GR1225 duplexer tuning for two frequencies

baltimorecs · Feb 10, 2016

I have one GR-1225 repeater that we are licensing for Itinerant use. I usually have my duplexers tuned by a local company here in Baltimore. I would just like to see if it would be possible to have the duplexer tuned for both 469.5 and 469.55 (tune to 469.525?) or is they too far apart? I would like to do that so we could switch between the two if one has other itinerant traffic on it.

I only have one cable and one antenna so I cannot use two separate antennas for this repeater.

mmckenna · Feb 10, 2016

If it's one of the small "mobile" duplexers you could probably get away getting it tuned for the center. There may be some slight degradation in performance, but it would likely be slight.

kayn1n32008 · Feb 10, 2016

baltimorecs said:
I have one GR-1225 repeater that we are licensing for Itinerant use. I usually have my duplexers tuned by a local company here in Baltimore. I would just like to see if it would be possible to have the duplexer tuned for both 469.5 and 469.55 (tune to 469.525?) or is they too far apart? I would like to do that so we could switch between the two if one has other itinerant traffic on it.

I only have one cable and one antenna so I cannot use two separate antennas for this repeater.

Yes it is possible. It is not as simple as tuning it for 469.5250, instead the notch needs to be widened out to allow a signal to be notched out on either frequency.

In the early 2000's I built a pair of portable VHF repeaters, 13 channels, repeater transmit spread across 153.xxxxMHz with the receive 5.26MHz above. The bottom of the notch was around 225KHz wide, giving around 70dB isolation. Ideal? Nope, but the customer needed a couple of repeaters they could drop on a mountain top and select the clear channel and do ops. These were the same channels as a 50+ fixed repeater network. Each portable repeater had a community tone panel that allowed the repeater to output the tone received on the input.

Each repeater was putting 5w out of the duplexer. Made them from a pair of Icom mobiles, Zertron community tone panel, ICS power supply and a Sinclair mobile duplexer all mounted in a Pelican case.

Sent from my iPad using Tapatalk

ElroyJetson · Feb 10, 2016

A typical mobile duplexer will meet specs while allowing several adjacent channels to pass thru, no problem at all. This is because the cavities are tuned to REJECT specific narrow ranges of frequencies, but even then they're not THAT narrow.

I've had customers use one duplexer on a multichannel repeater in scan mode, with all the channels operating over a span of 150 KHz. Though it's really an odd setup they had, it worked for them.

They basically had repeater functionality on all five of their channels (only one channel at a time, though) and had one repeater doing the job. The limitation was that, of course, only one channel could be active at a time. Given that they did not use their radios that much they were happy to accept these limitations.

ramal121 · Feb 10, 2016

I have done this before on UHF with simple notch filters. Can't remember the exact numbers but something like 100 kHz spread was less than 3 dB difference from the bottom of the notch. Your channels are less than this spacing. Unless your duplexer isolation is running right on the ragged edge, you should be able to tune it to the center between both freqs and have it work just fine. Give it a shot.

mikewazowski · Feb 11, 2016

I just tuned one the other night for a move of 500khz and it was just a very minor tweak as the notch is very broad.

GR1225 duplexer tuning for two frequencies

baltimorecs

Member

mmckenna

I ♥ Ø

kayn1n32008

ØÆSØ

ElroyJetson

I AM NOT YOUR TECH SUPPPORT.

ramal121

Lots and lots of watts

mikewazowski

Forums Manager/Global DB Admin

Similar threads