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Problem with RX on VHF Quantar

LCT123

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I have a Quantar vhf repeater that I purchased a few months ago and have not had any problem with it until now. It will not receive anything further away than about 2 miles. I originally had a separate antenna for receiving and another antenna for transmitting. The only change I made to the setup was to install duplexer, that came from my old repeater, and was already tuned to my frequencies. The SWR checked out at 1.49 and I plugged another radio into the same antenna and it works fine. I have also checked the mobiles and they are transmitting fine. Is there anything else I can or should check before I purchase another receiver board?
 

wgbecks

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I would not immediately suspect that the receiver to be faulty, especially if it seemed to work ok when you were using split transmit and receive antennas. That said, using a service monitor, you'd need to first measure the receiver’s sensitivity with it isolated from the duplexer to determine if its sensitivity meets specifications.

On the surface, it sounds more like you have receiver desensitization since migrating from split antennas to use of the duplexer. What is the
make and model of your duplexer and what is your frequency split? Keep in mind that the VHF Quantar's receiver preselector is approximately 4 MHz wide and is specked for a separation of approximately 1.5 MHz when using a typical duplexer.

If you're using the Quantar for a two-meter harm repeater with only 600 KHz of separation, then you're going to need a lot more duplexer isolation that what you were accustomed to when using with older repeater stations that had narrower front-end designs. Of course, you may
have transmitter noise or something in your transmission system that is contributing to desensitization aside from duplexer isolation.

Again, the only way to determine the problem with the apparent lack of sensitivity is to trouble shoot the system using a service monior to test the receiver stand alone and in full duplex operation with and without the antenna and transmission line connected.

BTW, an SWR of 1.49 is not all that good in that you only have about 14 dB of retrun loss and whereby the mismatch in load impedance may
contribute to a reduction in duplexer insertion loss and isolation performance.
 
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MTS2000des

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Any time you are transmitting into the same antenna with a duplex filter, any changes made really need a proper on site tune up. Start with an RX sensitivity test without the duplexer, transmitter off. Verify proper filter operation with an ISO-T. Changing hardware in a duplex system even a repeater or module can impact performance. Start with the basics.
 

LCT123

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The duplexer is a 4 cell Decibel Products that I would guess is about about 30 years. The frequency split is RX 157.530, TX 152.270. I was told that the duplexer should not have to be retuned after taking it out of the old radio if the frequencies stayed the same. The Quantar had been tuned before it was shipped to me and the only change was the addition of the Duplexer. The Motorola tech that put the old radio together for me has moved so I will probably have to take it to a local radio shop or find someone that can tune it on site.
 
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wgbecks

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I would not immediately suspect that the receiver to be faulty, especially if it seemed to work ok when you were using split transmit and receive antennas. That said, using a service monitor, you'd need to first measure the receiver’s sensitivity with it isolated from the duplexer to determine if its sensitivity meets specifications.

On the surface, it sounds more like you have receiver desensitization since migrating from split antennas to use of the duplexer. What is the
make and model of your duplexer and what is your frequency split? Keep in mind that the VHF Quantar's receiver preselector is approximately 4 MHz wide and is specked for a separation of approximately 1.5 MHz when using a typical duplexer.

If you're using the Quantar for a two-meter harm repeater with only 600 KHz of separation, then you're going to need a lot more duplexer isolation that what you were accustomed to when using with older repeater stations that had narrower front-end designs. Of course, you may
have transmitter noise or something in your transmission system that is contributing to desensitization aside from duplexer isolation.

Again, the only way to determine the problem with the apparent lack of sensitivity is to trouble shoot the system using a service monior to test the receiver stand alone and in full duplex operation with and without the antenna and transmission line connected.

BTW, an SWR of 1.49 is not all that good in that you only have about 14 dB of retrun loss and whereby the mismatch in load impedance may
contribute to a reduction in duplexer insertion loss and isolation performance.

Let me correct that last statement..

BTW, an SWR of 1.49 is not all that good in that you only have about 14 dB of return run loss and whereby the mismatch in load impedance may contribute to an increase in duplexer insertion loss and a reduction in isolation performance.
 

MTS2000des

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The duplex-er is a 4 cell Decibel Products that I would guess is about about 30 years. The frequency split is RX 157.530, TX 152.270. I was told that the duplex-er should not have to be retuned after taking it out of the old radio if the frequencies stayed the same. The Quantar had been tuned before it was shipped to me and the only change was the addition of the Duplexer. The Motorola tech that put the old radio together for me has moved so I will probably have to take it to a local radio shop or find someone that can tune it on site.
Duplexers can easily be influenced by being in transit, they become part of the antenna/feedline itself. Everything can influence performance when you are transmitting into the same antenna you are receiving from.

On site tuning, ISO-T testing is the industry standard method. You should be using a good tracking generator. If this isn't being done, this is all mickey mouse and you'll be wasting lots of time trying to troubleshoot. Even the jumper cables themselves can play a huge factor with performance in duplex systems.

If you take the duplexer out of line, and connect your Quantar and it has good sensitivity, your problem isn't the repeater: it's the duplexer and associated setup including feedline (please tell me you aren't using LMR400) can dramatically affect performance.
 

wgbecks

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The duplex-er is a 4 cell Decibel Products that I would guess is about about 30 years. The frequency split is RX 157.530, TX 152.270. I was told that the duplex-er should not have to be retuned after taking it out of the old radio if the frequencies stayed the same. The Quantar had been tuned before it was shipped to me and the only change was the addition of the Duplexer. The Motorola tech that put the old radio together for me has moved so I will probably have to take it to a local radio shop or find someone that can tune it on site.

OK. I had thought that you might be using the Quantar for a two-meter ham repeater. With 5.26 MHz spacing, the 4-cavity DB duplexer
should provide enough isolation, if properly tuned. It sounds like you need and on-site tech with a service monitor to check out the entire
system.
 

KevinC

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The duplex-er is a 4 cell Decibel Products that I would guess is about about 30 years. The frequency split is RX 157.530, TX 152.270. I was told that the duplex-er should not have to be retuned after taking it out of the old radio if the frequencies stayed the same. The Quantar had been tuned before it was shipped to me and the only change was the addition of the Duplexer. The Motorola tech that put the old radio together for me has moved so I will probably have to take it to a local radio shop or find someone that can tune it on site.
Is the power of the Quantar the same as the old repeater? Or more?
 

LCT123

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Is the power of the Quantar the same as the old repeater? Or more?
The old repeater was just two maxtrac's and a R.I.C.K. repeater maker with a 100 watt Amp. The Qunatar is doing about 110 watts
 

KevinC

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The old repeater was just two maxtrac's and a R.I.C.K. repeater maker with a 100 watt Amp. The Qunatar is doing about 110 watts
Ok. Just making sure the old one wasn’t a lot less power.
 

LCT123

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Duplexers can easily be influenced by being in transit, they become part of the antenna/feedline itself. Everything can influence performance when you are transmitting into the same antenna you are receiving from.

On site tuning, ISO-T testing is the industry standard method. You should be using a good tracking generator. If this isn't being done, this is all mickey mouse and you'll be wasting lots of time trying to troubleshoot. Even the jumper cables themselves can play a huge factor with performance in duplex systems.

If you take the duplexer out of line, and connect your Quantar and it has good sensitivity, your problem isn't the repeater: it's the duplexer and associated setup including feedline (please tell me you aren't using LMR400) can dramatically affect performance.
I had 3" jumpers that were a little short so I ordered some new ones that were suppose to be low loss. The older cable I had are the good ones, they are kinda of gold or beige color which probably doesn't tell you anything, but that's what the old radio was put together with. I think I need to take everything apart and start over and check those new jumpers as you pointed out. I could not find any of what I call the good jumper that were long enough and I don't even know the proper name for them. However, I am using Heliax for the antenna wire.
 

KevinC

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I had 3" jumpers that were a little short so I ordered some new ones that were suppose to be low loss. The older cable I had are the good ones, they are kinda of gold or beige color which probably doesn't tell you anything, but that's what the old radio was put together with. I think I need to take everything apart and start over and check those new jumpers as you pointed out. I could not find any of what I call the good jumper that were long enough and I don't even know the proper name for them. However, I am using Heliax for the antenna wire.
Details are important. So the interconnecting cables are not the same ones the duplexer used on the old repeater?
 

LCT123

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Details are important. So the interconnecting cables are not the same ones the duplexer used on the old repeater?
No they are not. I think I will take the radio and duplexer out of the cabinet and set them up with the older jumpers and see what happens. I will also see if I can find some better jumpers.
 

MTS2000des

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I had 3" jumpers that were a little short so I ordered some new ones that were suppose to be low loss. The older cable I had are the good ones, they are kinda of gold or beige color which probably doesn't tell you anything, but that's what the old radio was put together with. I think I need to take everything apart and start over and check those new jumpers as you pointed out. I could not find any of what I call the good jumper that were long enough and I don't even know the proper name for them. However, I am using Heliax for the antenna wire.
You do realize the jumper lengths, cable type, etc all affect duplexer tuning. Doing a repeater right with a duplexer is a black art. Your Quantar, if tuned well, will blow away the MaxTrash you had before RX sensitivity wise. Like a high performance automobile, only as good as the tires, the fuel, and the mechanic.
 

KevinC

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You do realize the jumper lengths, cable type, etc all affect duplexer tuning. Doing a repeater right with a duplexer is a black art. Your Quantar, if tuned well, will blow away the MaxTrash you had before RX sensitivity wise. Like a high performance automobile, only as good as the tires, the fuel, and the mechanic.
The repeater to duplexer jumpers aren’t length critical. Now the cavity to cavity one’s definitely are (assuming his duplexer is that style).
 

LCT123

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You do realize the jumper lengths, cable type, etc all affect duplexer tuning. Doing a repeater right with a duplexer is a black art. Your Quantar, if tuned well, will blow away the MaxTrash you had before RX sensitivity wise. Like a high performance automobile, only as good as the tires, the fuel, and the mechanic.
 

MTS2000des

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The repeater to duplexer jumpers aren’t length critical. Now the cavity to cavity one’s definitely are (assuming his duplexer is that style).
That is what I am referring to.
Really needs to do an on site duplexer tune and proper ISO-T test.
Many people think tuning duplexers on a bench and shipping them off are all it takes. Hardly the case.
Feedline choice is also critical in duplex systems.
 

LCT123

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Yes, I have learned that with this radio. I ordered longer jumpers in order to eliminate connectors and now you have me thinking I ordered the wrong cables.
 

prcguy

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Before anything the receiver needs to be tested independently to see if it meets sensitivity specs. Then you can move on to duplexer testing/tuning and other stuff that has been mentioned. I have a feeling the rx is just fine and when any problem arises you should always ask, what has changed? Sounds like this problem cropped up right after going from dual antennas to a duplexer so there is a good chance that is the problem.

You can waste hours or days of time chasing things that don't really matter. Instead answer the obvious question first, is the receiver ok?
 

LCT123

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Before anything the receiver needs to be tested independently to see if it meets sensitivity specs. Then you can move on to duplexer testing/ tuning and other stuff. I have a feeling the rx is just fine and when any problem arises you should always ask, what has changed? Sounds like this problem cropped up right after going from dual antennas to a duplexer so there is a good chance that is the problem. But test that receiver by itself first.
It sounds like I just need to load them up and take them to the nearest radio shop or find someone to come to me that knows what they are doing. Thanks for all the help and suggestions.
 
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