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Author Topic: Dual VHF Radios TX / RX Separation Guidelines  (Read 454 times)
AE7VA
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Posts: 82




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« on: December 07, 2017, 07:20:56 AM »

I have a project where I'm looking for any formula or rule of thumb for distance separation between two VHF transceivers that are communicating with the same mountain top repeater; eliminate or reduce front end overload / receiver desense. I primarily operate only single transmitter HF, so VHF FM activity is foreign to me.

The first transceiver is a Icom F5021 connected to a Signalink and computer and handles a VoIP connect. This transceiver transmits that VoiP connection to the repeater and takes the repeater's transmitted signal and dumps it back onto the Internet. It will be connected to a Ringer Ranger vertical antenna with 20 feet elevation.

The second VHF transceiver is a Yaesu FT1900 and I use to be the NCS. This is connected to a discone at 20 feet elevation.

The second Yaesu is installed and working. The Icom is not yet installed, but I would like to determine from the start how far away I should put the Ringo Ranger antenna away from the discone running both 5 watts or 25 watts.

I know from experience already that transmitting 50 watts off the discone will desense the receiver on my HT 50 feet away in my shack. But I don't need 50 watts to hit the repeater.

I was just wondering if there was a way to calculate the distance separation between antennas instead of trial and error of erecting/taking down the mast and moving it.

Any distance recommendations or suggestions?
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W8JX
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« Reply #1 on: December 08, 2017, 05:49:40 AM »

You will gain more isolation stacking them vertically 20 or more feet apart than horizontal separation at 10 times that distance. I would also suggest that you scrape dis-cone in vertical stack and use another vertical or vertical dipole as it will provide a better null for isolation in vertical stack. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K5LXP
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Posts: 5346


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« Reply #2 on: December 08, 2017, 08:28:46 AM »

It's going to come down to trial and error.  The dominant variables are the frequency separation, power/ERP, transmitter purity and receiver immunity.  You could probably calculate it if you had performance data on this equipment but it would be a bit of work with fancy test gear to get it.  Faster to hook it up and see at what power level things begin to degrade.  From there you can see how much margin you have, or need, and can pick the best remedy as necessary.

I would add that for very close separations (assuming 600kHz here) you need some pretty good gear or a fair amount of isolation.  If you don't need frequency agility, cavity filters would do the trick but are pretty spendy.  Barring that you're looking at tailoring antenna patterns and raw separation distance, or a mix of filters and antennas.  Finding a new home for the VOIP gear starts to look attractive.

Wondering what you're using to pipe VOIP over 2M?

Mark K5LXP
Albuquerque, NM
« Last Edit: December 08, 2017, 08:50:49 AM by K5LXP » Logged
N8EKT
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Posts: 595




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« Reply #3 on: December 08, 2017, 04:27:41 PM »

Vertical spacing is what is needed

10 feet of vertical separation of two half wave dipoles will provide about 35db of isolation

20 feet will provide about 45db

30 feet will provide about 55db

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