Two antennas on the same mast

[ILSAPP]

Hello,


I'd like to ask for your advice regarding mounting two antennas on the same mast.
I plan to use a 18ft mast and I want to place in it a 11 element Yagi 1296MHz antenna (will be used as RX only) as well as a Diamond X-50 (will be used as TX on 70cm).

Probably the best would be to space them vertically giving let say 6ft distance apart? What would probably be the best compromise for this case?


Thank you in advance!

 

[mmckenna]

Hello,


I'd like to ask for your advice regarding mounting two antennas on the same mast.
I plan to use a 18ft mast and I want to place in it a 11 element Yagi 1296MHz antenna (will be used as RX only) as well as a Diamond X-50 (will be used as TX on 70cm).

Probably the best would be to space them vertically giving let say 6ft distance apart? What would probably be the best compromise for this case?


Thank you in advance!

6 feet is probably more than you need if you are running reasonable (50 watt) radio on the X50.
The X50 is going to be radiating power out towards the horizon, not so much down towards the mast.

The 1296MHz Yagi will have very poor coupling on 144/430MHz, so probably even less RF getting in that way.

Your 1296 radio will probably have some sort of filtering outside the ham bands to get rid of the cellular stuff that may be in that range, depending on where you live.

But more separation won't hurt things. As long as the Yagi can see what you want to hear, it's probably just fine.

 

[ILSAPP]

Thank you for the quick and detailed reply.
Indeed on 70cm band I'll use 1W max so I guess that the distance between antennas can be further reduced than 6ft.
Yes the 1296MHz radio has filtering outside ham bands.
The problem with more separation is placing the 70cm antenna further down thus limiting its range.

 

[mmckenna]

The problem with more separation is placing the 70cm antenna further down thus limiting its range.

Reading this, I want to make sure you understood that my recommendation was to have the X50 at the top of the mast, and the 1296 Yagi below it.

If your plan is to put the 1296MHz antenna at the top of the mast, and the X50 below it, you need to make sure you stand the VHF/UHF antenna a couple of feet away from the metal mast, if that is what you are using. If it's a non-conductive mast, the coax going up to the Yagi is going to create the same issue.

You were going to put the dual band antenna on top, right?

 

[ILSAPP]

Initially I thought of using a horizontal support and place them in the same height but I've read that vertically separating antennas generally requires less distance compared to horizontal separation to achieve the same isolation. Then yes I thought of placing the yagi on top and the X50 below it because of LOS propagation with 23cm band.
It is a non-conductive mast.
I now realize that the X50 should be really mounted on top of the mast and the yagi below it.

 

[mmckenna]

Initially I thought of using a horizontal support and place them in the same height but I've read that vertically separating antennas generally requires less distance compared to horizontal separation to achieve the same isolation.

Yes, for vertically polarized antennas, vertical separation gives you more isolation.

I now realize that the X50 should be really mounted on top of the mast and the yagi below it.

You shouldn't need much separation between the two antennas, so the X50 at the very top and the 23cm antenna a foot or so down should be plenty.

 

[mmckenna]

I have installations at work where I have an 800MHz vertical at the top of a mast and a VHF Yagi directly under it. The VHF is running 50 watts, and the 800MHz antenna is running about 15. No issues.

I've done UHF verticals with a 2.4GHz antenna about a foot under it. UHF was running about 10 watts. Again, no issues.

 

[G7RUX]

Hello,


I'd like to ask for your advice regarding mounting two antennas on the same mast.
I plan to use a 18ft mast and I want to place in it a 11 element Yagi 1296MHz antenna (will be used as RX only) as well as a Diamond X-50 (will be used as TX on 70cm).

Probably the best would be to space them vertically giving let say 6ft distance apart? What would probably be the best compromise for this case?


Thank you in advance!

I would imagine that your proposed setup would work very well, although you probably don't need as much as 6ft spacing in this configuration, especially if you are intending to use the yagi as an HP antenna.

A few years back I did tests at a broadcast site with two same-band antennas mounted on the same mast both with VP but one about 20 ft above the other. There was remarkably little coupling between them and the experimental system worked very nicely.

I have a pair of HB9CV (one for 2m and the other for 70cm) antennas mounted on the same handheld boom and the cross-band coupling between those is also surprisingly low.

 

[ILSAPP]

Many thanks @mmckenna and @G7RUX for your valuable input, much appreciated!
I'll do some tests (both VP antennas) starting with a low separation and see the effects of cross-band coupling between the antennas. Glad to know that with more critical cases a short separation revelead to be enough.

 

[G7RUX]

If it comes to it then filters would help improve the isolation...I use a high-pass and low-pass pair for my 2/70 satellite crossband stuff and they give superb additional isolation.