Anyone running 2 dual band antennas near each other? Spacing?

[ClemsonSCJ]

I’m in the final stages of getting my shack set up and I’ve got a plan for everything except antenna placement. I’ve got a tripod and a 10ft fence top rail to use as a mast, but I’ve now decided I want to set up an iGate/digipeater in addition to my “conversing” radio. I would like to place both antennas at the top of the mast but I’m not sure how far I should separate them to keep from interfering with one another. The idea I had in mind was to get some sort of T bracket at the top of the mast, run 2 short lengths of pipe outward and then 90 deg elbow to angle back up. Basically will look like a giant tuning fork. I’ve done some research and the main points I’ve read are 1) no less than 1/4 wavelength apart but also the more space the better. Obviously since I’m at the top of a mast I wanna keep this setup as small as possible…so at what point are there diminishing returns?

 

[mmckenna]

There are a lot of factors involved:

Transmit power.
Feed line losses
Antenna gain
Horizontal spacing between antennas
Vertical spacing between antennas (if one is mounted higher than the other)
Receiver design/filtering.

Since it would be hard to quantify all that without some test gear, it's best to just separate them as much as you can. Unless they are really far apart, you should expect some receiver desense (receiver will go deaf) when the other radio is transmitting.

Vertical separation will give you more isolation, so if you can put one at the top of the mast, and one below and stood off from the mast at least 1/4 wavelength, then you'll do a bit better.

Would be better to mount them in separate locations.

Filtering can be used on the iGate radio, but that can get a bit expensive and will require tuning. You could use properly tuned filters to block out everything outside 144.390MHz ± a few tens of KHz.

 

[prcguy]

I would go a minimum of 2 wavelength spacing at the lowest frequency to try and avoid serious changes in the antenna patterns. For 2m that would be about 12ft 10in. At 1 wavelength spacing or about 6ft 5in your going to get some lobes and nulls in the patterns. Closer is worse.

For two radios on the same band, one on each antenna at that close distance they will completely blitz each other, even at low power.

 

[ClemsonSCJ]

There are a lot of factors involved:

Transmit power.
Feed line losses
Antenna gain
Horizontal spacing between antennas
Vertical spacing between antennas (if one is mounted higher than the other)
Receiver design/filtering.

Since it would be hard to quantify all that without some test gear, it's best to just separate them as much as you can. Unless they are really far apart, you should expect some receiver desense (receiver will go deaf) when the other radio is transmitting.

Vertical separation will give you more isolation, so if you can put one at the top of the mast, and one below and stood off from the mast at least 1/4 wavelength, then you'll do a bit better.

Would be better to mount them in separate locations.

Filtering can be used on the iGate radio, but that can get a bit expensive and will require tuning. You could use properly tuned filters to block out everything outside 144.390MHz ± a few tens of KHz.

I think what I may do, since the only additional cost will be the tripod and mast, is to put the digipeater antenna at the end of the house over the garage. My main antenna (antenna for the actually talking radio) is going to be on a tripod at the apex of the roof directly over my shack. This works out perfect cause it keeps my coax run super short, so that’s why I wanted the 2 on the same mast. But if I get another tripod and mast I can put it at the end of the garage which will be about 3ft lower and about 20ft away. It should only add about another 15ft to my coax run so not much difference in the grand scheme of things and sounds like it will keep the 2 antennas from interfering with one another almost entirely.

Since you mentioned vertical spacing and another site I looked at mentioned vertical spacing as well, I do have one question about that. So the site that mentioned it said that a foot of vertical spacing is worth several feet of horizontal spacing. This is obviously due to the donut shape of the radiation pattern of omnidirectional antennas so it’s much easier to get out of that radiation pattern when you move vertically. However, most base 2m/70 cm antennas are about 5ft long. So assuming your reference point is both of them level with each other…a foot of drop still has 4ft of overlap on the vertical plane. So when talking about x-amount of feet vertical movement, do they mean once you have gotten the top of one antenna level or lower than the bottom of the other, or are they talking about from dead even with each other?

Also, the equipment will be a UV-50X2 with a DBJ-1 antenna for the digipeater. That radio runs about 70ish watts so it will probably be closer to 60 watts at the antenna. The other radio is a Yaesu FTM-300 with a Diamond X50 antenna, so running about 50 watts out the radio and roughly 40 watts at the antenna. All coax is going to be LMR-400 with the exception of about 3ft coming out of my ceiling and going to the shelf with the radios. That will likely be something a little more flexible. So I can get it routed neatly inside the house.

 

[ClemsonSCJ]

I would go a minimum of 2 wavelength spacing at the lowest frequency to try and avoid serious changes in the antenna patterns. For 2m that would be about 12ft 10in. At 1 wavelength spacing or about 6ft 5in your going to get some lobes and nulls in the patterns. Closer is worse.

For two radios on the same band, one on each antenna at that close distance they will completely blitz each other, even at low power.

One of the things I read pertained mainly to vehicle mounting but I would imagine it would apply to base as well granted this is even correct…but it said that obviously a minimum of 1/4 wavelength apart but also to make sure you don’t space at the exact increments of the common antenna wavelengths. So basically if you decide you’re going to do 1/2 or a whole wavelength apart, add or subtract a few inches so that you’re not dead on 1/2 or whole wavelength apart. I don’t recall why it said to do this but I do know it only pertained to operating on the same bands and was not applicable if you were inquiring about spacing of antennas for 2 totally separate bands (such as one 2m/70cm and one cb or other band).

 

[prcguy]

What you need to do is look up antenna patterns for antennas spaced at close distances like 1/4 wave and 1/2 wave, you will be shocked and will stop considering doing this. Antennas that close are in the near field and the patterns will be very messy and with a lot of coupling between the antennas.

There is nothing magic about an exact 1/2 wavelength spacing vs a 1/2 wavelength and a few more inches except that few extra inches reduces coupling by a little bit. The radiation patterns will still be terrible and you will still have massive coupling where it will be impossible for one radio to receive anything if your transmitting on the other. I mentioned two wavelengths apart because that is where the patterns just start becoming fairly uniform and omni directional instead of clover leaf's or worse.

One of the things I read pertained mainly to vehicle mounting but I would imagine it would apply to base as well granted this is even correct…but it said that obviously a minimum of 1/4 wavelength apart but also to make sure you don’t space at the exact increments of the common antenna wavelengths. So basically if you decide you’re going to do 1/2 or a whole wavelength apart, add or subtract a few inches so that you’re not dead on 1/2 or whole wavelength apart. I don’t recall why it said to do this but I do know it only pertained to operating on the same bands and was not applicable if you were inquiring about spacing of antennas for 2 totally separate bands (such as one 2m/70cm and one cb or other band).

 

[ClemsonSCJ]

What you need to do is look up antenna patterns for antennas spaced at close distances like 1/4 wave and 1/2 wave, you will be shocked and will stop considering doing this. Antennas that close are in the near field and the patterns will be very messy and with a lot of coupling between the antennas.

There is nothing magic about an exact 1/2 wavelength spacing vs a 1/2 wavelength and a few more inches except that few extra inches reduces coupling by a little bit. The radiation patterns will still be terrible and you will still have massive coupling where it will be impossible for one radio to receive anything if your transmitting on the other. I mentioned two wavelengths apart because that is where the patterns just start becoming fairly uniform and omni directional instead of clover leaf's or worse.

So do you think if I put up another tripod and mast over the end of my garage which is about 20ft away and 3-4ft lower than the other antenna, that would more or less eliminate any interference between the 2?

 

[prcguy]

That would eliminate all antenna pattern problems but you will still have interference on one radio when transmitting on the other radio when on the same band depending on frequency spacing and power. I can do that here with some high end radios and high power a MHz or more apart and other radios fail miserably.

So do you think if I put up another tripod and mast over the end of my garage which is about 20ft away and 3-4ft lower than the other antenna, that would more or less eliminate any interference between the 2?

 

[ClemsonSCJ]

That would eliminate all antenna pattern problems but you will still have interference on one radio when transmitting on the other radio when on the same band depending on frequency spacing and power. I can do that here with some high end radios and high power a MHz or more apart and other radios fail miserably.

Well I suppose if there’s a positive there, pretty much every repeater around me has a 2m and 70cm operating frequency and everyone seems to use the 70cm band the most. I’m guessing that’s because of what I too have noticed, that the 70cm repeaters pick up FAR better than the 2m repeaters which I’m a bit perplexed by seeing as how the rule of thumb is that lower frequencies travel farther and penetrate objects easier, and the landscape seems to lend itself better to 2m despite it performing rather poorly. Anywho, that’s a whole other topic for a whole other time. All that was to say that 99% of my activity on 2m will be primarily for APRS and not much else.

 

[R8000]

Also, the equipment will be a UV-50X2 with a DBJ-1 antenna for the digipeater. That radio runs about 70ish watts

So you are going to use that China spur-master mobile radio we discussed in the other thread. And now you want to have more than one radio on the same band and not have interference ???
You were given good advice by folks who work with this stuff for a living. You ignored it.
Good luck.

 

[ClemsonSCJ]

So you are going to use that China spur-master mobile radio we discussed in the other thread. And now you want to have more than one radio on the same band and not have interference ???
You were given good advice by folks who work with this stuff for a living. You ignored it.
Good luck.

Feel free to read the post directly above your reply, I believe it is #9. Thanks.