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GMRS Repeater


mpoole
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I recently purchased a used Vertex Standard VXR-7000U repeater and a Fumei Duplexer factory tuned to 462.000/467.000.  The repeater is using 462.550 for community communications utilizing a Dr. Fong DBJ-UHF commercial antenna with 5db gain and custom tuned to 465MHz mounted 20 feet off the ground with 80 feet of Rg8x cabling.  On my initial install of the repeater and duplexer, the SWR was 1.06 but wattage dropped from 52 to 33 using the duplexer. 

 

I started to notice the repeater signal showing a lot of static and quality lower than simplex TX with my hand-held radio.  I rechecked the SWR this morning to find it now at 2.4 through the duplexer.

 

My question to more experienced operators:  Is it normal to loose 20 watts of power when using a duplexer?  What would make the SWR change during 2 months of use?

 

I have removed the duplexer and currently using 2 antennas, the TX being a Dr. Fong UHF/VHF DBJ-1 mounted on the opposite roof line of my house from the RX antenna.

 

Any incites on how to address the quality issues would be greatly appreciated.  

PS....I do not have the equipment to re-tune the duplexer to improve SWR.

 

Mike

WRJE502

 

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Your duplexer is mis-tuned! It should be tuned to precisely the frequencies you are going to be using.

 

The entire purpose of the duplexer is to allow only a single frequency to pass with minimum loss, while supressing any other frequencies. The easiest way to explain this is with a picture. This image is from a VHF duplexer, but the principles are the same. Note that the green line represents the lower frequency of your pair and the red line represents the higher frequency.

 

It is important to note that any frequency lower or higher than the tuned frequency is quickly attenuated. This is precisely why you are losing so much power when you transmit:

 

GPbcD.png

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Have you checked the SWR past the duplexer (checking the feedline and antenna?) Also check your jumpers, its possible you accidentally hooked swapped them and is often the problem.

RG8x isn't very good for short repeater runs let alone the length you are running, While I doubt thats causing your swr issue, it will greatly reduce the range of the repeater, especially at longer lengths. Personally I used LMR400 then upgraded to 600 which in my opinion is fine for running 30-40 feet up, though there are guys here who will demand 1/2" or 3/4" helix which for the type of install and the cost, I just dont feel its worth it.

If the SWR is good past the duplexer, then it would be reasonable to believe the duplexer is bad. Most flatpacks that I have seen are only rated to 45W for UHF band, and work best at wattages below 25W. `If you are putting 52W in you may have caused some damage to the internals, and could have damaged the repeater itself. So, I would try running low power, around 20W or so, and see if that gives you better SWR readings.

Last, I would recommend changing out the flatpack for a band pass/reject style duplexer. The difference will shock you for two reasons. #1, the Chinese flatpacks arn't made with the best quality and often have issues with them and are never tuned correctly. and 2) BP/BR duplexers seem to just work better than the flatpacks do.

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Makes sense. If I get this duplexer tuned to 462.550/467.550 then I should see no power loss and the signal quality will improve?

 

Thanks for your help!

You can tune it with a nanovna. It should be tuned in it's operational location, as simple road vibrations can cause it to fall out of calibration.

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Makes sense.  If I get this duplexer tuned to 462.550/467.550 then I should see no power loss and the signal quality will improve?

 

Thanks for your help!

 

Absolutely you will notice a huge difference, especially on the receive frequency. You will still have some power loss, but by having the duplexer tuned to precisely your transmit frequency, it will be far less loss.

 

Here is a screen shot of a tuned Bp/Br duplexer (band pass/band reject). Note that the upper red line is the "reject" trace. It has a 5MHz skirt and is rejecting the transmit frequency from the low side, and the receive frequency from the high side. This is from a 70cm duplexer.

 

GPbvp.png

 

Here is a blog illustrating how to "sort of" tune a duplexer with a NanoVNA:

http://wa8rra.org/duplexer-tuning/

 

Here is an excellent video showing the major weaknesses of NanoVNA versus a professional Textronix unit.

https://www.youtube.com/watch?v=GipCVEsiqXc

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Makes sense.  If I get this duplexer tuned to 462.550/467.550 then I should see no power loss and the signal quality will improve?

 

Thanks for your help!

 

There is always some loss, however your SWR should be low. High SWR means you have a lot of power returning to the transmitter. Granted 2.4 is still bad, I've seen it as high as 19:1. The BP/BR duplexer loses 3-8W depending on how much power I put in.

 

 

You can tune it with a nanovna. It should be tuned in it's operational location, as simple road vibrations can cause it to fall out of calibration.

Not on a flatpack at 460M. The nano VNA stops reading at levels below -60DB. I tried this several times and while I think it'll be in the neighborhood and could be used to test the duplexer, I wouldn't attempt to tune a 460 duplexer with it.

 

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I'm using the same duplexer from amazon but in my setup I'm utilizing two antenna and only using the duplexer on the receive side as a filter and terminated the other connection with a dummy load believe it or not it works it doesn't get killed by tx antenna with only 6 feet of horizontal separation I'm getting the power I want and have a good ears at the same time.

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There is always some loss, however your SWR should be low. High SWR means you have a lot of power returning to the transmitter. Granted 2.4 is still bad, I've seen it as high as 19:1. The BP/BR duplexer loses 3-8W depending on how much power I put in.

 

 

 

 

Not on a flatpack at 460M. The nano VNA stops reading at levels below -60DB. I tried this several times and while I think it'll be in the neighborhood and could be used to test the duplexer, I wouldn't attempt to tune a 460 duplexer with it.

 

I tried it. It’s exactly what WRAK968 said. -60dB would be Max with the noise floor, and the sweep is real slow. I’ve tuned lots of similar devices, I took mine over to friend with a service monitor to see how close I was. Best I could do was about -60 on 466.52 vs the 467.575 I was tuning for. If you nailed it you would be lucky. My repeater had RX desense because I tried the NanoVNA, now it’s a 20 mile repeater once I put the Amazon duplexer on the service monitor.

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Well it comes down how much money you want to put into the system

Cavity duplexers get pricey. 

I have used those Fumei duplexers and they do a good job so long they are tunned precisely.

It is  granted there will be RF loss but that happens with coaxial lines alone

Important thing in mind to keep is that you want to have acceptable SWR to prevent damage to the transmitter. 

I have heard so much negativity from hams and gmrs folks alike about those darm chinese duplexer but it is all about name brands.

they are not perfect chinese made devices but not too different than the US made

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Well it comes down how much money you want to put into the system

Cavity duplexers get pricey.

I have used those Fumei duplexers and they do a good job so long they are tunned precisely.

It is granted there will be RF loss but that happens with coaxial lines alone

Important thing in mind to keep is that you want to have acceptable SWR to prevent damage to the transmitter.

I have heard so much negativity from hams and gmrs folks alike about those darm chinese duplexer but it is all about name brands.

they are not perfect chinese made devices but not too different than the US made

From my experience I agree 100% I have a Fumei duplexer now tuned at -90 to -95 dB and with good 7/8 heliax it transmits almost exactly what the theoretical map say 50W at 55’ in my location will cover. It’s so good that now I just got a Decibel Products BpBR and I’m afraid the losses will be higher than the China flat pack.

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Thanks for all the great information you guys provided.  I'm taking the duplexer to a radio dealer to get it tuned next week.  I will also have them check out the radio as they were a Vertex Standard dealer shop in years past.   It will be an all day trip, but hey, road trips can be fun.

 

Thanks again!

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Thanks to everyone who responded to my inquiry, very good info here.  I've just returned from taking the duplexer to a professional radio shop for tuning to 462.550.  The Fumie duplexer still has a 20 watt lose on TX.   It appear the Fumie is junk and the advise of the radio technician is to stay away from the cheap China duplexers.....all of them.  His minimal recommendation was a ICOM at a cost of $395.

 

I think I'm going to investigate using two antennas instead of a duplexer.  As long as I have signal separation to prevent bleed-over it will be less expensive and provide full TX power from the repeater.  We are moving forward to the next stage of our project.

 

Thanks again for your support!

Mike

WRJE502

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Without diving into the ins & outs of tuning a duplexer (I've got my doubts on a 20 watt loss being blamed on poor design)  -  I'll just go along with your split antenna system and say this:

 

Good practice theory says that for UHF you should use at LEAST 10 feet of vertical separation between the Transmit and receive antennas. Horizontal separation would require at least 1000 feet for antennas on the same plane, which really isn't practical. The higher your transmit power level (or the worse your receiver's selectivity) the more vertical separation you'll need. I'd still put some type of bandpass cavity on the receive side to improve selectivity & filter out unwanted noise.

 

Somewhere out on the internet there's a few charts showing the recommended vertical spacing for different frequencies. (As you go higher in MHz, the need for physical separation decreases as a function of wavelength.)

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I'm using the same duplexer from amazon but in my setup I'm utilizing two antenna and only using the duplexer on the receive side as a filter and terminated the other connection with a dummy load believe it or not it works it doesn't get killed by tx antenna with only 6 feet of horizontal separation I'm getting the power I want and have a good ears at the same time.

Thanks for this info.  I've ordered a dummy load and will utilize this option if the 3 meter separation still causes problems.  I'm currently running two antennas at 24' separation horizontally and have no issues at 50 watts.  I'm not on a tower and in a wilderness area so I only have to cope with my own TX/RX.

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Well it comes down how much money you want to put into the system

Cavity duplexers get pricey. 

I have used those Fumei duplexers and they do a good job so long they are tunned precisely.

It is  granted there will be RF loss but that happens with coaxial lines alone

Important thing in mind to keep is that you want to have acceptable SWR to prevent damage to the transmitter. 

I have heard so much negativity from hams and gmrs folks alike about those darm chinese duplexer but it is all about name brands.

they are not perfect chinese made devices but not too different than the US made

Running 1.85 currently as I'm using a dual tuned VHF/UHF antenna to TX.  That will change with the move to another location and two GRMS tuned antenna.  Hope to get below 1.5k SWR.

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Since you already own the duplexer and have it tuned, you can still use it on the receiver side by putting dummy load on the HIGH side. It should improve the receiver's performance.

Good info, just ordered a dummy load and will utilize this setup if I get bleed-over on the TX.    Much appreciated!

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Without diving into the ins & outs of tuning a duplexer (I've got my doubts on a 20 watt loss being blamed on poor design)  -  I'll just go along with your split antenna system and say this:

 

Good practice theory says that for UHF you should use at LEAST 10 feet of vertical separation between the Transmit and receive antennas. Horizontal separation would require at least 1000 feet for antennas on the same plane, which really isn't practical. The higher your transmit power level (or the worse your receiver's selectivity) the more vertical separation you'll need. I'd still put some type of bandpass cavity on the receive side to improve selectivity & filter out unwanted noise.

 

Somewhere out on the internet there's a few charts showing the recommended vertical spacing for different frequencies. (As you go higher in MHz, the need for physical separation decreases as a function of wavelength.)

We are looking at this.  If vertical separation isn't enough, we'll put the duplexer on the RX side only with a dummy load on the TX side and filter out the bleed-over.  Should work fine and allow us with a full 50 watt TX instead of the 33 we were getting through the duplexer after being professionally tuned.

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