Desk Trac rx sensitivity issue HELP!

[FCM]

Hi,

I have a desktrac that is currently setup on vhf ham frequencies. The rx sensitivity is terrible! I tried adjusting the squelch to see if this changed anything and it did not. If you are within a 1'2 mi of the repeater its fine, but any farther , even with 75 watts from my mobile the received signal is all scratchy and unreadable. Is there anything I can try before pulling this and taking it into the big M? I even installed a preamp on the rx side to help and nothing. I am using two seperate antennas, no duplexer. I don't have any service monitor or any advanced test equipment other than a dmm. Any ideas would be helpful!..Thanks!!!



Frank

[kb4mdz]

Did you just program this for ham freqs? I think it likely you're below the normal passband of the pre-selector section of the RX; some tuning or the RX leg is probably in order.

Was it checked for basic operation before reprogramming?

Yup, you'll need a service monitor, and a PC running correct software, and a knowledgeable tech. Good luck and godspeed.

[Satelite]

Hello:
Using two antenaes to eliminate a duplexer can be done BUT:
You need quite a bit of either vertical or horizontal separation of the antenaes to keep from desensing the repeater.
If you post the rx and tx freqs someone here on the bd can tell you how many feet youd need to have to avoid desense.
Now if you go to the repeater and tx from it with no rx in and it sounds great to a mobile far out then id suspect desense.
Others here can steer you better than I can as I don't have experience with two anteanna setups but do with duplexer and one antenna setups.
Just a thought on your info but possibly needs checked into.
Satelite

[Jim202]

Hi,

I have a desktrac that is currently setup on vhf ham frequencies. The rx sensitivity is terrible! I tried adjusting the squelch to see if this changed anything and it did not. If you are within a 1'2 mi of the repeater its fine, but any farther , even with 75 watts from my mobile the received signal is all scratchy and unreadable. Is there anything I can try before pulling this and taking it into the big M? I even installed a preamp on the rx side to help and nothing. I am using two seperate antennas, no duplexer. I don't have any service monitor or any advanced test equipment other than a dmm. Any ideas would be helpful!..Thanks!!!



Frank

I can only guess that you are like we all were with the first repeater that got thrown together.

If I can make a few suggestions, it might make your grief a little easier to tolerate the frustrations your going through. Don't try using any LMR type coax for repeater service.

Beg borrow or trade to get some cavities to make the repeater have less desense.

Use only double shielded coax cable for jumpers between the antenna feedline and the receiver and transmitter.

Make sure you have the shield kit for the receiver.

Over the years, I have helped many hams that were like you. They all were trying to learn and put a repeater together for the first time. It would be to your advantage to get with the local ham community and make friends. It won't take long to learn who the technical geeks are. Did I say make friends?

See if you can get one of them to become a mentor for you. We all had to start some place and a mentor can sure short circuit all the years it takes to learn some of the tricks of the trade so to speak.

You have taken the first step by coming to this site and asking for help. Keep up the effort.

Jim

[FCM]

Thanks for the reply guys. We are using two antenna seperated about 8 ft or so apart. Frequency is 146.700 tx - offset. We are using rg58 coax jumper from the tx side to a 100 watt amp and running hardline from the amp to the lightning arrestor and out to the antenna. On the rx side we are sharing the antenna with a uhf repeater with no issues. Note.. the suspected desense problem existed from the beginning, before we added the amp. Output was 25 watts previously. Transmit is fine , no issues there at all. Yes, I programmed the desktrac to ham frequencies directly without any previous testing. It was previously setup on 154... Everything works great when you are in close range to the site..1/2 mile or so, and worsens when you move away. I also heard about loose rx cable issues in the receiver and mini uhf connector issues. I can check those. Thanks for all the responses and ideas on this topic. Maybe I should just take it to the shop and let them check it with the proper equipment and move on from there. Any more ideas Ill take them for sure!

This is a great board and great people , I used it back in 2000 to learn how to set up a stx portable, rss, rib, labtool, super lab and all that fun stuff!
Yes,,stx portable (the brick)..

Thanks
Frank

[MassFD]

On the rx side we are sharing the antenna with a uhf repeater with no issues. Note.. the suspected desense problem existed from the beginning, before we added the amp.

I hope you are not realy sharing an antenna with a UHF repeater, if so theres at least part of your problem. VHF and UHF require band specific antennas, how are you spliting the antennas. Please tell me it's not a coax Tee.

To trouble shoot athe desense issue, put someone at the repeater site and take a radio out to where the RX becomes bad. Have the other person shut down the repeater Xmitter while monitoring the RX audio. If the RX audio improves greatly with the Xmitter disabled you are being desensed by the Xmitter.

[Jim202]

Thanks for the reply guys. We are using two antenna seperated about 8 ft or so apart. Frequency is 146.700 tx - offset. We are using rg58 coax jumper from the tx side to a 100 watt amp and running hardline from the amp to the lightning arrestor and out to the antenna. On the rx side we are sharing the antenna with a uhf repeater with no issues. Note.. the suspected desense problem existed from the beginning, before we added the amp. Output was 25 watts previously. Transmit is fine , no issues there at all. Yes, I programmed the desktrac to ham frequencies directly without any previous testing. It was previously setup on 154... Everything works great when you are in close range to the site..1/2 mile or so, and worsens when you move away. I also heard about loose rx cable issues in the receiver and mini uhf connector issues. I can check those. Thanks for all the responses and ideas on this topic. Maybe I should just take it to the shop and let them check it with the proper equipment and move on from there. Any more ideas Ill take them for sure!

This is a great board and great people , I used it back in 2000 to learn how to set up a stx portable, rss, rib, labtool, super lab and all that fun stuff!
Yes,,stx portable (the brick)..

Thanks
Frank

First off, RG-58 is trash cable when it comes to repeater operation. There is not enough shielding to prevent leakage.

Second, your antennas don't have enough isolation at only 8 feet apart.

Bottom line is that you need at least 70 db of isolation between the RX and TX ports. It would be even better if you had closer to 100 db of isolation. How ever you get this isolation, that is what it takes for a good repeater to function.

You can obtain isolation in a number of ways. Frequency separation is one. But when running repeaters, your limited in what that separation can be. You can get isolation from antenna separation. Go do some homework on just how many feet that takes. It is large. The use of a duplexer can supply most of the isolation. Last, the use of good coax, like "HELIAX" or another cable like it. The use of coax with just a single copper braid is not a good choice at all.

Go fix you isolation problem and then come back with additional questions.

Jim

[Satelite]

Hello:
The op mentions 146.700 MHz tx with offset but doesn't mention how much offset .
It would be nice to know his rx freq so we could see how much separation he really has.
It was mentioned an 8 foot seperation on the rx and tx antennas and my thought is its way too close for 100 watts output.
Pretty much a qarauntee for desense troubles in my opinion.
RG58 would be one of my last picks on a 100 watt vhf repeater.
Id at a minimum go with 1/2 inch hard line in something like Heliax or Andrew for example.
And no I wouldn't consider LMR400 on a repeater from my past experience with it on repeaters was lousy . ( Started out ok but give it time and its trouble )
I know its probably a budget cost thing but you will do yourself a huge favor if youll change out the cheaper coax and consider a duplexer or get the antennas properly separated as 8 ft doesnt sound right to me =not even in the ball park I don't think.
Good Luck
Satelite
Ooops I see Jim202 posted as I was typing but he and I see it the same way .

[Al]

For Frank:
For 70 dB of isolation at 150 Mhz(assuming no coupling between antennas produced by a common metallic support), you need 2600 ft. horizontally and 59 ft. vertically. At 146.7 Mhz, the required minimum distances would be a couple of feet more.

[FCM]

Thanks for the reply guys. No Im not using a coax t! LOL! Im am purchasing a 6 cavity duplexer this week and will setup as a single antenna operation. I should have done a better job of describing the connections. Here is what I have to clarify..uhf repeater has a comet duplexer in the line, it is a vhf/uhf antenna. The vhf side goes to the rx side of the vhf repeater. The vhf trans goes from the desktrack on a 3 ft of rg58 to the amp. 60 watts. From the amp to the lightning arrester and out is 1/2 inch heliax.

Here is what I intend to do the fix all this ..Put the uhf back on its own antenna first and remove the comet duplexer.
Install the 6 cavity duplexer on the single antenna and replace the short coax jumpers with something more than rg58.
Someone asked the frequencys..146.7tx and 146.1rx
Antennas will remain at 8 ft apart vhf/uhf

Hopefully this will do it. I didn't know about the seperation issue, now I do.

Thanks. Ill keep you posted.

Frank

[Will]

It IS very possible that the nearby transmitter 'burned out' your receivers frontend transistor. Very common problem.

[Satelite]

Hello:
Also make sure the duplexer you order can do the 600 kHz spacing between 146.100 rx and 146.700 tx freq and that its not designed for example 3 or 5 MHz spacing.
Youll need a good duplexer to do the .600 kHz spacing on the split you posted between the rx tx freqs.
Also id check its wattage input rating and see that it meets or exceeds your systems expected wattage output.
Otherwise ooops wont work for you.
If you go to Ebay and search VHF duplexer and read the sellers specs on their vhf duplexers you will find most are selling a duplexer that can do 3.5 MHz min rx to tx spacing and a couple did have the .6 kHz spacing you will need and watch the wattage input too that will meet your application.
The .600 kHz type you need for the freqs you posted will be a bit more in cost but if you put a 3.5 MHz split in youll regret wasting the money spent cuz its not going to work for you.
Also check the db isolation they provide as if you don't get enough protection then its going to be an issue to cause more troubles for you.
I myself learned many years ago that 60 db isolation in heavy rf enviroments doesn't work very well and since I bit the bullet and put in a 120 db isolation at 5mhz uhf spacing and man I loved every moment after that with the repeaters operation.
I will admit I did over kill the the isolation at 120 db and could have gotten by most likely at 80 db but ill never under spec one again because it was a PAIN IN THE @$$ and a lesson I never forgot.
So do read the specs and cover your needs per your specs so you too will smile at the end results.
Good Luck
Satelite

[FCM]

Thanks for the info.we got a telewave tprd-1556. Minimum split is 400 khz. 350 max power 100 db isolation