Discussion in 'General Technical Questions and Answers' started by K7DVS, Aug 9, 2020.
Would a repeater on 444.800 interfere with a repeater on 444.750?
Not usually, but it all depends on a user's distance from the two. For example if you were using the 444.750 and it was far away, or had a weak signal to you, then the 444.80 could cause problems if it was much stronger or closer. But the proximity of the frequencies should not normally be a problem.
Do you mean if they were co-located? They might. It depends on the isolation between their antennas and the kind of duplexers they use.
PLL controlled repeaters are not as clean as old school Crystal Oven controlled ones.
Finally , a professional.
Thanks Dereck, among my peers I'm known as a pre-fessional. heh heh
Yeah, before you can answer this you need to know the details, because it really depends on the setup. If the idea is to keep a transmitter out of a receiver, there are so many variables.
For example: Say you have a transmitter on one antenna and a receiver on another antenna 30 feet above. Say the TX is putting out -33 dBm of conducted noise on the receiver channel, I'm assuming it well exceeds the FCC 90.210 masks, that it's not some ham-market thing.
If the TX feeds a notch duplexer, like a lot of hams use unfortunately*, and the other repeater's receiver frequency isn't attenuated much, then that noise goes mostly unhindered to the receive antenna, losing maybe 40 dB along the way. It goes down the RX coax and might end up at the receiver at about -80 dBm, so you'd have pretty terrible noise-on-channel degradation.
But if the TX has a good pass duplexer or at least some pass filtering like a pair of Telewave 5" cavites, that could reduce that noise by 50 dB putting it down around -130 dBm where it'll have no effect.
So if you really want an answer, tell us what you have we can make an educated SWAG.
*It's actually OK to use a notch duplexer at a repeater site IF transmitter pass filtering is added between the isolator and the duplexer, and the receiver will probably need pass filtering too to keep other carriers from blocking the front end. It's technically OK, but somewhat half-assed.
One thing I would point out, is that if the repeaters are close in frequency, as the OP suggests, that the two duplexers will be working to suppress that noise at the other party's RX frequency.
As an example, lets say that both repeaters are using notch only duplexers, and use the frequencies in the OP. Lets assume that both duplexers have 85dB of notch at the TX frequency associated with their own repeater, and that the conducted noise is -33dBm. That means that the noise from each TX will be around -118dBm at its associated RX input. This is typically around the minimum sensitivity of a normal repeater, so desense would be either non-existent or very minimal. So each repeater would perform normally.
Now, if these two repeater antennas are spaced 30' apart vertically on a tower, we might expect to see over 70 dB of isolation between the two antennas at 440MHz. But the duplexers are only 50KHz apart, meaning that the alternate RX is pretty much in the existing notch, maybe 6-10dB less than it would be on the tuned frequency. Lets assume that the duplexer actually offers 75dB on the alternate frequency, this means that the level of signal getting to the other repeater RX would be -33-75-70=-178dBm, which is way less than any FM RX could possibly detect.
We actually have many sites where there are two repeaters separated by only a few tens of kHz, and they all work fine. We have two mountaintop sites where there are two 2M repeaters separated by 20KHz, and they work fine. The biggest issue becomes when someone gets close to the repeater site, and gets to where the IF of their mobile RX starts to pick up the unwanted repeater. This can happen somewhere around a mile or so from the repeater site with a decent mobile rig.
^^^ I think your numbers sound good and are probably closer than mine for the 50 KHz spacing he has. Also if he has a pass/reject duplexer with wider notches it would be even better, so I think you're absolutely right. I don't think I've ever seen close to 70 dB between antennas but I have only measured a few pairs and they probably had some offset or some other coupling.
Maybe a better question would be: In the northern part of Montana, somewhat close to Calgary- Alberta, Canada- is it permissable to install a 444.800 repeater, and how much power might be permitted? - A 'Eureka' moment, if ever there was one ( pun and reference to sci-fi series intended ). 444.750 positive offset has been listed since at least 2002 in Calgary, sponsor VE6KQ at that time.
The restrictions on US operation near the Canadian border are only in the 420-430 segment. That portion is LMR in Canada.