Feeding 2 VHF Yagis from one coax?
 

I have a temporary tower set up with a 6M yagi on it. I'd like to add a
2M yagi. Is it possible to feed the two antennas with one feedline
similar to feeding multiple HF antennas with one feedline?



Coax from Shack RG-8 RG-8
=========================T============2M Yagi
I
I RG-8
I
6M Yagi

So basically, I would add a T connector at the 6M Yagi and run another
piece of RG-8 from the T to the 2M Yagi (the T connector would have a
male PL-259 on one port connected directly to the 6M Yagi, the other 2
ports are female UHF and one would be connected to coax from shack and
the other would have the jumper to the 2M antenna). Now the $64K
question...should the run of RG-8 from the T to the 2M be a certain
length, such as 1/4 or 1/2 Wavelength at one of the bands and if so
which band? i.e., 1/2 wavelength at 6M or 1/4 wavelength at 2M, etc.
Both antennas use a gamma match as the feed, so the short section of
coax seems like it might be perceived as a open-ended stub attached to
the 6M antenna??? This is for high speed meteor scatter, so I'm not
overly concerned if the antenna patterns get a bit screwed up...

Scott
N0EDV

Feeding 2 VHF Yagis from one coax?
 

Hi Scott

No doubt others will give you much better info than I.

The gamma macth feed is irrelevent. If you have a resistive 50 ohm load
for each antenna at its operating frequency it makes the maths of using
tuned coax lengths a little easier. I would have to sit down and think
about it a bit deeper but on the surface it looks like;

Use a 1/4, 3/4, 5/4 etc wavelength of coax on 2M to attach the 6M
antenna to the T piece. The logic is that the lowish impedance of the 6M
antenna on 2M will be transposed to a high impedance at the T piece, the
net result being little effect to feeding the 2M antenna.

Do the reverse when feeding the 2M antenna.

Problem - There is a relatively close harmonic relationship between 6M
and 2M. ie a 1/4 wavelength on 6M is close to a 3/4 wavelength on 2M. If
you think about it you will note that any 6M length will also be a 2M
length! You cant actually get away from the problem! This feeding
technique is only really usable when there isnt a 3:1, 5:1 etc frequency
relationship.

You are welcome to a spare coaxial relay I have! I dont however have
spare N connectors. If you want it pls email me direct.

Cheers Bob W5/VK2YQA East Texas

Scott wrote:


I have a temporary tower set up with a 6M yagi on it. I'd like to add a
2M yagi. Is it possible to feed the two antennas with one feedline
similar to feeding multiple HF antennas with one feedline?

Feeding 2 VHF Yagis from one coax?
 

Oh yes, I forgot about those things! I have one for 2M and 432 and it
works quite well. This idea is just for a temporary set up. The new
tower should be going up this summer...this idea was just to get me by
until then...

Scott
N0EDV

wrote:

On Sat, 08 Apr 2006 11:02:06 +0000, Scott
wrote:


I have a temporary tower set up with a 6M yagi on it. I'd like to add a
2M yagi. Is it possible to feed the two antennas with one feedline
similar to feeding multiple HF antennas with one feedline?



Coax from Shack RG-8 RG-8
=========================T============2M Yagi
I
I RG-8
I
6M Yagi

So basically, I would add a T connector at the 6M Yagi and run another
piece of RG-8 from the T to the 2M Yagi (the T connector would have a
male PL-259 on one port connected directly to the 6M Yagi, the other 2
ports are female UHF and one would be connected to coax from shack and
the other would have the jumper to the 2M antenna). Now the $64K
question...should the run of RG-8 from the T to the 2M be a certain
length, such as 1/4 or 1/2 Wavelength at one of the bands and if so
which band? i.e., 1/2 wavelength at 6M or 1/4 wavelength at 2M, etc.
Both antennas use a gamma match as the feed, so the short section of
coax seems like it might be perceived as a open-ended stub attached to
the 6M antenna??? This is for high speed meteor scatter, so I'm not
overly concerned if the antenna patterns get a bit screwed up...

Scott
N0EDV


What you need is called a Diplexor. Diplexors can be implemented in
many ways (using coax sections s only one). Their goal is to keep
the signal on one path seperated from another. However they introduce
small measurable losses and that may not suit your scatter work.

I've built and used diplexors for 145/437 (sats) use to deal with the
two antennas one coax connection of some deal band radios.

As to patterns, if the antennas are seperated enough there would be
little effect.

Myself I prefer two coax as the losses are lowest. Second best is
coax relays at both ends (you still incur connector losses) . Third
best is Diplexor as there are losses through a diplexor and isolation
limits.

Thats the short version.

Allison


Allison