Repeater antenna questions OFFSET vs OMNI

[mvrx]

Hello guys,

Me and my fellow hams are preparing to build a repeater (2m & 70cm), solar powered, sharing the same site/tower. So far, we've acquired all but the antennas.
I found a few, but decided to go with the same guys that built our duplexers.

They list the antennas needed for our repeater here :
VHF - 136-174 MHz Dipole antennas D1 VHF, D2 VHF, D4 VHF, D8 VHF : Radial :
UHF - 400-490 MHz Dipole antennas D1 UHF, D2 UHF, D4 UHF, D8 UHF : Radial :

You can see the tower where we'll have to mount the antennas here :
Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting

The antennas will be side-mounted.

My questions are :
1. What is the difference between OMNI vs. OFFSET for this antenna type?
I know you can modify the dipoles spacing and orientation to achieve different patterns, but is it something "common" for the exposed dipole antennas?

2. If we're going to point each dipole to a 90° difference, will the antenna work?
We need to cover all the area around the repeater.

3. Is there a problem if all dipoles (4 VHF + 4 UHF) will share the same pipe/mast?

Thank you and excuse my poor English. I'm from Europe.

 

[zz0468]

1. What is the difference between OMNI vs. OFFSET for this antenna type?
I know you can modify the dipoles spacing and orientation to achieve different patterns, but is it something "common" for the exposed dipole antennas?

That's one of the advantages to that type of antenna. You don't state in your post which version you're getting, but for example, the 4 dipole array is typically 6 db gain as an omni, when the 4 dipoles are oriented in 4 directions, or 9 db in the primary direction when the dipoles are oriented in the same direction. It's also possible to get close to 6 db omni when side mounting to a tower by setting all 4 dipoles in the same direction, aimed toward the tower, and with carefully selected spacing. Consult the antenna manufacturer for details.

2. If we're going to point each dipole to a 90° difference, will the antenna work?
We need to cover all the area around the repeater.

That would work just fine. But see my comment above.

3. Is there a problem if all dipoles (4 VHF + 4 UHF) will share the same pipe/mast?

It depends on how you do it. You could install all the UHF elements facing east, for example, and the VHF elements facing west and interaction would be minimal. Neither antenna would be perfectly omnidirectional. Again, check with the manufacturer for suggestions. Most antenna manufacturers are staffed with some pretty sharp people. They can help with installation specific questions better than we can here.

Thank you and excuse my poor English. I'm from Europe.

Your efforts at English are far better than my efforts would be in your language.

 

[mvrx]

Thanks for your fast and detailed reply.

You don't state in your post which version you're getting...

It's the D4 version for both bands, so each antenna has 4 dipoles, just because I wanted to point each to a different direction and try to cover a 360° radius.

...check with the manufacturer for suggestions.

Already did that, and to my surprise the guy failed to provide the requested information. It's confusing to see that it can work in 2 ways but only get a graphic representation for the OFFSET.
My question, after I agreed upon the costs, was simple : "how we should configure the 4 dipoles for an omni configuration?"
Their reply : "You can calculate diagram of 2 diversity on difference sides of mast by MANNA.Our D4VHF has assembled power divider and can't diverse on opposite sides."
What does "can't diverse on opposite sides" mean?

That's the main reason why I've opened a new thread.

Most antenna manufacturers are staffed with some pretty sharp people. They can help with installation specific questions better than we can here.

In US maybe.

Again, thanks for your reply. I just need to make sure I won't buy something that cannot be used for my needs.

 

[zz0468]

My question, after I agreed upon the costs, was simple : "how we should configure the 4 dipoles for an omni configuration?"

I have no idea what they mean by that. But configuration for omni is easy. In the offset configuration, the dipoles are aligned on one side of the mast, one over the other at a very specific spacing. In the omni configuration, that spacing is maintained, but the dipoles are oriented in 4 directions.

Their reply : "You can calculate diagram of 2 diversity on difference sides of mast by MANNA.Our D4VHF has assembled power divider and can't diverse on opposite sides."
What does "can't diverse on opposite sides" mean?

I'm not quite sure what they mean by that, but it's possible what they're trying to say is the power divider/phasing harness provided is not long enough the wrap around the mast so that the dipoles can be oriented in different directions. This is a wild guess on my part.

The fact that you're wanting to combine the two antennas on a single mast may be the complicating factor. I've seen this done, usually it's one over the other on a long mast.

You could start with the VHF antenna, aim the dipoles in 4 directions, then install the UHF in between, oriented and spaced in such a manner as to maximize the spacing between it and the VHF elements.

The VHF elements could have some detrimental impact on the UHF elements, because they're going to be close to the 3rd harmonic in length. So try to make sure that adjacent VHF/UHF elements (in terms of their vertical position) are on opposite sides of the mast as much as possible. By that I mean, say, the top most VHF element aimed East, the topmost UHF element aimed west. The next VHF element aimed north, the next UHF element aimed south. And so on...

Hard to describe without a picture.

 

[W2IBC]

How many bays are you going to use on the VHF & UHF 1? 2? 4? 8?

also what kind of coverage area are you trying to get? are you wanting manily local or wide area?

any way you can put the VHF at the top of the tower say 4 bay at the top of the tower on its own pole and side mount the UHF with 8 bay?

if they have to be all side mounted then point them all in the same direction put the VHF on one side of the tower and the UHF on the other. also me personally I would try and get them about 2-4ft away from the tower as well which means you would have to make some kind of mount.

 

[mvrx]

The VHF elements could have some detrimental impact on the UHF elements, because they're going to be close to the 3rd harmonic in length. So try to make sure that adjacent VHF/UHF elements (in terms of their vertical position) are on opposite sides of the mast as much as possible. By that I mean, say, the top most VHF element aimed East, the topmost UHF element aimed west. The next VHF element aimed north, the next UHF element aimed south. And so on...

Got it, thanks. Something like all dipoles 1 to 8 pointed like (S - W - N - E - N - E - S - W)
Anyway, mounting all 8 dipoles (4 VHF + 4 UHF) on the same mast is not mandatory.
That tower is unused and we can mount the antennas anywhere, as long as it's not on top. The reason for that is weather and lighting, the tower being the highest point in that area of 1776 meters (5826 feet)

How many bays are you going to use on the VHF & UHF 1? 2? 4? 8?

4 bays for each band, a total of 8 bays.

also what kind of coverage area are you trying to get? are you wanting manily local or wide area?

As wide as possible, otherwise I could have just mount the dipoles as OFFSET and point to the direction of interest.

any way you can put the VHF at the top of the tower say 4 bay at the top of the tower on its own pole and side mount the UHF with 8 bay?

We're trying to avoid mounting anything at the very top of that tower. As explained above, protection from lightning is needed.

if they have to be all side mounted then point them all in the same direction put the VHF on one side of the tower and the UHF on the other. also me personally I would try and get them about 2-4ft away from the tower as well which means you would have to make some kind of mount.

The idea is to obtain a "Quasi-Omni" pattern. Sure, the horizontal spacing is to be calculated using this information :
http://www.repeater-builder.com/antenna/pdf/antenna-mounting-guidelines.pdf

I guess we'll have to order the dipoles and do some testing in the backyard and see how it will perform as an OMNI.

Thank you all!

Regards,
Razvan