below your message, all the answers I got. Hope they help you as helped me
few days ago. In few words, as you will notice below, great care must be
used to avoid interactions with any upper antenna. In my situation the C3XL
by Force 12. Interactions are still there, but a compromise must be
accepted. At moment 30 and 17 are OK for SWR but 30 interacts with C3XL,
when turning the Yagi.
12 meters show higher SWR ( 1.9) but seem independent from Yagi rotating.
At 12.40 07/10/01 -0700, you wrote:
>I have a similar problem. Could you share the responses you
>received with the rest of us?
>73 de Jim Smith VE7FO
>Sante - IK0HBN wrote:
> > Thanks to all answered my message. This is a GREAT reflector and answers
> > never missed in 5 years...when asking help.
At 07:18 AM 10/1/01 +0200, you wrote:
>I need your help. I must redo 2 dipoles hanged down to the same coax. They
>got broken due to the age of wires, so I choice using thicker wire.
>Involved bands were 30 and 17 meters. Having to do the job I have thought
>adding even the 12 meters band so to have that band running here. My
>a) Is it possible sharing a single balun for 3 dipoles? I guess YES.
>b) Have I trying trimming them when ALL linked to the balun, or should I
>cut one dipole at time, removing it, and after putting up another to be
I would try cutting them all a bit long, connect them to the balun, and
then adjust individually.
>c) If the first option should be preferred, of course trimming one band
>legs, resonance on the other 2 bands will move, so from which band I have
>to start trimming? IE. Do I begin bringing at resonance the 30 meters band
>first with the other 2 at place (linked to balun) or should I start from 12
>meter? - My question take place from the evidence that cutting one band,
>the others two move enough.
If you can spread them apart a little more at their ends, their interaction
will be significantly reduced, and it should be not too bad because of the
frequencies involved, which are not harmonically related to one another.
>I have a MFJ-269 for the job.
> From the balun the 3 dipoles run slightly separated so to have them at
>ground about 1 meter between them.
>Any possible help/hint will be greatly appreciated here.
For years, I had inverted-vee dipoles for 40, 20 and 10 meters off a single
balun, spread out over about 45 degrees of azimuth. They worked quite well.
73, Pete N4ZR
See comments below...
| I need your help. I must redo 2 dipoles hanged down to the same
| coax. They
| got broken due to the age of wires, so I choice using thicker wire.
| Involved bands were 30 and 17 meters. Having to do the job I have thought
| adding even the 12 meters band so to have that band running here. My
| questions are:
| a) Is it possible sharing a single balun for 3 dipoles? I guess YES.
Yes. As long as the SWR is kept reasonable (same comment for any balun even
on a single band antenna).
| b) Have I trying trimming them when ALL linked to the balun, or should I
| cut one dipole at time, removing it, and after putting up another to be
You have to trim with all elements connected. They form a system and pieces
of the system will interact. The trimming will counteract the interaction.
| c) If the first option should be preferred, of course trimming one band
| legs, resonance on the other 2 bands will move, so from which
| band I have
| to start trimming? IE. Do I begin bringing at resonance the 30
| meters band
| first with the other 2 at place (linked to balun) or should I
| start from 12
| meter? - My question take place from the evidence that cutting one band,
| the others two move enough.
I've tried it both ways and concluded that I had less trouble when I first
cut all dipoles to their normal lengths and connected them in parallel.
Then I adjusted the longest element first and trimmed it to lowest SWR. I
then proceeded to the other elements, with the shortest element adjusted
In most of my efforts I used twin lead for the elements if I could find the
old "transmitting" twin lead where the conductors were strong enough to hold
the weight of the antenna and feedline. In the case of a 2-band antenna one
piece of twin lead was used, with one of the twin lead conductors cut back
to represent the dipole element on the higher frequency. Each "half" of the
twin lead became a dipole.
I have also used individual wires in an inverted Vee configuration, each
going to its own support on the ground or a tree.
Every single effort has resulted in different lengths. There is
considerable interaction present. No two are alike.
Good luck and 73,
Apex, NC USA
As I recall from a triband wire vertical project, the higher
band elements needed to be LONGER than normal due
to interaction effects.
One of my favorite WARC band antennas is a 20M dipole
(34 ft total length) fed with ladderline and a balanced antenna
tuner. This provides a figure 8 pattern from 10 through 30M
with beamwidths from 50 degrees on 10M where it acts as
two half waves in phase to 90 degrees on 30M where it is a
short dipole (2/3 size).
Performance on 30M could be improved slightly by using a
40 ft center fed dipole instead which will be two half waves
on 12M (and 1.25 WL on 10M as a double extended Zepp).
de Tom N4KG
At 08:18 PM 10/1/01 +0200, you wrote:
>Thanks to all answered my message. This is a GREAT reflector and answers
>never missed in 5 years...when asking help.
>By the way, one afternoon job left me with 30 and 17 meters legs perfectly
>matched, but no way to have a SWR less than 2.1 on 12 meters legs. I must
>say that all the sistem is under a C3XL tribander and to achive a good
>match on 30, the boom must be turned perpendicular to the dipoles. Beside
>the little noise of turning beam to 90 degrees, now I must find the way to
>have even the 12 meters working. At moment, no way to add another
>balun/coax, so the stuff must share the same coax. MFJ read impedance of 26
>ohm and a Xs of 21, very high as I could see. As usual, any following hints
>will be greatly appreciated.
Hmmm -- not so good, Sante. I particularly dislike the interaction between
the 30m dipole and the C-31. As you can guess there is potential for
severe effects on the beam's performance on both 15 (2nd harmonic) and 10
meters. That MFJ reading on 12m is also very odd -- I would expect
something closer to 50 ohms, unless there is a LOT of mutual coupling.
can't you spread the ends of the dipoles out (in azimuth) so they are less
close to one another?
73, Pete N4ZR
I once had three parallel dipoles (I think for 40, 20 and 15) using one
coax feedline and one balun, mounted so that the three wires were only a few
inches apart (using wooden spacers, a real low-tech installation). I didn't
find there was much, if any interaction among them. I think I tuned the
longest one first, then the others. Just start with some extra length in
the wires in case you need it, hi.
73 and GL, Andy, ae6y
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com