Grain bin/silo repeaters

[SteveShannon]

2 hours ago, tweiss3 said:

It depends on a lot of things. A mobile duplexer, or the little duplexer that comes in the RT97, yes, you will need the isolation. The notch is pretty big and may not be specific enough to provide isolation between repeaters. On sites that have multiple same band repeaters, some have combiners, some have duplexers & combiners to get enough isolation. 

 

I’m just trying to understand, not argue, but I’m aware it could feel like I’m arguing.  I feel like I’m just not getting it.

If the duplexer on my RT97 prevents the integral transmitter (at 467 MHz) of the RT97 from affecting the integral receiver tuned to 462 MHz, why wouldn’t it prevent the transmitter from one RT97 (at 467 MHz) from affecting the receiver (tunes to 462 MHz) in another repeater, which is further away?

Or are you talking about the transmitted signals adding together to form a different signal? That I could see.

[VETCOMMS]

WRVD brought up something I was wondering about.  Thx for that.

I'm also curious about the different duplexers.  I've heard the term notch used.

Is there a thread about duplexers that already exists so we don't beat a dead horse about duplexers?

[Radioguy7268]

7 hours ago, Sshannon said:

I’m just trying to understand, not argue, but I’m aware it could feel like I’m arguing.  I feel like I’m just not getting it.

If the duplexer on my RT97 prevents the integral transmitter (at 467 MHz) of the RT97 from affecting the integral receiver tuned to 462 MHz, why wouldn’t it prevent the transmitter from one RT97 (at 467 MHz) from affecting the receiver (tunes to 462 MHz) in another repeater, which is further away?

Or are you talking about the transmitted signals adding together to form a different signal? That I could see.

You ask good questions.  Those tiny little flatpack duplexers are designed to "notch" out a single frequency pair (receiver side notches out the transmit frequency, and the transmit side notches out the receive freq).

What you have to get your head around is that when those notch duplexers are measured for isolation, they're only getting 55 or 60 dB of isolation. That means that some of the transmit power is always going to be leaking into the receiver - causing desense.  When you've got a low enough power setting, the residual amount leaking into the receiver is small, and doesn't cause much trouble - at least, not enough to make a difference when your low power repeater still covers a mile or two, even with just a little bit of desense.  The problems begin when you start trying to crank up the power, or even (gasp) toss on a BTech power amplifier to get 50 watts feeding into that flat notch duplexer. Now your tiny little bit of leakage is suddenly drowning out the receiver and causing you all sorts of trouble. Yeah, now you can transmit out to 7 or 8 miles, but you can't even talk back in with your portable until you are within a 1/2 mile.

You're also right on the idea of mixing for nearby transmitters. Close channel spacing creates a nightmare of intermod products. The basic formula for figuring out intermod is 2* A +/- B, A being the first frequency, B being the 2nd frequency.  Remember, if you've got a small flatpack notch duplexer, and you were tweaking and tuning it tight just to notch out one specific frequency pair and get 65 dB of isolation, and now you're finding out that you need to notch out an entire range of frequencies. That is is why serious duplexers use bandpass cavities, and cost thousands, not hundreds. They also boast isolation figures of 80 dB or better. Gaining 20dB of isolation is Huge with a capital H.

Probably better to head over to repeater-builder.com and read up on their duplexer section. They've got it all written up already. They'll even toss in the math if you want to geek out on it.

[SteveShannon]

12 minutes ago, Radioguy7268 said:

You ask good questions.  Those tiny little flatpack duplexers are designed to "notch" out a single frequency pair (receiver side notches out the transmit frequency, and the transmit side notches out the receive freq).

What you have to get your head around is that when those notch duplexers are measured for isolation, they're only getting 55 or 60 dB of isolation. That means that some of the transmit power is always going to be leaking into the receiver - causing desense.  When you've got a low enough power setting, the residual amount leaking into the receiver is small, and doesn't cause much trouble - at least, not enough to make a difference when your low power repeater still covers a mile or two, even with just a little bit of desense.  The problems begin when you start trying to crank up the power, or even (gasp) toss on a BTech power amplifier to get 50 watts feeding into that flat notch duplexer. Now your tiny little bit of leakage is suddenly drowning out the receiver and causing you all sorts of trouble. Yeah, now you can transmit out to 7 or 8 miles, but you can't even talk back in with your portable until you are within a 1/2 mile.

You're also right on the idea of mixing for nearby transmitters. Close channel spacing creates a nightmare of intermod products. The basic formula for figuring out intermod is 2* A +/- B, A being the first frequency, B being the 2nd frequency.  Remember, if you've got a small flatpack notch duplexer, and you were tweaking and tuning it tight just to notch out one specific frequency pair and get 65 dB of isolation, and now you're finding out that you need to notch out an entire range of frequencies. That is is why serious duplexers use bandpass cavities, and cost thousands, not hundreds. They also boast isolation figures of 80 dB or better. Gaining 20dB of isolation is Huge with a capital H.

Probably better to head over to repeater-builder.com and read up on their duplexer section. They've got it all written up already. They'll even toss in the math if you want to geek out on it.

Thank you! That helps.

I’ll take another look at Repeater-builder.com.  I think I’ll enjoy the math, but I signed up last year for the IEEE Antennas and Propagation Society and so far I haven’t really been able to get into it.  I don’t even understand some of the article titles, much less the math. Hopefully it’s a bit different than repeater-builder, which hasn’t lost me yet.