Desense using M1225's for "desktop" repeater

[oz_com]

Opinions welcome:

I have configured two M1225's as a 25-Watt "desktop" UHF repeater (i.e., not rack mounted). While the controller and wiring are functional, the system sufers from severe desense on receive. I would welcome any thoughts on whether it's worth sending off the duplexer for re-tuning... or should I just accept that the environment is unworkable.

The problem is space. The antenna is located in a second story office window. (Roof access is not available.) The only location for the TX/RX radios + duplexer is about two wavelengths below the antenna. I can move them horizontally, but only about 20 feet.

The system has unacceptable desense. At the gas station a mile away, a 1-watt P1225 is almost full quieting into the TX radio on the talkaround frequency, yet at the same location, a 40-watt mobile is completely unreadable on the repeater input frequency with the TX radio actively retransmitting.

In addition to the space issue, cable quality is a known issue. Currently the duplexer is fed with short runs of RG58, and while I am aware of the need for something double-shielded, I am really wondering if that's going to produce the 16db (probably more) needed on the input.

The duplexer is a Fiplex DHL-4533, rated for 65 dB isolation at the 5 MHz spread utilized (max power handling 40 W). It was tuned by the seller. The only thing I can test is the TX insertion loss, which seems to be fairly close to the 1.5 dB specified figure.

One final note: I have run across FAQ's online for do-it-yourself duplexer tuning (instructions that involve "attenuating" a signal without a need for a signal generator and injector), but the problem is that one seemingly cannot attenuate enough a transmitter to simulate a weak signal at close range. I replaced my handheld's antenna with a 1-Watt dummy load, and even when placed inside nested metal boxes, the unit produced a signal strong enough to fully quiet the repeater input.

If I could play with the duplexer tuning myself, I would; if I could put the antenna on a mast, I would. Except for better cables, I think all the free or cheap options are exhausted.

Your opinion: is it worthwhile to have the duplexer re-checked professionally? Or is the system likely to ever work right with the antenna so close to the repeater?

Thanks for any perspectives.

-T.

[Doug]

In my humble opinion the first thing is I'd do is change out the RG58 to a good quality double shielded cables with appropriate connectors to match the antenna connector on the radio and the connector on the duplexers making sure there are no adapters being used.
You may want to post your general area as well as there are several board members that may be able to check your duplexer tuning for you or possibly give you a hand if your uncomfortable setting this up to get it working.
Doug

[tvsjr]

It sounds like you have a whole lot of, by themselves, fairly minor issues... which are adding up to a big problem.

1. The duplexer really needs to be tuned by someone who knows what they're doing. I've seen quite a few "professionals" that didn't. Your duplexer is a knockoff... it may be good, may not be. They're made by the same company that produces the sub-$100 HT1000esque radios. I've never used one, so I can't say for certain.
2. Your cables to the duplexer need to be replaced. No adapters and proper coax.
3. The radios need to be benched. Your TXer may have some sort of problem and be splattering all over the place, thus causing your desense.
4. The antenna needs to be swept. It may have a problem. Have you checked SWR from the TXer to the duplexer and from the duplexer to the antenna? High reflected power could cause your issue.
5. Any other strong transmitters in the area? You could be dealing with a mix product.

Your environment isn't the best but is certainly workable, but you need to have someone with proper test equipment and skill to walk through the system step by step and figure out what's happening and how to cure it. It's going to take a service monitor, antenna analyzer or return loss bridge, and a wattmeter at a minimum to get started on the project.

Where are you located? You might find a fellow Batlabber that could help you out.

[oz_com]

Thanks... some good ideas.

1. The duplexer really needs to be tuned by someone who knows what they're doing. I've seen quite a few "professionals" that didn't. Your duplexer is a knockoff... it may be good, may not be. They're made by the same company that produces the sub-$100 HT1000esque radios. I've never used one, so I can't say for certain.

No argument here! Again, not something I would try as I lack the equipment. The seller was an Extra-class ham, so I am looking for "confirming" evidence before rushing to have the duplexer re-tuned.

Also,

change out the RG58 to a good quality double shielded cables with appropriate connectors to match the antenna connector on the radio

I also am considering this more, since those RG58 cables are terminated by PL-259's and the dreadful mini-uhf adapter, which may be the leakiest point. The duplexer takes TNC and again, using an adapter.

Can anyone recommend an online seller of good coax and connectors?

3. The radios need to be benched. Your TXer may have some sort of problem and be splattering all over the place, thus causing your desense.

I'll pause before considering this -- the shop that programmed the TX radio indicated, on the work order, that the alignment was checked.

4. The antenna needs to be swept. It may have a problem. Have you checked SWR from the TXer to the duplexer and from the duplexer to the antenna? High reflected power could cause your issue.

The antenna is a homebuilt J-pole, and tuned to low SWR at roughly 1.2:1. However I was reading recently an article on Repeater-Builder.com about how no-ground-plane designs often don't do well with repeaters (though an explanation wasn't given). The antenna sits on top of a 4' metal file cabinet and the radios and duplexer are bolted to the lower side. Just for laughs, I may try replacing the J-pole with a quarter-wave mag mount, since the experiement wouldn't cost anything.

SWR as checked from TX-to-duplexer and duplexer-to-antenna seems reasonable. I *assume* the duplexer would have been tuned with a 50-Ohm dummy load.

5. Any other strong transmitters in the area? You could be dealing with a mix product.

No, but plenty of weak ones -- a desktop computer (serving as repeater controller), cable modem, network switch, and wireless router. I know the cable modem has a spur about 20MHz off, but it's pretty weak. Something else that may bear investigation. I could try an experiment by unplugging everything except the computer.

Your environment isn't the best but is certainly workable,

That's encouraging actually. Since I haven't seen a lot of implementations of desktop repeater systems, it's hard to judge the relative effects of poor cables vs. RF-rich environment.

Realistically, I have very modest expectations of a system made out of two mobiles and a toy duplexer. I do expect, though, to talk-in on a handheld from further away than three city blocks. Maybe I should lower ERP altogether? Next free experiment may be swapping out the TX radio for a handheld and see if that changes anything.

Thanks for the perspective,

-T.

[tvsjr]

Moving to proper forum...

[Will]

Actuly the Radius M100 and the Maxtrac have a much better receiver.

[chartofmaryland]

You have a desense problem,

First, in your explanation of the duplexer it seems to me to be a Band-reject or notch which can only be tuned with a tracking generator accurately. Ask the seller how the duplexer was tuned, if he says watt meter ask him for $100 back as that is what a shop may charge you if you can find one to tune it properly.

Second, to confirm there are some easy ways to see if a mobile notch duplexer has been tuned properly and is malfunctioning. Check the arrangement of the tuning slugs / screws in relation to each other. They should be inline with respect to distance from the body of the duplexer if the unit was tuned properly. Drop the output power of the transmit radio if the model will allow it, until your desense problem is reduced. If you can get better performance with lower power then you know isolation is your culprit.

Third, if you have limited resources, you can build an Iso-tee to check isolation. Most are made from a dual female / single male UHF tee with the center pin removed for the purpose of isolation. With a female / female barrel connected to the now pin absent male of the tee, you can inject a low power signal, say a portable on 1/2 watt, and create a 40 db down signal. You will have to play with it as this is normally done with a signal generator and will not be as accurate or easier to do.

Turn off your transmit radio, terminate your antenna port on your duplexer into a dummy load, and turn on the receiving radio. Adjust your portables power or your UHF barrel until the receiver of the repeater is just about noise check this with speech until you can barely understand yourself, this is the threshold of the receiver. Then, turn on your transmit radio for the repeater. The change in your signal needed to key the repeater, and keep it keyed is the difference in isolation you are needing to have a properly isolated system.

Most of the time a retune will correct your problem. If you don't have the cash and can stand the smaller coverage, drop the power until there is enough isolation. 1 watt can cover a good sized area on UHF from a high point so if you can get 12 watts and good performance, as long as your radio can run at that low of an output, its going to last longer and you don't have to change much more.

The above covers your duplexer operation. You cannot do too well with a no ground plane antenna for a repeater. Think about that and look at a decent one from Tessco or one of the other suppliers. This with a quality feedline and respectful jumpers will improve your system.

CoM

[fineshot1]

Have you confirmed that there is no local noise being generated on the input frequency.

I ask becasue it sounds like from your description that this is an office or home environment.

Lots of noise generators in office & home environments, ie: routers,modems,pc oscillators,etc.etc...

[oz_com]

Yes, there is electronics noise being generated *close to* (within 20 MHz of) the input frequency, though quite not on the input frequency -- good suggestion in any case,

-T.