Best Ham repeater receiver and 12KHz duplexer

Post by **HumHead** » Wed Jul 09, 2003 9:14 am

Two questions:

1) It was asked before, not never really answered: Any opinions on the best receiver for a 900MHz ham repeater?

2) If you try to use the ARRL bandplan, where the heck do you find a duplexer that can support the 12MHz spacing? I've looked casually, and haven't seen anything under 25MHz. Could I just say the heck with the duplexer and vertically stack the antennas? If so, how much seperation would I need at 900?

Thanks!!!

Post by **xmo** » Wed Jul 09, 2003 10:26 am

MSF5000 has an outstanding receiver. I have two of them up on 900. Here is one waiting for you on ebay:

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... egory=1502

Maxtrac is a suitable, compact, easy to modify unit that will also get the job done for you.

Duplexer availability is one of the reasons the 25 MHz split is taking over nationwide. You could just put up a 25 Meg split machine uncoordinated. After all, the coordination process is voluntary and the only purpose is to resolve interference possibilities. The FCC says if two repeaters interfere and one is coordinated, the uncoordinated system must resolve the issue. Now, if your coordinator won't coordinate any 25 Meg split systems, how is there going to be a coordinated system for you to interfere with???? Unless he changes his mind and puts someone else there out of spite, so I would notify him (certified) of your intended operation so you have proof of being there first.

If you don't choose to go that route, most of the easy radio conversions like MTSX series portable & Spectra just won't work for you. How are you going to get growth and activity if every radio conversion takes new filters and has to be done by a rocket scientist?

If you do choose to go with a 12 M split, duplexers are available. The Sinclair MR456 series is available as close as 9 MHz, The Q4220E series as close as 3.6 MHz. There are models from most manufacturers. Here's one from Celwave:

http://www.celwave.com/RFSGlobal/datash ... =Duplexers

John G · Post by **John G** » Wed Jul 09, 2003 1:13 pm

It doesn't take much vertical seperation at 900 MHz to give quite a bit of isolation. See http://www.decibelproducts.com/engineer ... lation.htm for more information. I have a 900 duplexer but I don't think I will use it. I plan to use vertical seperation. I also want to try diversity receive since it seems to work so well for cellular. Has anyone tried it yet on 900? I know it takes two receivers, antennas, feedlines and some kind of diversity switch but I think that the results would be worth it. I have an old cell site transponder I plan to experiment with. Any other ideas about diversity?

Post by **HumHead** » Thu Jul 10, 2003 12:31 pm

Thanks for the input guys!

If I'm looking for around 70dB of isolation, it looks like I'll still need around 20 ft of vertical seperation, so it looks like a duplexer it is.

As far as radios, I've been looking at either the GTX or Maxtrac. Both seem to be readily available at decent prices, and the GTX looks like it doesn't need much, if any, hardware modification. The ability to do simplex looks like a plus too.

I have to admit to thinking about going 25KHz uncoordinated. It's not like the band is exactly swamped

, but the ARRL bandplan doesn't leave much room for 25MHz splits without playing in the weak signal portions of the plan (Heck, the whole band is only 26MHz wide.). What ranges / pairs are people who are running 25MHz spacing typically using?

Will the GTX or Maxtrac support the 12KHz split "out of the box" or would that mean filter or other major changes?

Thanks again!

Post by **xmo** » Thu Jul 10, 2003 1:58 pm

Look at this thread: http://batboard.batlabs.com/viewtopic.php?t=1069

There you will see that the GTX will do the ham band with RSS mods and that the receive filters roll off toward 920. This being the case, you may not be happy with the GTX on an ARRL split.

The Maxtrac will go anywhere in the ham band but you must do hardware mods. The VCO must be opened and 'adjusted' and the front end filters must be replaced. These tasks are not technically complex but they will challenge your soldering skills.

SERA, the largest coordinating body in the US has gone to 25 MHz split. The SCCRBA bandplan has been 25 MHz for some time. You can look these up on the web.