Odd repeater splits or inputs?

[DarkHelmet]

Recently I've come to have at least two other co-channel users on our input frequency of 467.5750.  These are not FRS users, but rather maritime users of ship based repeaters while parked at the Port of Tampa.  This is just under a mile from our site in downtown Tampa at 500' up and clobbers anyone coming in on it.

 

As I can't readily change the output frequency to anything other than 462.5750, I setup the receiver to scan between 467.5750 and 467.7250.  This way the receiver will treat the 725 input as a priority (it's still not scanning when 575 is busy of course, it's only one receiver).  This way if we're seeing interference or noise, we can just go to the alternate input in the radio and avoid the interference until the ship leaves port. 

 

457.525 MHz, 457.550 MHz, 457.575 MHz, 467.525 MHz, 467.550 MHz and 467.575 MHz are permitted frequencies per US287 footnote in https://transition.fcc.gov/oet/spectrum/table/fcctable.pdf so it looks like they are 100% legal :-)

 

Has anyone else experienced this sort of interference before, and/or is anyone running an odd split or alternate input frequency repeater?

[wayoverthere]

Not personally, but there have been similar issues on the west coast...Here are a couple threads:

 

https://forums.mygmrs.com/topic/1984-seattleport-angeles-repeater-owners-ix-from-nxdn-idas-equipped-yacht/?fromsearch=1

 

https://forums.mygmrs.com/topic/1971-san-diegola-600-repeater-owners-ix-from-nxdn-idas-equipped-yacht/?fromsearch=1

[mbrun]

There appears to be an accidental typo in the GMRS frequencies listed. All GMRS frequencies begin with 462 and 467, none are 457.

 

The issue of odd splits has been discussed. While some believe that there is nothing that prevents use of a odd split, so long as only the main frequencies are used, it most certainly is unconventional. The use of unconventional pair of frequencies, does have the side effect of limiting use of a conventional pair by another would-be repeater user. I don’t recall hearing anyone confirm that they are actually doing this. I do recall one reason folks consider during this is to make it more difficult for those that seek to cause intentional interference on the radio. It makes it more difficult for them get their kicks (but does not prevent it).

 

Is the off-shore traffic sufficiently frequent that even with different PL codes, communications cannot reasonably occur through the repeater? If the interference is frequent and disruptive enough I would consider shifting frequencies.

 

Just some thoughts.

 

 

Michael

WRHS965

KE8PLM

[DarkHelmet]

There appears to be an accidental typo in the GMRS frequencies listed. All GMRS frequencies begin with 462 and 467, none are 457.

The frequencies I listed are the permitted maritime frequencies, they are paired 10 MHz split. I listed them as they may be high out, low in and you may find the users on their 457 MHz pairs too.

 

Is the off-shore traffic sufficiently frequent that even with different PL codes,

communications cannot reasonably occur through the repeater? If the interference is frequent

and disruptive enough I would consider shifting frequencies.

When you have a strong signal on the input (multiple ships in this case) PL only keeps it from keying up the repeater.

It will still affect the users transmitting through the repeater as the repeater receiver will capture on the stronger signal.

Digital modes will suffer just as analog too.

 

 

.

[WRAK968]

You may have no choice but to shift frequencies, THOUGH, a good duplexer should be blocking out anything from 457 on the receive side of the repeater, in fact it should be blocking anything below 467.0000.

 

The big concern with using offset frequencies is that in the end you end up blocking 2 repeater pairs. This may not be a big deal if there are no other repeaters in the area, however if you do have other repeaters, it can cause more interference issues. Another thing to point out is that a lot of pre-programmed 95E radios will not allow you to do an offset like this, meaning there will be radios which would not be able to reach your repeater.

In my opinion, you may be best checking/upgrading your duplexer to block lower frequencies, and perhaps moving the repeater away from the harbor if needed.

[DarkHelmet]

You may have no choice but to shift frequencies, THOUGH, a good duplexer should

be blocking out anything from 457 on the receive side of the repeater, in fact it

should be blocking anything below 467.0000.

A duplexer isn't going to reject something that far off frequency generally.

It's a moot point here as we're not using a duplexer in this system.

 

In this case it's not the output causing issues on 457, but rather the users on their radios at the port causing

interference to the 467.575 input. A 5 watt radio a mile away is enough to cause severe interference to a mobile user

5 miles away.

 

The big concern with using offset frequencies is that in the end

you end up blocking 2 repeater pairs. This may not be a big deal if there are no

other repeaters in the area, however if you do have other repeaters, it can cause more

interference issues. Another thing to point out is that a lot of pre-programmed 95E radios will

not allow you to do an offset like this, meaning there will be radios which would not be able to

reach your repeater.

 

 

In my opinion, you may be best checking/upgrading your duplexer to block lower frequencies,

and perhaps moving the repeater away from the harbor if needed.

 

 

Well that capability is a concern, but I don't anyone using a purpose built GMRS radio. I've not seen one IRL, and the majority of

the lower end radios are the baofeng uv5r and others which can do any split. Some times they even will occupy multiple

frequencies at the same time

 

As it's just another input frequency in use, there's the likelihood of how many simultaneous GMRS conversations are going in the same area at the same time.  In Tampa, it's really not that many. 

 

A duplexer isn't a bandpass filter, and there's not much we can do about on frequency interference.  Changing the frequency would involve ordering new interconnect cables and retuning the cavity/isolator for this channel.  I'd also have to do a new IMD study for the stack (8 different channels here).  I think this is the easiest path.  On the plus side, it might encourage people to get better radios that can program odd splits.

 

[K5DVT]

On 4/28/2021 at 3:55 PM, DarkHelmet said:

 

A duplexer isn't going to reject something that far off frequency generally.

It's a moot point here as we're not using a duplexer in this system.

 

 

A duplexer actually will do this. A proper BpBr will anyway. It sounds like you are using a scanner or some sort of mobile radio as a receiver. That would be your problem. You have just about little to no receiver protection.

[DarkHelmet]

On 1/26/2022 at 1:31 PM, K5DVT said:

A duplexer actually will do this. A proper BpBr will anyway. It sounds like you are using a scanner or some sort of mobile radio as a receiver. That would be your problem. You have just about little to no receiver protection.

Not only do you necrobump the thread, but you do it by spewing this horseshit. 

A BpBr filter is nothing but a notch cavity with a anti-resonance reactance across it to improve close in pass.  This will give a "band pass" with notches high and low from it in most designs.  As you get further away from the pass frequency, past the notches, you'll see the pass band loss decrease.  At 10-20 MHz away the band pass the match will be poor and the notch will be almost non-existent; 6 dB at best. 

This repeater doesn't have a duplexer on it, it's fed from a window filter on receive and transmit goes into a combiner which is a number of true bandpass filters all feeding the same antenna. 

Here's what the transmitter sees https://gallery.keekles.org/d/31221-1/chan+04+to+antenna.gif

You can see the RX filters here http://gallery.keekles.org/d/31182-1/dual+filters.gif

And with the RF pre-amp, this is an old (circa 1990) bipolar pre-amp with poor input match (-10 dB).  This shows how important input match is.  http://gallery.keekles.org/d/31185-1/Filters+with+pre-amp.gif

The TX and RX antenna have over 45 dB of isolation between them.  This was measured as part of the system commissioning.

This issue is one of _ON_FREQUENCY_ interference.  There is nothing that can be filtered.  https://flscg.org/2021/05/tampa-gmrs-interference/

 

[Bugkiller]

For the OP

Took a minute and went to your website.  This seems to be an as yet unresolved problem (saw the gnashing of teeth comment)   As someone who goes through the Tampa area occasionally, I’m watching this closely,.  One thing I noticed is that the information here (mygmrs) has not been updated in about a year, is it still current?  

Though this is a bit of a zombie thread, it’s still good to know, as this is ongoing.