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Author Topic: Spacing of Antennas?  (Read 1228 times)
KD6MLH
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Posts: 1




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« on: October 11, 2009, 10:35:28 AM »

Hello all.

I recently received a new HT for my birth day, (VX-8R) it does 6M/2M and 440 it also does my APRS.  I currently have a 2M 440 mobile in my truck and a HF rig. I am looking to mount another antenna on the roof of my pick up truck I am thinking of mount it in frunt of the the current one. (the HF antenna is o the right rear corner) So I will have a 2M 440 in the middle of the back half of my extended cab,  the  6M 2M 440 will be in the middle front Half of the roof.  I don't want to turn it in to a beam on  2M,  the antennas are 1/2 waves on 2M.

So this brings me to the questions, Does any one have any ideas about interaction of the too antennas on the roof?
Should I space them more or less?
Has any one had a problem with directionality?

I have never had more then 1 antenna on a roof any help would be grate,
Tyghe KD6MLH
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K0BG
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« Reply #1 on: October 11, 2009, 11:32:08 AM »

There is always interaction, even if they're 20 feet apart. There are really two worries.

First, if you transmit enough power out of one, and you're receiving on the other, there is always a chance you zap a front end. This is more likely to happen to the HT, than the mobile radio.

The other is desense. Transmit on one, and you'll likely not receive on the other, even on different bands sometimes.

To answer your question directly, place them as far apart as you can.

Alan, KØBG
www.k0bg.com
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KC7YRA
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Posts: 256




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« Reply #2 on: October 11, 2009, 04:53:17 PM »

I currently have 5 roof mounted, NMO antennas on the roof of my F-150 and a large screwdriver type HF antenna on the bed.

I have seen some VERY slight interaction where as 1 radio will not receive a signal that another is.  This only happens with signals that are so far in the noise, I can't understand them anyway.  

Like Alan said, mount them up and get they as far apart as you can comfortably.  All of mine on similar bands try to maintain at least 24" of separation.  About the only thing I notice that I would call a "concern" is the de-sense issue.  If I am talking on one radio, it pretty much wipes all the others out.

Brad
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N7MSD
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Posts: 8




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« Reply #3 on: October 15, 2009, 02:48:16 AM »

This question dovetails into a question I'm having right now, and I think you may have answered it, Brad.

I do D-STAR as well as 23cm FM.  I eventually want to get the ID-1; in the mean time, I'll just tie my IC-T81A quad-bander in for 23cm.  For the lower bands, at present, I can use that radio as well as either a IC-91AD or ID-800H (both dual-banders).

I bought a Comet SB-97 tribander (2m/70cm/23cm) at a closeout price, but am faced with the problem of how to get the RF from 2 radios up to the roof.  I have 2 solutions:

1. Use a diplexer, in this case a Comet CF-413B or CF-4130B, or (if I were able to find it) the discontinued Diamond MX-37.  These all do the same thing: split 23cm (and 33cm?) from everything below and is practically custom-made for this application.  This is the simplest, cleanest, and cheapest solution, but not expandable for the future.

2. Get a second antenna.  Ideas would be the Comet UHV-4 or -6, Diamond HV7A, a conventional 6m/2m/70cm tribander, or--very interesting IF IT WORKS--a MP/Niljon Super Mobile Ultra antenna that's claimed to work to 6 GHz.  The catch is far more expense and coupling between antennas--which is where this discussion comes in. (Note that I'm not that interested in HF mobile and it's associated nightmares at the moment.)

With that, I'm open to whatever you guys suggest.

Thanks, Mike
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KC7YRA
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Posts: 256




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« Reply #4 on: October 20, 2009, 08:31:55 PM »

Mike,

It seems like it would work.  In my mind it makes it seem like a PITA.  

If it were me, I would go with another antenna.

Purely from the standpoint that I cannot operate 2 radios as exactly the same time and I'm sure the diplexer is more expensive than a 2nd antenna.

Brad
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KC7YRA
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« Reply #5 on: October 20, 2009, 08:38:14 PM »

Ok Mike,

It is late and I read your post a little wrong.

I thought you were getting at trying to operate 222mhz.

I doubt very seriously that you would have any interaction between any radio on 2/70 and another on 23CM.  Many multitudes of MHZ away.

Get a small NMO mount antenna for 23CM and have fun.  VERY cheap and will let you expand your operation in the future as you were talking about.  

Brad
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CLEARSIGNAL
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Posts: 2




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« Reply #6 on: October 28, 2009, 06:33:45 AM »

Hi Mike,
In case you haven't seen it, there is a new response to your NMO question on the VHF/UHF forum.
Dr. Jack Nilsson, N8NDL
jackn3@msn.com
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