Any radio/antenna engineers out there?

[d72hemi]

OK here are the basics. I need to know what the nearest two 1.5dBic gain antennas can be while operating on the SAME UHF frequency at 20W of power. If there are any engineers out there that would like to take this on or at least give me a place to start, I could provide every detail that you need.

Thanks

Ian

[73-charger-383]

I work with a bunch of guys that'll know.......i'll ask tomorrow..

[Todd Wilson]

Lets hear the details........... You dont want to put antenna's very close to each other on any freq as you can get bleed over.


Todd

[73-charger-383]

golfreakindarnit....i forgot to ask........i work at digi-key.........shoot an email to the tech department  .....someone should be able to help you..if not, i'll try to remember to ask them tomorrow......

http://www.digikey.com

go to contact us and do a live web chat.........it's free.....

[d72hemi]

Lets hear the details........... You dont want to put antenna's very close to each other on any freq as you can get bleed over.


Todd

You asked for it so here it is. I have two pair's radios/antennas that I am concerned about. The radios are AN/GRC-171 operating in AM, 20W out, near 290 MHz and 325 MHz. The antennas are TACO D-2221 rated at 1.5 dBic gain. These antennas are placed on top of 75 foot towers 150 feet apart. The system was never intended to be used with multiple radios on the same frequency, and I personally believe that the power being received at the antenna and thereby sent to the receiving radio, from the transmitting radio is way too high. After all the AN/GRC-171 is set to break squelch at only 3uV. I have personally seen an AN/GRC-171 with a similar antenna tower setup talk over 200 miles to another AN/GRC-171 with a ground based antenna.

I also have the AN/GRC-171 equipment specialist working on this. I am not trying to step on toes, I am more than confident that he and his team can figure this out, I just always approach a problem at least 2 ways (backup plan), out of habit. After all I live by, "plan for the worse, hope for the best, you are always covered that way".


If any of you can calculate the power level of what is being received at an antenna only 150 feet away that would help a lot, even if you don't have the time do the math, please send me or direct me to the equation(s) that I need to figure it out. I have searched the internet for weeks trying to just find that. I will need the calculations to prove to my bosses they are TOO CLOSE! Otherwise it will just sound like "I know this guy, and he said...."


Thanks for any information you can supply,

Ian

[dodgecharger-fan]

The army says this about setting up that gear:

REMOTE COMMUNICATIONS FACILITY
8-116. A remote communications facility includes an equipment building
with associated antenna platform(s). The ground radio equipment used for
communications between air traffic controller and aircraft is installed in the
building. The remote communications facility can either be a separate
transmitter site, receiver site, or both. Transmitter and receiver sites are
located a minimum of 1/4 mile to 1 mile apart to reduce or eliminate mutual
interference. UHF and VHF multi-couplers and vertically stacked antennas
are used to enhance frequency isolation and reduce total number of antennas
needed.

Page 8-23 of this doc: http://www.globalsecurity.org/military/library/policy/army/fm/3-04-303/fm3-04-303.pdf

And as an engineering type with no specific antenna design skills, I see something in this doc that is basically a guideline:
http://www.columbiaelectronics.com/an_grc_171_v__1__2____4_uhf_radio_sets.htm

The collocation performance spec is given at 24dB isolation.
To me, that means you need to isolated the two antennas by some means - the easiest being distance, but, speaking about antenna technology in general, other technical ways exist such as:
driving the beams in phase quadrature and spacing them close enough together to cancel out some of the effect: http://www.freepatentsonline.com/4480255.html
polarization: http://ieeexplore.ieee.org/iel5/8636/27414/01220332.pdf?tp=&isnumber=&arnumber=1220332
and http://www.dopsys.com/Interference.htm

These methods may not apply to what you're working on specifically, but the concept of isolation does apply.


So, to answer more directly, I think what you've got there would work but at a much-less-than-optimal level unless they've employed some method to provide that 24dB isolation (the freq separation is there. So at least that's good. You always want to keep your freqs separated or they'll band together and revolt on you....    I thought it was funny.  )


Looking purely at the distance that you've given, I'd almost bet that someone read the safety specs on antenna tower installation and went with that:
"Space towers at a distance that is at least twice the height of the tower." In your case 75*2 is 150... that computes.
The idea is that if they fell directly at each other, they wouldn't hit.

but that safety guideline does not take into consideration any interference generated by the antennas on top of the towers and the effect it has on the performance of the system.


If you could ask, innocently and without leading them to what you're after whomever decide on the 150ft number how they got there, I wonder if this safety spec. would come up.
It's just a hunch of mine, but it fits so well, I'd have to ask the question even just to eliminate the possibility.


Now that you've got me drawn completely in to this, please make sure you update us with what you find out or I'll go insane. 

[Todd Wilson]

Well I asked some of my fellow ham radio operators about it.  It appears that you really dont have all the info that would be needed to figure it out for sure. You'd need a meter to measure the received power.  The ohms of the antenna. And go from there. Chances are if its a good radio overloading the front end wont happen. If the radios are on seperate channels and you get bleed over then they are too close for sure. If you had real metered #'s coming into the feedline from a signal then you could figure out its its more then your radio can handle. In theory the antenna's are too close together but if it appears to be working and the radios are good units you may get away with it.

I dont really see how basic math from a set of #'s from a book will solve your issue. It wont provide real world #'s without meters plugged in to measure things.

As with all radios you should try and use the lowest power setting possible to transmit.  If its a problem you could put an antennuator inline but that will knock down your long distance signal some.


Todd