.84 1.11
: 21.00-21.45 5.90-5.69 4.96-4.70
: 24.95 4.52 2.75
: 28.0-29.7 4.83-1.88 3.38-1.48-2.55
: Take the measured SWRs and use the charts at the end of Chapter 16
: in the ARRL Handbook to find the SWR at the coax/ladder-line
: junction and the additional loss caused by standing waves. For
: instance, a transmitter end SWR of 3:1 on 40m means an SWR of
: about 4:1 at the other end of the coax with a total loss of
: about 2dB in RG58, less than half an 'S' unit (Sterbie unit?).
: G5RV radiation efficiency can definitely be improved but not by
: as much as one might think. Commercial versions are way over-
: priced. But it is not bad as an all-band HF antenna.
: 73, Cecil, KG7BK, OOTC (not speaking for my employer)
--
Michael Haydon
Your response presupposes that I care which direction my "dipole favors,
which I do not.
Cecil Moore
>From the looks of these swrs, a tuner will be need for solid
state rigs
>anyway. A recent article in QST proved that many tuners have
considerable
>losses. Add that to the 2 to 4 db lost from the coax and you
have even
>more loss. Why is the G5RV considered a mutliband antenna? Is
there some
perfect. Mine
started out as a G5RV and is now perfect. :-) A tuner of some
sort is needed
for almost any multi-band antenna. Except for 30m and 15m, the
range of the
SWRs are within the efficient matching ranges of most popular
tuners. My SB201
says that anything less than 3:1 is a "good" SWR. For someone
who wants "plug
and play", a G5RV and a tuner are hard to beat. I can guarantee
you that the
G5RV will radiate more RF than a lot of antenna systems that
run ladder-line
all the way to the shack directly into a 4:1 balun. Baluns
don't like 3000+
j3000 impedances and balun losses can easily be greater than
the coax losses
in the G5RV. If one doesn't know what impedance one's balun
sees, one is
probably better off using a G5RV and no balun, IMO.
73, Cecil, KG7BK, OOTC
Cecil Moore
>... But it is not bad as an all-band HF antenna.
>
I've been chastised for not ragging on the G5RV. IMO, saying it's not a bad
antenna falls short of endorsing it. The common advice is to replace the coax
with ladder-line, use a 4:1 balun, and an antenna tuner. Let's see where
following that advice can lead.
We know that on 75m, the G5RV 300 ohm series
section transforms the antenna impedance to near the purely resistive current
node. Simulations tell us that the SWR will be higher on 300 ohm line than on
the G5RV 50 ohm line. ELNEC says about 30:1. I've measured about 15:1 at the
transmitter end. To make the math easy, let's say we have a current node at
the transmitter end and run it through a 4:1 balun. The balun sees 300/15=20 ohms
and transforms it to 20/4=5 ohms which the antenna tuner is called upon to match.
We have a balun designed to match 50/200 ohms trying to match 5/20 ohms and an
antenna tuner designed to match 10-300 ohms trying to match 5 ohms. I submit
that this configuration probably has more losses than the G5RV. Assuming 1/2 WL
of coax on the G5RV, the impedance seen by the antenna tuner would be around
10 ohms for an SWR of 5:1. Seems to me that no balun and 10 ohms for the antenna
tuner to match is lower loss than a mismatched balun and 5 ohms for the antenna
tuner to match.
Cecil Moore
> Your response presupposes that I care which direction my "dipole favors,
>which I do not.
Nope, my response presupposes that the hams to whom you are giving advice
care where *their* RF goes. When I put up my G5RV (first and only) I didn't
know most of the RF was being wasted over the oceans. I thought it wasn't
working until I read a G5RV application note from Antennas West and
realized that my N/S orientation was at fault. Rotating it by 15 degrees
made all the difference in the world, e.g. Europe, Pacific Rim, S.America.
W8JI Tom
Are you paying attention?
In article <DMBt4...@iglou.com>, n4...@iglou.com (Steve Ellington)
writes:
>
>From the looks of these swrs, a tuner will be need for solid state rigs
>anyway. A recent article in QST proved that many tuners have considerable
>losses. Add that to the 2 to 4 db lost from the coax and you have even
>more loss.
I test new tuner designs several times a year. I find the WORSE CASE
losses are ~20% on 160 and ten, and much less at the mid ranges. I have no
idea what the QST test indicated, but my method was accurate. Let me say
this, if the loss is 20% with 1000 watts, 200 watts of heat is generated.
That makes for a very hot tuner even with very little loss!
But what does that have to do with the constant hacking away at the G5RV?
The open wire dipole so "wildly" endorsed needs a tuner also, and it can
be (and often is) at an extreme impedance that maximizes tuner loss. The
loss in the G5RV coax isn't 2 to 4 dB in the coax on 80, 40 or 20. It can
be lower than that on the high end of the HF spectrum!
>Why is the G5RV considered a mutliband antenna?
Because the SWR is low on 80, 40 and 20 (for an 80 meter basic design).
>Is there some magic swr the qualifies it?
No, just the right things coming together at those points.
>Put up any old loop in any configeration that is at least one wavelength
around and it >will have better matches to coax then this.
So you are saying I can squish up a loop, and weave it in and out of other
conductors, and it will always have a low SWR? Or that it can be 1-1/4 wl
circum and have a low SWR?
Almost any loop like you describe won't be better on 80, 40 and 20. The
SWR is still in the 2:1 range (+ or - losses or ground effects that make
the SWR look better or worse). The loop is fine in some applications, and
so is the G5RV. The loop becomes BW restricted on higher overtones, plus
the impedance rises. For my application (DX'ing), the pattern stinks.
Every antenna has advantages and disadvantages. The best antenna is
determined by what the user wants. I want a clean predictable low angle
pattern and reasonable SWR on the lower bands. The G5RV provides that
quite nicely in less space. I can keep my G5RV antenna centered over a
good ground system and away from other stuff. I can load it like a T on
160 and work Europeans almost the same as I can with my 135 ft vertical. I
get 160, 80, 40 and 20 with a low SWR and great DX performance with only
two supports!
A loop is better? Not for me.
73 Tom
Steve Ellington
end of the ladder line will provide the ultimate multiband match to the
50 ohm coax. I believe its the Janurary issue in an article about baluns
by W2FMI. He sells baluns through Palomar and this may be worth trying.
: antenna tuner designed to match 10-300 ohms trying to match 5 ohms. I submit
: that this configuration probably has more losses than the G5RV. Assuming 1/2 WL
: of coax on the G5RV, the impedance seen by the antenna tuner would be around
: 10 ohms for an SWR of 5:1. Seems to me that no balun and 10 ohms for the antenna
: tuner to match is lower loss than a mismatched balun and 5 ohms for the antenna
: tuner to match.
Cheong Wai Seng
Can anybody e-mail me or post it in this Newsgroup please?
Mark Pettigrew
> working until I read a G5RV application note from Antennas West and
> realized that my N/S orientation was at fault. Rotating it by 15 degrees
> made all the difference in the world, e.g. Europe, Pacific Rim, S.America.
>
> 73, Cecil, KG7BK, OOTC (not speaking for my employer)
Do you have a copy of the relevant bits from the app note, or more details of the
source?
Thanks
Mark
G0WLR
Cecil Moore
>Do you have a copy of the relevant bits from the app note, or more details of the
>source?
the $7.50 price IMO. Antennas West, Box 50062-C, Provo, UT 84605,
(800)926-7373 Of particular interest are the radiation patterns for all
bands 75m-10m.
73, Cecil, KG7BK, OOTC
