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42db isolation between 2m repeater antennas, is that enough?

MrClean

MrClean

New Member
Mar 11, 2016
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According to the commscope isolation calc at 146mhz using a 1/2 wave set of antennas (that's there baseline calc) at 65' on horizontal plane I should be at approx 42db isolation.

What could I see for isolation db if I run just plane 1/4 wave (aka unity gain) antennas?

My proposed setup is a simple 10w 146mhz repeater with 2 separate antennas and 2 radios on opposite ends of my house. I want to do a 600 kHz spread though my local said I could do 1 MHz also if needed thought it would be odd.

1 antenna can be lower than other as well if that would help.I know I will get some desense, just concerned about killing front ends and such.

With 10w testing I get almost 20 miles range that I test driven.

I can't go vertical at my location per lease.

Help appreciated.
 

MrClean said:
According to the commscope isolation calc at 146mhz using a 1/2 wave set of antennas (that's there baseline calc) at 65' on horizontal plane I should be at approx 42db isolation.

What could I see for isolation db if I run just plane 1/4 wave (aka unity gain) antennas?

My proposed setup is a simple 10w 146mhz repeater with 2 separate antennas and 2 radios on opposite ends of my house. I want to do a 600 kHz spread though my local said I could do 1 MHz also if needed thought it would be odd.

Help appreciated.
Click to expand...

Not even close to being enough, at best your repeater will be able to hear nothing weaker than S9+60 signals when its transmitting.

Basically what you need to do is work out how much isolation you need to begin with and to do that you need to know the the antenna gain and all losses between the PA and the antenna. If you're running 100ft or so of RG213, 10W TX and the antenna you're planning you'll have an ERP of roughly the 10W.

You need to work out the RX efficiency as well which includes the antenna gain, feeder losses and any insertion losses from things like cavities. So if you're again running the same antenna at the other end with the same length of RG213 its going to be roughly 0dB gain.

You then end up with a formula which is

ERP (in dBm) + TX gain + RX gain.

So in our case 10W is +40dBm, we've got 0dB of transmit gain and 0dB of RX gain so:

40+0+0=40dBm.

If we'd got 3dB of gain in the TX antenna system and 3dB of gain in the RX antenna system we'd have 40+3+3=46dBm.

Next thing you need to decide on is what the weakest signal is you want to receive.

So lets assume 10W transmitter power, no gain from RX or TX antenna system and you want to receive S3.

10W is +40dBm.
S3 is -109dBm

You therefore need 149dB of separation.

Now you get 42dB of isolation from antenna separation so you need to find 107dB of isolation from somewhere. Those duplexers you are looking at aren't going to come anywhere close on their own at 600kHz spacing. You'll be lucky to see 30dB.

If you want a simple cheap and effective solution, cross band repeating is the one to do and you don't need any cavities or duplexers. You have one end TX on VHF and RX on UHF and vice versa at the other end. You wouldn't need to separate antennas, you could just use a dualbander on each end.
 
M0GVZ said:
Not even close to being enough, at best your repeater will be able to hear nothing weaker than S9+60 signals when its transmitting.

Basically what you need to do is work out how much isolation you need to begin with and to do that you need to know the the antenna gain and all losses between the PA and the antenna. If you're running 100ft or so of RG213, 10W TX and the antenna you're planning you'll have an ERP of roughly the 10W.

You need to work out the RX efficiency as well which includes the antenna gain, feeder losses and any insertion losses from things like cavities. So if you're again running the same antenna at the other end with the same length of RG213 its going to be roughly 0dB gain.

You then end up with a formula which is

ERP (in dBm) + TX gain + RX gain.

So in our case 10W is +40dBm, we've got 0dB of transmit gain and 0dB of RX gain so:

40+0+0=40dBm.

If we'd got 3dB of gain in the TX antenna system and 3dB of gain in the RX antenna system we'd have 40+3+3=46dBm.

Next thing you need to decide on is what the weakest signal is you want to receive.

So lets assume 10W transmitter power, no gain from RX or TX antenna system and you want to receive S3.

10W is +40dBm.
S3 is -109dBm

You therefore need 149dB of separation.

Now you get 42dB of isolation from antenna separation so you need to find 107dB of isolation from somewhere. Those duplexers you are looking at aren't going to come anywhere close on their own at 600kHz spacing. You'll be lucky to see 30dB.

If you want a simple cheap and effective solution, cross band repeating is the one to do and you don't need any cavities or duplexers. You have one end TX on VHF and RX on UHF and vice versa at the other end. You wouldn't need to separate antennas, you could just use a dualbander on each end.
Click to expand...

How about 1 of these single 'cavity' for RX and 1 for TX of these single 8" cavities??

http://rover.ebay.com/rover/1/711-5...0001&campid=5336136228&icep_item=301909457655

For use on 90-150mhz, with N connectors..

s-l1600.jpg


Seller (who is a HAM) says it should add 25db isolation to the RX side, and using a 2nd one on the TX would help also..

He is willing to give me a pkg price for both..

I planned on using my LDF4-50A 1/2" Heliax for the TX, and LMR400 for the RX side, I have RG213 for the 'jumper' from radio to whatever 'filter' I may use...

Keep in mind each radio will have its own power supply and antenna, on separate ends of the house. The coaxial run for antennas won't be long.
 
MrClean said:
How about 1 of these single 'cavity' for RX and 1 for TX of these single 8" cavities??

http://rover.ebay.com/rover/1/711-5...0001&campid=5336136228&icep_item=301909457655

For use on 90-150mhz, with N connectors..

s-l1600.jpg


Seller (who is a HAM) says it should add 25db isolation to the RX side, and using a 2nd one on the TX would help also..

He is willing to give me a pkg price for both..

I planned on using my LDF4-50A 1/2" Heliax for the TX, and LMR400 for the RX side, I have RG213 for the 'jumper' from radio to whatever 'filter' I may use...

Keep in mind each radio will have its own power supply and antenna, on separate ends of the house. The coaxial run for antennas won't be long.
Click to expand...


I didn't run Conner's numbers but they sound about right so if you need an additional 107 dB of isolation why are you thinking about using something that offers only 25 dB? I have NEVER seen a single cavity filter offer anything remotely near what is required for isolation. I had a commercial repeater that had eight such cavity filters to achieve the required isolation however it did use the same antenna for RX as it did TX. Repeater building is not for the newcomer and there is more to it than most people think. IIRC there is/was a book called The Repeater Builders Handbook that had a wealth of info about how to design and build a repeater. In any event you are going to need several cavity filters on the receiver AND transmitter to ensure that therer is only one desired TX frequency making it's way to the antenna and that the receiver has enough rejection of that frequency so as to not de-sense during TX.
 
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Slightly off topic but am I correct in assuming that people running repeaters are doing so 100% free of anything that resembles compensation? If so it seems like they're taking on a lot in the name of a hobby, I hope the hams that use these repeaters treat them well.
 
I read alot of items online and at the RPT Builder website..

I had an active GMRS repeater in W. Texas though the 5mhz spread was easier to deal with a cheapy duplexer and it was FLAT out there (we were getting 60 miles).. Here in the mountains VHF is better and the repeaters near me are 30 miles away at the minimum on 2m (which prompted me to get my ham 'ticket' recently.. I don't like relying on others for my comms should SHTF.. Since I was able to locate a non-used 'pair' up here I figured I would give it a 'go'.

Working for $9/hour don't lend itself to buying a $700-$1k cavity set at this time, hence my various ways of asking similar questions..

FWIW, I do have a new dual-band KW TM-V71A for the house and KW 705 using in the mobile currently (which is front panel programmable which is nice).. From the house I can bang a repeater using 50w 60 miles up in the 'Catskils' of NY.. and the ones 30 miles away easy.. The truck gets to 2 of the repeaters 30 miles at 25w..

I also figured though I would give the 'local' hams near this mountain some 'option' for a local repeater.. but apparently can't without some cash flow..

The only option would be to sell the GMRS setup (dual GM300's) as separate radios and try to cover some of the VHF cavities costs I guess. I'll end up with a spare Comet CA-712EFC UHF antenna as well..

My VHF setup was going to be a pair of Mot GM300 set at 10w TX with the ID'omatic IV running the CW every 10 and such... heck I already got 1 fresh GM300 just in an programmed up (since I learned how program it myself)..
 

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