Batboard

How can one use these frequencies in the same building in two repeaters?(MTR2000s)

4xx.3000
4xx.2875

The standard TX combining equipment requires 50 MHz channel seperation.

Can the MTR do Narrow Band?? I'm guessing the 2875 channel is supposed to be a narrow band channel. What about the 3000?

The MTR2000 will do 12.5 KHz. However both of these will be used at 20 KHz.

motisking wrote:How can one use these frequencies in the same building in two repeaters?(MTR2000s)

4xx.3000
4xx.2875

The standard TX combining equipment requires 50 MHz channel seperation.

2 ways, 1 kind of simple , the other real expensive. I'm assuming that the 2 channels you refer to are indeed 12.5 kHz apart from each other TX and RX.

First way... Order the repeaters WITH the duplexer option and the external preselector option. Put up 2 antennas (high quality DB / Celwave/ Sinclair) using nothing but Hardline (size appropriate for the length of the run..) on the SAME support (tower, mast, side of building) getting as much vertical spacing as possible and placing the antennas directly in line with each other. I've been successful running 85 watts on each station with as little as 10 feet tip to end spacing with the same 12.5 kHz spacing you have. The more vertical spacing, the better. The idea here is the duplexers (high power Celwave/DB products/TX/RX Systems, etc..) are pass / reject type. Since your channel spacing is so close, you will benefit from this due to the adjacent channel on each repeater falling within the notch frequencies of the other duplexer. The preselector adds another 20-30 dB of reject of the transmit frequencies to the receiver.

Second way, TX combiners, hybrid will do 12.5 KHz spacing, loss is about 6-7 dB per channel (100 watts in = 15-25 watts out) and a receive multicoupler with a pass/notch filter used as a preselector. With careful engineering, you could combine the 2 TX freqs and the RX onto a single antenna using a duplexer. This way gets tricky and could cause unwanted noise in your RX. Good bit more expensive if you don't have combining parts and filters laying around.

I hope others post their suggestions, there is a great resource of techs on the board that have many more years of experience doing this than I do....IMHO!!!!

Dave

I would reccomend this being the spacing is very close:

2 antennas on the tower. One will be a transmit of one repeater and the other will be for the receivers of both repeaters and the transmit of the other one. It should be configured like this:

Antenna 1 - 4xx.300 TX

Antenna 2 - 4xx.2875 TX and 4xx.300 & 4xx.2875 receivers thru a splitter & duplexer

With this configuration only 1 duplexer is required and the only losses incurred will be thru the duplexer, splitter(rx only), and transmission line. The duplexer & +5 MHz tx/rx spacing should allow enough isolation so the 4xx.300 tx won't interfere, but an additional bp/br filter can be put on the tx line if required.

IMO it would be really difficult to have an efficient system using a combiner as most of the power would be lost in heat and you would still require 2 antennas.

Moto, are these two repeaters going into a common radiax system? A little more info would help. I have several systems close spaced on radiax in L. A.

"The standard TX combining equipment requires 50 MHz channel seperation." I think they mean 50Khz spacing, .05mhz, normal for most hybrd combiners.

No radiax being used.

The antennas, two DB408, will be on the same tower with a max vertical separation of 40 feet on a 80 foot tower.

It looks like the way to go is two duplexers, the units that ship with the MTR2000, and add band pass cavities, to close for notch filters, and mount one antenna directly above the other with as much vertical separation as possible.

Thanks all.

thinking about this for a bit, here's what you'll need...

one duplexor on the output of each transmitter notching the other's signal. one duplexor on the output notching the RX frequency (since they are that close together, only one duplexor should be needed.

Then the recieve side just needs the one duplexor notched for the two frequencies, or two duplexors if the notch isn't wide enough.

My first question is to ask who coordinated this
mess. Frequency coordination is supposed to
help avoid situations such as this, or at least I
thought that this was the way things are supposed
to work.

Tom in D.C. wrote:My first question is to ask who coordinated this
mess. Frequency coordination is supposed to
help avoid situations such as this, or at least I
thought that this was the way things are supposed
to work.

This configuration is for a heavily RF populated area. In other words i'll take what I can get!

Tom in D.C. wrote:My first question is to ask who coordinated this
mess. Frequency coordination is supposed to
help avoid situations such as this, or at least I
thought that this was the way things are supposed
to work.

Tom....

My experences in the last couple of years has left me shaking my head. The term "short-spaced repeaters" can now be measured in hundreds of feet....

Dave

Sometimes coordination is a joke.

Not long ago, I discovered another user COCHANNEL licensed fpr the SAME tower as my 452. system. (and I owned the site).

the coordinating body (wont mention their name) didn't have much of a reply to my gripe, except that I should think about a community tone deck.

the outcome? they had around 300 mobiles to my 10. I went to another freq.
they are on my tower, paying (ahem) BIG rent.

guess it did work out in the end

Mike in CT

My two bits worth:
It seems that you wish to operate two repeaters which are only seperated by a few channels. I presume that you have the normal 5 MEG spacing between TX/RX
(Not that you have a reprater transmitting a few hundred KC from your receiver, also known as reverse split in Ham jargon)

No problem.
You need a two channel Hibred-Ferrite combiner which can combine on the same channel if you wish, however this comes at a cost as you loose 3+DB in the combiner.
A receive power splitter, or multicoupler and a pass/reject duplexer.
I have built several such systems in the UHF band.
I even have a spare hibred combiner.

I'm basically with Bernie as far as the solution here, but there are some additional concerns that jump out at me:

1) By my math, your frequencies are only 12.5 KHz apart. How are you going to run 20KHz spaced deviation on frequencies 12.5KHz apart? Your channels will actually overlap at higher deviations. That's a real recipe for problems. I would double check the licensing / coordination, and see if that was actually even allowed. (Not that I've never seen a coordination for nearby 25KHz channels 15KHz apart )

2) In these situations, intermod is probably a much bigger consideration than desense. If you talk to the engineering experts at TX/RX or EMR, they will probably tell you to stay away from duplexers unless you are sure that you are clear of intermod products out to the 21st order.

3) I don't know of any cavity filtering solution that can deal with a 12.5KHz spacing and, if you are actually planning to overlap your channels, it is downright impossible. (The usual bare minimum is 400KHz, with very high quality cavities)

4) Another important consideration is not to ask "How much TX power am I allowed?" but rather "How little TX power can I get away with?". If the repeaters are properly designed and assembled, they will include harmonic filtering for intermod protection. Unfortunately, the same cannot be said for the mobiles and portables using the system. If you get a close-in unit on one repeater, and a distant unit on the other, the two repeater outputs can combine in the close-in mobile unit's output stage. The resulting 3rd order IM product will fall exactly on the second repeater's input, and de-sense the second repeater. This type of interference can be a bear to find once it starts occurring. The only solution is to keep the TX powers as low as possible to minimize the strength of the resulting IM products.

The only real option that I see for a clean install is to run the two transmitters through a hybrid-ferrite combiner (which can work with zero separation) to one antenna, with the absolute bare minimum TX power. For RX, run the other antenna to the receivers, through a preselector, or band pass cavities, followed by a receive multicoupler.

Of course, every inch of collinear vertical separation that you can get on the antennas will help. If you get 20-25' you will probably be in really good shape.

I've put four close spaced repeaters onto one set of antennas with this approach. It is a bear in terms of TX power loss, but it is really the only way to do it cleanly without putting up the biggest intermod factory in the neighborhood.

Yikes! I just noticed the post dates.

Talk about a day late and a dollar short.

My two bits worth:
I have been thinking about this post for a while, and like some members I asumed that the second repeater was on a reverse split, which in my opinion is impossible.

I re red it and it seems to me that it is a simple problem.
Since I live in Hawaii, I live on "Hawiian time"
Yes! a day late and a dollar short is my motto.

I could certainly make such a Kludge work, as far as the other problems are concerned, such as if the radios would be able to communicate is another matter. Personally, I think that NB sucks, but that wasn't the question.
I don't think that intermod would be a problem, If I had the exact channels, as well as everything else on the site I could do an intermod run.