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UHF Antenna spacing on a repeater without a duplexor?

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#1
Good evening, I have a cheap setup for now. I know that the best option for my new system is a duplexor and one antenna, but for now, i have two antennas and 60ft of masts. This is for use on the GMRS band.

I can get the antennas horizontally spaced by about 35ft, and i have that right now. But of course I had an issue with the tx antenna killing the received signal. So i took a 5ft mast off the tx antenna and put it on the rx, making a 10ft difference vertically. This seems to be much better, but not amazing of corse.

So, what would be the best way to set this up with what I have? I hear it may be best to just mount the tx under the rx with about 5 to 10 ft of space between them (using a stand off so it would be like 6 inches to the side. Still I'd imagine 5ft vertical isn't much at all). So, I figured its best that I ask you guys because I have no idea what I'm doing and everyone here seems to be very kind and helpful.
 
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#3

SteveC0625

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#4
Let me point out that horizontal separation, in addition to vertical separation is most likely needed. It's all in the charts in the above referenced link.

But, once you start investing in high quality coax, connectors, antenna mount arms and such, you will instantaneously surpass the cost of a starter duplexer. And you would need two runs of coax, one from each radio to it's respective antenna.

While good quality coax is a must, I suspect that you'd get much better performance with a $150 duplexer and the antenna at 20 ft than you are now, even using the coax you currently have. The low antenna height means a shorter, single run of coax which will negate at least some of the loss due to your current setup. Depending on what coax you are currently using (you didn't say BTW), you might even see a drastic improvement in performance.

In a perfect world, you'd have a $2,000 repeater with a built-in duplexer, super high grade hard line, and a 7 db gain antenna on the top of that 60' mast. However, I'm wondering if the mast you have would handle the weight of the hard line and gain antenna.

For starters, I think you'd be much better served with the antenna much lower and a duplexer in place particularly if you want to keep it on the inexpensive side for now.
 
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#5
The needed vertical separation will depend on the antennas you use. Higher gain antennas will give a narrower vertical radiation pattern and that will result in slightly better isolation. But don't think that you will achieve anything close to what a duplexer will give. Two high-gain antennas, two runs of 1/2" hard line along with the needed connectors will exceed the cost of a high quality duplexer.
 
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#6
I'm working with equipment I already have. I understand a proper dual antenna setup is more expensive then a duplexor, and that is why I do plan to do that once i get some money. What i have for now is the best i can do with what i have, several 5ft masts, two high gain antennas (7db i think, cant remember off my head), two 100ft LMR400 cables (I will cut them shorter once i figure out exactly where I will have the antennas them selves), and a couple Motorola CDMs.

That link was helpful and I think I can figure this one out. Either way, i can put something up, give it a test, and learn. Thank you guys =)
 
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#7
Never going to work well, however assuming you are using vertical antennas they will likely have a null straight up and/or straight down. That said you are going to need 15 or more feet of vertical separation. The problem with the above is the two antennas will have different 'ranges' with the one on top covering a much larger area. If the receive antenna is on top you will have a 'mouse'-hearing much further than it talks and the other way you will have an 'alligator' that can be heard much farther than it can hear. Both are poor in practice.

You can add a little to the isolation by using a solid metal sheet at least a half wavelength square right below the top antenna.

The reason most installations use a single antenna is to make the transmit and receive 'patterns' nearly the same.
 
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#8
Never going to work well, however assuming you are using vertical antennas they will likely have a null straight up and/or straight down. That said you are going to need 15 or more feet of vertical separation. The problem with the above is the two antennas will have different 'ranges' with the one on top covering a much larger area. If the receive antenna is on top you will have a 'mouse'-hearing much further than it talks and the other way you will have an 'alligator' that can be heard much farther than it can hear. Both are poor in practice.

You can add a little to the isolation by using a solid metal sheet at least a half wavelength square right below the top antenna.

The reason most installations use a single antenna is to make the transmit and receive 'patterns' nearly the same.

You can improve your operation by the use of some cavities in both the RX and TX lines at the radios. The use of some pass and notch cavities will greatly improve the performance.

Your actually creating a poor mans duplexer in this fashion. But if you only use one cavity in each line, you should not get into problems with the cable lengths. If you put multiple cavities in each line, then the cable lengths will be critical.

Put the pass cavity on the TX side and the notch cavity on the RX side. Set the notch for the TX frequency on the receive input and the pass cavity on the TX input. Set the pass cavity for the TX frequency.

Hopefully your using some double shielded cable going to the antennas. Again hopefully your not using LMR type coax for the cables. Over time you will start to hear scratchies on the receiver when the repeater is transmitting. This is caused by moisture getting into the cable and causing oxidation to take place between the aluminum foil shield and the braided copper shield. It's a well documented issue that shows up.

Another comment I will pass along about the LMR coax cable in duplex service. You will have some people say that this problem will not occur if you weather seal up the connectors correctly. Well if this was the case, why are not the cellular companies saving money and using this type of cable on all their towers? They tried it and found that no matter how well the connectors get weather sealed, the problem develops. So enough said on this issue. How do I know this? I spent about 20 years engineering and building cellular tower sites for a number of cellular companies. They all use Heliax type cable or this Comscope aluminum, solid outside shielded cable. Takes a special prep tool to get the cable ready for the connector. Even Motorola is using it when they install Public Safety antennas these days.
 
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#9
Let me point out that horizontal separation, in addition to vertical separation is most likely needed. It's all in the charts in the above referenced link.

But, once you start investing in high quality coax, connectors, antenna mount arms and such, you will instantaneously surpass the cost of a starter duplexer. And you would need two runs of coax, one from each radio to it's respective antenna.

While good quality coax is a must, I suspect that you'd get much better performance with a $150 duplexer and the antenna at 20 ft than you are now, even using the coax you currently have. The low antenna height means a shorter, single run of coax which will negate at least some of the loss due to your current setup. Depending on what coax you are currently using (you didn't say BTW), you might even see a drastic improvement in performance.

In a perfect world, you'd have a $2,000 repeater with a built-in duplexer, super high grade hard line, and a 7 db gain antenna on the top of that 60' mast. However, I'm wondering if the mast you have would handle the weight of the hard line and gain antenna.

For starters, I think you'd be much better served with the antenna much lower and a duplexer in place particularly if you want to keep it on the inexpensive side for now.
Using even a mobile duplexer(ASSuming an at home repeater) in an environment that is not saturated with adjacent channel RF, using 1/2" hardline, will run circles around dual antennas with LMR400.

Never going to work well... The problem with the above is the two antennas will have different 'ranges' with the one on top covering a much larger area. If the receive antenna is on top you will have a 'mouse'-hearing much further than it talks and the other way you will have an 'alligator' that can be heard much farther than it can hear. Both are poor in practice.
If the OP already has HAAT advantage, vertical spaced antennas will have little difference in 'range'. Having done coverage analysis of a site, already at a HAAT advantage site, there was very little difference on predicted coverage with an antenna at 40m and and an antenna at 140m. The major difference, with 100m(328') height difference was shadows at 40m were filled in at 140m. Increase in footprint was less than 6mile radius.

What will be significantly different will be antenna pattern. If one antenna is above the top of the tower, it will have more of an omni pattern than an antenna side mounted. The side mounted antenna will become somewhat directional due to tower shielding.

The reason most installations use a single antenna is to make the transmit and receive 'patterns' nearly the same.
Actually, many systems use a single antenna because it is cheaper than buying two runs of feed line, and two antennas. It is also, at a low RF site, much easier to engineer a repeater to use one antenna than two.

You can improve your operation by the use of some cavities in both the RX and TX lines at the radios. The use of some pass and notch cavities will greatly improve the performance.

Your actually creating a poor mans duplexer in this fashion. But if you only use one cavity in each line, you should not get into problems with the cable lengths. If you put multiple cavities in each line, then the cable lengths will be critical.
At this point he may as well just use a duplexer. Also, critical length coax is required only between the pass/reject cans.

why are not the cellular companies saving money and using this type of cable on all their towers?... They all use Heliax type cable or this Comscope aluminum, solid outside shielded cable... Even Motorola is using it when they install Public Safety antennas these days.
They use hardline because of attenuation at 700/800/1700/1900/2100/2500MHz, that and it is a full duplex application. 100m of LMR hardline has extremely high losses at these frequencies.

Figure that a rural cell site is 100m tall, minimum loss(NOT including connectors) of LMR400 just for on the tower is:

700MHz 10.3dB
800MHz 11.0dB
1700MHz 18.0dB
2100MHz 20.2dB

1-5/8" hardline losses for the same run

700MHz 1.87dB
800MHz 2.01dB
1700MHz 3.07dB
2100MHz 3.53dB

It is simply an attenuation issue with regards to cellular.

The PIM issues with LMR400 are a completely different issue all together.
 
Joined
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65,126
Location
Virginia
#10
It wasn't mentioned, but how much transmitter power is being used?... in an order of simple solutions, antenna separation and power reduction are first considerations. Are we talking two separate units, a transmitter and receiver?.. Obviously separating them will also help. Maybe my comments are all too simplistic in light of the other's, above...........:)
However, there is a great chance for a learning experience here... as mentioned, installing pass and notch filters will definitely improve things... They don't have to be expensive- fabricating some cavities out of sheet copper, a handful of BNC connectors- and, of course, literature about cavities (I have used orange juice cans for 400Mhz cavities in a laboratory 'pinch')... it is certainly a do-able and cheap experiment, -- and while their insertion losses may be- who knows?... you will certainly learn a lot about duplex'rs, filters and isolation techniques...
Good luck guy.... :)
.
.......................................................CF
 
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