Batboard

Greetings to all,

Maybe it is the late nights and not feeling well but I need some help with an antenna seperation issue.

I have a friend/business associate that has purchased a company that was pretty much going out of business. In the purchase of this company he has obtained a repeater pair and repeater. The problem is where the current repeater is located he had to move it off of the tower and re-file the repeater to a different location.
Now the snafu is that the person/company that owened the repeater before had a duplexer go bad about 6 months ago and was able to get a local radio shop to borrow him a duplexer to keep it alvie for a little while. So now with the buy out the radio company came in and said it is ours we are taking it back and they did, of course rightfully so.
So Timmy (my friend) has found a new location for the repeater and was checking into the prices of duplexers and is now questioning keeping the repeater active.

The repeater FAQS= It is a 100 watt repeater that is synth and in good almost excellent condition (continous duty). With the repeater came a 7dB Gain antenna (for transmit), and a 3dB gain antenna (for recieve). The freqs are only 0.585 Mhz in seperation between the transmit and recieve freqs.

So Tim does not want to spend money into the repeater and everything unless he knows this new location he had to move it to will work good. So the question of how or what can he do to test it to see if it is worth putting money into it.
I advised him that with that close of a repeater pair it is going to cause some troubles in the repeater getting back to itself. I suggested that maybe he could do the following as of right now in order to test (but that it would not be a good test of what to do):

1.) Program the repeater for one DPL on recieve and a different inverted DPL on transmit.

2.) Use the two antennas that came with the repeater. Place the 3dB gain for recieve at the top of the structure/location. Then place the 7dB gain for transmit about 2 stories (or a little less) below it. {In addition I told him that he may even have to invert the transmit antenna.}

3.) Look to borrow a duplexer some where. (No Luck) But we were able to borrow some filtering so place the filters on the antenna lines to try and isolate the interference between TX and RX.

I know that it is a half a$$ed set-up and I told him that this will not work correctly unless he purchases a duplexer for the equipment. However he claims he can not justify the cost unless he can test it first. Now the radio company that was allowing the other guy to use there duplexer will not do the same unless Timmy signs a 1 year service agreement and these people have tried to screw Timmy over in the past so there is no dice there.
In checkng high and low no one else will allow us to borrow a duplexer for a few days just some filtering as stated above.

Can anyone offer anymore auggestions or what a person can do in this case???

By the way I did not mention it before but it is for BUSINESS only, it is not for public safety!

The normal Tx/Rx spacing for a VHF amateur repeater is 600 kHz, and your business machine is only 585 kHz. This really is "close enough for government work". Go buy a 6-can duplexer on eBay.

Now, a few points to ponder. There are plenty of inexpensive "mobile" duplexers for sale. These are usually notch-only (i.e. they reject the Tx signal at the receiver port and vice versa) and probably won't handle the 100 watts from your repeater. So don't even go here.

Look for a reputable brand, such as Sinclair, Celwave, Phelps Dodge, TxRx, DBProducts, Wacom, to name a few.

There are also several "adequate" 4-can duplexers that will claim to have sufficient isolation. Too much is never enough. One can will typically provide 35-45dB of isolation (some are better than others), so two cans give you about 70-90dB. Depending on how good the receiver selectivity is, how clean the transmitter is, and the type of antenna, 80dB "might" work well but it could also leave you with some desense - receiver degradation and interference to weak signals when the transmitter is on. You don't want this - it's a "Martha Stewart Bad Thing" so try to get a 6-can unit. These will give you in excess of 100dB of isolation, which really is optimum.

Duplexers come in several flavors: band-pass (which broadly tunes the frequencies to pass the receive signal to the receiver and vice versa), band-reject or notch (which attempts to minimize the transmitter's signal at the receiver input, and vice versa), and BP/BR (a combination of both, probably the best there is). They're all easy to tune up with the proper test equipment.

Duplexers also cover discrete frequency ranges. For example, one might be configured for 150-170 MHz with one set of cables, but will work fine with a longer set of cables at 130-150 MHz. The manufacturer, if they're still in business, should be able to either sell you a new cable set, or at least tell you the proper cable lengths if you feel like making them yourself and can get the same cable that they use. Also some companies will offer to retune a duplexer to your frequencies for a nominal fee if you can't find someone local to do it. But this could get costly if they determine some part of it isn't functioning properly and need to repair/replace cans.

Very few people have spare duplexers just waiting around to be loaned out (there are exceptions - I have a spare UHF duplexer at my house), and I can't see someone pulling theirs from a working system, just to retune it and let you try it for a week. If you want to buy a new one from one of the several companies out there, ask about their return/refund policy before you buy. But make sure you're sitting down as the price could be in the $2000-$3000 range for a new quality unit.

As far as your equipment is concerned, get it checked out to make sure it meets factory specs as far as sensitivity and output power are concerned. Also check the transmitter for spurious output signals, as these could pass right through most duplexers and hurt the repeater operation. The antenna and feed line should be checked with a good wattmeter on as close a frequency as possible to your licensed ones. You want minimal reflected power, under 1%, so if you can feed it 100 watts, you should get less than 1 watt coming back, preferably much less than that. On my UHF repeater system, I pump out 60 watts at the bottom and get back 0.1 watts (it used to be 0.05 until an antenna was moved closer to mine).

I'm sure others will add their $0.02 worth soon enough, but this should get you going in the right direction. If you see stuff on eBay and have questions about it's suitability or reputation, send a PM.

Bob M.

The repeater and antennas were checked by a radio dealer for functionality and a report was issued as part of the sale of the business and the radio equipment with it. (Timmy is a pretty smart guy and does a very good job with radio stuff for not really being in the radio hobby.)

As for the duplexers I know of places to the can get "cans" really cheap or at a really good price. The problem is that Timmy does not want to spend money on a radio system that he may not use all that much unless it can be proven to him or at least shown that it does do a good job.
As for the cheap route for testing with purchasing something off of eBa, well lets just say that that is a four letter word he does not speak.

I just posed the question about using two antennas, different DPL's, and some filtering on the antenna lines to see if anyone else has done it or could offer some good suggestions.
I am pushing him to just buy a duplexer and shell out the money to get the system at a good setting and to make some money off of it as a community repeater. It boils down for him that he has been burned by a few bad radio dealers with bad radio systems he use to rent space on, and now he has his own and can not nor does not want to do the same thing that happened to him on someone else.

Well we might do some antenna mounting on Friday so we will have to see how it goes from there.

My two bits worth:
I see, we want a perfectly operating radio system, perhaps one that can earn some income with out spending any money.

Now lets get real.
At your seperation 50' vertical spacing should work, less with some NOTCH filters, or pass/reject cavities.
This pair is too close for pass cavities alone, however additional filters may be required because of other site considerations.
It is allways good practice to use a cavity in the transmitter line to protect other users on the site.

In any case you will need some one with the proper test equipment, as well as experience to make all the required tests.

Keep in mind on a site you also have to consider the other users on the site.
No point if some one else has a transmitter 50kc from your receive, or visa-versa.

Forget about playing games with the PL.
Unless this is a link, you want the receiver to be as sensative as possible.
That means you need ISOLATION between the transmitter and receiver.
(as well as everything else)

So, what happend to the old duplexer?
Perhaps some one decided it was defective because they didn't understand how it works?

I have rarely seen a duplexer that was un repairable, unless it was unsuitable, or a piece of junk in the first place.

I probably didn't address everything in your original post, but Bernie touched on a few that I feel I can add to.

All the PL/DPL settings in the world won't help keep the transmitter signal from interfering with your own receiver or anyone else's. If you have a dirty signal, or insufficient isolation (from antenna spacing or duplexer), the receivers will suffer and won't hear much of what you want them to hear.

The notch-only duplexers, when combined with antenna separation, might result in enough isolation to let the repeater function well, assuming the repeater equipment is up to the task. But as Bernie stated, if the machine is in a crowded/busy environment, this won't be enough for everyone.

I realize that the inexpensive venue to buy duplexers can sometimes result in a bad experience. There should be some reputable people right here on BatLabs who have equipment to sell, or can recommend someone with whom they've had a good experience. I think that until you've been burned once, you haven't been fully exposed to the eBay experience. It's unfortunate but true.

Depending on how much the site and other equipment will cost, the duplexer isn't "cheap insurance" but it is "good insurance" and will be worth the expense down the road.

Don't forget to ground everything really well, and put a lightning arrester on the coax where it meets the duplexer. Sometimes I wonder whether these units save your equipment or prevent the lightning from hurting itself (we must be environmentally friendly these days) !

Bob M.