UHF Repeater Antenna Spacing Question

Cowley639 · May 25, 2016

Hello all,

I am looking to put up a repeater and antenna on top of a building that is centrally located in town (roof of a college basketball arena) at approximately 75 feet AGL. My current repeater site (on top of our building at approximately 25 feet AGL just isn't giving me the range we are looking for. I have received permission from the school to put my repeater antenna on the roof.

The school operates a UHF repeater for their maintenance/custodial staff that is located on the top of the building as well.

My question is; How far apart do the antennas need to be spaced to not interfere with each other? The school uses 464.3250/469.3250 with a 114.8PL, my repeater would be 464.5500/469.5500 with a 114.8PL (I would have no problem with changing PL tone on my system if that would help as well?).

Both repeaters have celwave mobile duplexers and are 50w continuous with approximately 30-35 watts to the antennas. The school is using a Hustler G-Series" G6-450-3 antenna. My repeater has a Comet CA-712EFC antenna.

There is a section on the southwest corner of the roof (it is a flat roof) where there is a raised section of brick (aprox 3' above the roof itself) that the school has mounted a conduit wall mounting bracket into the brick then attached a short piece of 1-1/2" pipe to that bracket, then the antenna is mounted to the pipe. The antenna is aprox 6-8" above the 3' tall raised brick side wall. In total there is approximately 10-12 feet of usable space for me to mount my antenna, which would be to the east (or to the right of the of the school's antenna if you are facing the north). The school's antenna is mounted on the southwest corner of the wall.

Any advise, help would be greatly appreciated!

Thanks!!

WA0CBW · May 25, 2016

Data from an old Decibel Antenna catalog for horizontal spacing for no gain dipole antennas indicates about 40db isolation for 30 feet of spacing. If using gain antenna then the sum of the antenna gains should be subtracted from the isolation value. 100 feet of spacing will give you about 55db of isolation.
BB

Cowley639 · May 25, 2016

The school's repeater antenna is 6dB, My repeater antenna is 9dB.

Cowley639 · May 25, 2016

I guess I am confused. The city police repeater and fire/ems repeater are on top of the same water tower they are both UHF, and are both on 453.xxxx/458.xxxx at the same power output, and are separated (horizontally) by at the very most 6 feet.

Your telling me I need 100 feet of spacing?

Im confused...

WA0CBW · May 25, 2016

Since the TX of one repeater is about 5Mhz away from the RX of the other repeater you can determine how well it will work by simply removing the duplexer from your repeater and see what kind of desense you get with your repeater. I doubt that it will be usable.
BB

Cowley639 · May 25, 2016

Wouldn't the TX of the two repeaters be 464.325 and 464.550? The receive would be 469.325 and 469.550? Your making it sound like the TX of one repeater is 464 and the RX of the other is 469. Wouldnt both repeaters be TXing on 469 and RXing on 464? 550-325=225kHz of spacing between both RX freqs and TX freqs.

How is the city making it work on the water tower then? Also how do all shared repeater sites make it work? I keep seeing all these sites with several different repeaters on them.

Do I need to pick a different frequency? I filed for a business Itinerant license. I applied for 464/469.500, 464/469/550, 453/458.8000, 453/458.8125. The only reason why I picked the 464/469 frequency is because my antenna's center frequency is tuned for 465.000.

ecps92 · May 26, 2016

453.8000 and 453.8125 are NOT Business Channels. those are Public Safety allocations (DPW, PD, FD, etc)

Cowley639 said:
Wouldn't the TX of the two repeaters be 464.325 and 464.550? The receive would be 469.325 and 469.550? Your making it sound like the TX of one repeater is 464 and the RX of the other is 469. Wouldnt both repeaters be TXing on 469 and RXing on 464? 550-325=225kHz of spacing between both RX freqs and TX freqs.

How is the city making it work on the water tower then? Also how do all shared repeater sites make it work? I keep seeing all these sites with several different repeaters on them.

Do I need to pick a different frequency? I filed for a business Itinerant license. I applied for 464/469.500, 464/469/550, 453/458.8000, 453/458.8125. The only reason why I picked the 464/469 frequency is because my antenna's center frequency is tuned for 465.000.

WA0CBW · May 26, 2016

Most mobile duplexers are reject only. That means they reject one frequency and pass ALL others. They offer little or no rejection to adjacent frequencies. They normally wouldn't be used in multi transmitter locations. The Public Safety repeaters you mention are probably band-pass/band-reject or maybe combiner/multi-coupler designs. More importantly may be that moving the location of a repeater is considered a major change to your FCC license and requires re-coordination of your frequencies.
BB

kayn1n32008 · May 26, 2016

Cowley639 said:
I guess I am confused. The city police repeater and fire/ems repeater are on top of the same water tower they are both UHF, and are both on 453.xxxx/458.xxxx at the same power output, and are separated (horizontally) by at the very most 6 feet.

Your telling me I need 100 feet of spacing?

Im confused...

Those repeaters very likely have significantly better filtering(proper pass/reject duplexers. Possibly something like: http://www.sinclairtechnologies.com/catalog/product.aspx?id=125 ) They also could be using separate receive and transmit antennas, and have the receivers multi-coupled to one antenna, and the transmitters combined to the other.

Because you and the other repeater are both using mobile, reject only, duplexers it is likely you will need proper pass/reject duplexers to happily live on the roof of the school, or at a minimum some additional band pass filters, added to each repeater, to notch out each other's transmit frequencies.

kayn1n32008 · May 26, 2016

Cowley639 said:
How is the city making it work on the water tower then? Also how do all shared repeater sites make it work? I keep seeing all these sites with several different repeaters on them.

Usually where there are multiple repeaters, in the same dand, on a single site, there use community receive/transmit antennas.

One antenna is used for transmitters, the other is used for receivers.

The repeater transmitters are all combined and the RF is sent to the transmit antenna, and all the receivers are multi-coupled to the receive antenna.

prcguy · May 26, 2016

Many years ago I had a fleet of mobile repeaters operating on consecutive frequencies with notch only duplexers and have successfully run several at the same site.

A mobile UHF notch duplexer has about 100KHz of usable BW depending on how its tuned and will still offer a good amount of rejection 200KHz away from center. Since the both repeater transmitters are within 225KHz of each other and both receivers are the same apart, the duplexer rx side in one repeater will offer a good amount of rejection to the other repeaters transmitter and its tx side will notch out a good amount of transmitter noise that would land on the other repeaters receive frequency. In other words it will probably work ok if the antennas are not right on top of each other.

The real problem will be if there are no isolators on the repeaters and when both repeaters key up you will create lots of intermod that could land on the repeater input frequencies. My mobile repeaters all had isolators
prcguy

Cowley639 said:
Wouldn't the TX of the two repeaters be 464.325 and 464.550? The receive would be 469.325 and 469.550? Your making it sound like the TX of one repeater is 464 and the RX of the other is 469. Wouldnt both repeaters be TXing on 469 and RXing on 464? 550-325=225kHz of spacing between both RX freqs and TX freqs.

How is the city making it work on the water tower then? Also how do all shared repeater sites make it work? I keep seeing all these sites with several different repeaters on them.

Do I need to pick a different frequency? I filed for a business Itinerant license. I applied for 464/469.500, 464/469/550, 453/458.8000, 453/458.8125. The only reason why I picked the 464/469 frequency is because my antenna's center frequency is tuned for 465.000.

nd5y · May 26, 2016

The FCC would probably not be happy with somebody obtaining a license for temporary itinerant repeaters and installing them at a permanent location like a water tower or building.

Cowley639 · May 26, 2016

ecps92 said:
453.8000 and 453.8125 are NOT Business Channels. those are Public Safety allocations (DPW, PD, FD, etc)

My apologies, I meant 451.8000 and 451.8125

Cowley639 · May 26, 2016

I was able to come across a DB4076 bandpass reject duplexer here locally that is for sale. Would that help me out any as compared to the mobile Celwave duplexer i currently have?

I just returned from the gym (that I am wanting to put my repeater on) I took a look at the School's repeater and duplexer. It is a Vertex VXR-9000 with a celwave mobile duplexer, the antenna is fed with LDF4-50A 1/2" heliax. The facility guy that let me look at the repeater said the school's repeater gets rarely, rarely used (mostly at night when the custodial staff is cleaning), and he said most of the staff doesn't even carry radios anymore, they just use the phone system. We would be using my repeater primarily durning the daytime hours, occasionally at night but not near as much.

So again, I am taking from your comments that a better quality duplexer would help me? Would it work using the 464/469.5500 repeater pair with their 464/469.3250 pair 225kHz away? If so, back to my original question; how far do the two separate repeater antennas need to be spaced apart?

radioman2001 · May 26, 2016

I think a little background is in order, yes you can actually put 2 TX/RX antennas as close as 10 ft horizontal BUT with a good base station pass/reject duplexer (85-110db isolation). A mobile duplexer is reject only so you are going to get RF down to you RX'r no matter how far away you are especially 225kc. Gain antenna's flatten out the dougnut of signal so now you are actually putting even more RF into you antenna. With gain antenna's you can go verticle separation and actually get more isolation. Roughly 10 ft verticle is equal to 100 ft horizontal. So a good base duplexer like you are getting and 10 ft verticle separation (a 6db anternna is roughly 6ft tall)should easily get you the isolation you need for no desense. For starters try 10ft verticle and 30 ft horizontal otherwise 60 ft horizontal only.

Quote"The FCC would probably not be happy with somebody obtaining a license for temporary itinerant repeaters and installing them at a permanent location like a water tower or building."

464.550 is an itinerant (except in Detroit area) which means you can operate up to a year in a fixed location. Done it many times and when the inspector came around we moved it then 1 year later moved it back.
As an alternative apply for the 90/90 channels.

Cowley639 · May 26, 2016

radioman2001 said:
I think a little background is in order, yes you can actually put 2 TX/RX antennas as close as 10 ft horizontal BUT with a good base station pass/reject duplexer (85-110db isolation). A mobile duplexer is reject only so you are going to get RF down to you RX'r no matter how far away you are especially 225kc. Gain antenna's flatten out the dougnut of signal so now you are actually putting even more RF into you antenna. With gain antenna's you can go verticle separation and actually get more isolation. Roughly 10 ft verticle is equal to 100 ft horizontal. So a good base duplexer like you are getting and 10 ft verticle separation (a 6db anternna is roughly 6ft tall)should easily get you the isolation you need for no desense. For starters try 10ft verticle and 30 ft horizontal otherwise 60 ft horizontal only.

Would using the 451/456.8000 repeater pair help any coupled with a DB4076 bandpass reject duplexer? Or would I still need 10ft of vertical and 30ft of horizontal?

Cowley639 · May 26, 2016

Also for what's its worth;

My repeater is currently set-up on a 25ft mast at our building on GMRS 462/467.6000 (while I wait for the IG license to be approved) My city PD repeater is on 460.2 and is aprox 50 feet away from my my repeater (across the alley) on a 100 foot tower at 100 watts. So we have 50ft of horizontal and 75ft of vertical separation with no problems at all (using a mobile duplexer). The building I am wanting to put the repeater on is aprox 1000 line of sight from my office (where the repeater is currently located).

freddaniel · May 27, 2016

The 464.325 and 464.550 repeaters will probably work OK nearby using the mobile duplexers, as the reject frequencies are within 225 KHz and most likely will not cause desense. Better pass/notch duplexers will work better, should desense on either repeater be discovered. Be sure to use Heliax feedline and not LMR400. Use RG223 or RG400 jumpers with Silver Plated connectors.

The real problem is with the repeater antennas at 5 to 20 ft apart, both repeaters will feed transmitter power into the other, when they both transmit at the same time. This will cause intermod on a number of frequencies, which may affect the local repeaters as well as any others within several miles. A simple fix is to place a dual junction isolator on both transmitters before the duplexers.

Cowley639 · May 27, 2016

freddaniel said:
The 464.325 and 464.550 repeaters will probably work OK nearby using the mobile duplexers, as the reject frequencies are within 225 KHz and most likely will not cause desense. Better pass/notch duplexers will work better, should desense on either repeater be discovered. Be sure to use Heliax feedline and not LMR400. Use RG223 or RG400 jumpers with Silver Plated connectors.

The real problem is with the repeater antennas at 5 to 20 ft apart, both repeaters will feed transmitter power into the other, when they both transmit at the same time. This will cause intermod on a number of frequencies, which may affect the local repeaters as well as any others within several miles. A simple fix is to place a dual junction isolator on both transmitters before the duplexers.

The school repeater (that is currently installed) on the roof uses 1/2" heliax to the antenna and RG142 jumpers between the repeater and celwave mobile duplexer.

My repeater will also use 1/2" heliax to the antenna and RG214 jumpers between the repeater and the DB4076 duplexer.

I will be getting back on the roof in the next few days to measure the maximum amount of spacing I will be able to get between the two antennas.

Can you suggest where to get a Dual-Junction isolator? Price? Availability?

Thanks again.

freddaniel · May 27, 2016

If you search on Ebay for "dual circulator isolator" most of the listings come up. Their are 25 available of different makes and different 1st loads, and the prices range from $125 to $265. I believe most come with a money back warranty. All of these are 450 to 470 MHz models. These generally last forever, unless hit by lightning, and hold their resale value. These UHF models are not always available on ebay.

Relating to the distance between antennas. If UHF antennas are closer than 2 wavelengths [4 feet] apart horizontal, the radiation patterns on both antennas will be distorted to the point coverage will be affected. When space is available, more the 5 feet is best.

UHF Repeater Antenna Spacing Question

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