I just completed the installation and tuning of a multiband inverted vee for
160, 80, and 75 meters. Essentially, the antenna consists of a center PVC
insulator with 6 terminal posts (2 per band) and the three dipoles separated
by wooden dowel-rod spreaders at spacings of about 6 inches. All three sets
of wires are direct fed with one RG/8u.

I have adjusted the lengths of each antenna to move the resonant point to
preferred frequencies for each band: 1.825 on 160, 3.525 on 80, and 3.775 on
75. However, at resonance the SWR on all antennas remains relatively high --
an MFJ259 analyzer says resonant SWRs are around 1.7-2.0:1 and
characteristic impedances are 100 ohms or slightly higher. Is a 2:1 balun
the solution, and which balun method? Seems to me I have the following
choices:

1. A "balun kit" from someone like thewireman.com and build my own. This
could be in either the form of a voltage or a current type -- which type
would be best?

2. A coax balun in the form of a 1/4 wavelength of RGB/11u 75 ohm coax --
replacing a portion of the RG/8u feedline with RG/11u. But since I am using
this for two bands... what length would offer the best compromise for both
80 and 160?

3. A coiled-coax balun which I have seen described as "about 16 turns of
RG8x on 3 inch PVC stock".

All of these options are easily do-able, but which looks like the best
route? Your opinions are welcome.
------
-larry
K8UT

"Larry Gauthier \(K8UT\)" wrote in message ...
I just completed the installation and tuning of a multiband inverted vee for
160, 80, and 75 meters. Essentially, the antenna consists of a center PVC
insulator with 6 terminal posts (2 per band) and the three dipoles separated
by wooden dowel-rod spreaders at spacings of about 6 inches. All three sets
of wires are direct fed with one RG/8u.

I have adjusted the lengths of each antenna to move the resonant point to
preferred frequencies for each band: 1.825 on 160, 3.525 on 80, and 3.775 on
75. However, at resonance the SWR on all antennas remains relatively high --
an MFJ259 analyzer says resonant SWRs are around 1.7-2.0:1 and
characteristic impedances are 100 ohms or slightly higher. Is a 2:1 balun
the solution, and which balun method? Seems to me I have the following
choices:

1. A "balun kit" from someone like thewireman.com and build my own. This
could be in either the form of a voltage or a current type -- which type
would be best?

I guess that could work...

2. A coax balun in the form of a 1/4 wavelength of RGB/11u 75 ohm coax --
replacing a portion of the RG/8u feedline with RG/11u. But since I am using
this for two bands... what length would offer the best compromise for both
80 and 160?

I guess this could also, but I'm not sure about the best length as a
compromise.
You shouldn't really need one on the lowest band, unless it's coupling
to something else.

3. A coiled-coax balun which I have seen described as "about 16 turns of
RG8x on 3 inch PVC stock".

This is 1:1. No transformation, so would only help decouple the
feedline.

All of these options are easily do-able, but which looks like the best
route? Your opinions are welcome.

The best route would be to separate the dipoles enough to avoid the
coupling you are seeing between them. That way, you wouldn't need to
do anything else. I always space paralleled dipoles as far apart as
possible. With your bands, I'd run the two set of 80 legs at right
angles, as a turnstile. "I run this myself".
Then the 160 legs somewhere in between. You should only see coupling
effects on the higher bands. Usually not the lowest band. If you see
effects on the lowest band, it's probably coupling to something else
in the area. "power lines, guy wires, etc"..If it were me, and I
couldn't space the legs out, I'd probably prefer the coax transformer
as a first shot. I'd probably try a 1/4 wave section for 80m, "appx 44
ft or so for 75 ohm .66VF coax" and see if I could match 160 as is by
varying the lengths or spacing. If that didn't work, I guess you could
try a 2:1 balun. MK

Larry,
I don't think a balun would do any good in your situation,
but it wouldn't hurt to try one, I guess.
There are other 'tricks' to lowering the SWR besides using
a balun. Seperating the various 'legs' of the antenna more,
changing the angle between the 'legs', and so on. Depending
on how you have the antenna mounted, changing it's 'shape' is
quicker/easier/cheaper than adding a balun, so I'd try that
first.
If you do try a balun, a 'current' balun is almost always the
prefered type to use. As long as it's rated for the power you
plan
to use, just make it 'cheap' for your self. And, since coax is
cheaper than buying a balun, I'd give the coaxial 'choke' a try
first.
I doubt if it helps with the SWR problem, though. Can't hurt to
try it, so why not?
'Doc

Larry,

Where did you measure the impedance? Probably not at the feedpoint. You
would have to use a Smith chart to determine what the actual antenna
impedance is. A 2:1 balun *at that point* should work, though. They are hard
to find. Amidon has one, but it is about $60. I have fed multiple dipoles
off the same feedpoint, and gotten an impedance close to 50 Ohms; however, I
had something like 15 degrees separation between the dipoles.

Tam/WB2TT
"Larry Gauthier (K8UT)" wrote in message
...
I just completed the installation and tuning of a multiband inverted vee
for
160, 80, and 75 meters. Essentially, the antenna consists of a center PVC
insulator with 6 terminal posts (2 per band) and the three dipoles
separated
by wooden dowel-rod spreaders at spacings of about 6 inches. All three
sets
of wires are direct fed with one RG/8u.

I have adjusted the lengths of each antenna to move the resonant point to
preferred frequencies for each band: 1.825 on 160, 3.525 on 80, and 3.775
on
75. However, at resonance the SWR on all antennas remains relatively
high --
an MFJ259 analyzer says resonant SWRs are around 1.7-2.0:1 and
characteristic impedances are 100 ohms or slightly higher. Is a 2:1 balun
the solution, and which balun method? Seems to me I have the following
choices:

1. A "balun kit" from someone like thewireman.com and build my own. This
could be in either the form of a voltage or a current type -- which type
would be best?

2. A coax balun in the form of a 1/4 wavelength of RGB/11u 75 ohm coax --
replacing a portion of the RG/8u feedline with RG/11u. But since I am
using
this for two bands... what length would offer the best compromise for both
80 and 160?

3. A coiled-coax balun which I have seen described as "about 16 turns of
RG8x on 3 inch PVC stock".

All of these options are easily do-able, but which looks like the best
route? Your opinions are welcome.
------
-larry
K8UT

Larry Gauthier (K8UT) wrote:
Is a 2:1 balun the solution, and which balun method?

A 2:1 balun on a single toroid is hard to come by.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----