Duplexer Tuning

[KJ4UAW]

Better place to get audio is the high side of the volume control unless you like poking around the radio with an o-scope. (One side should be at zero volts and the middle is the wiper). Use your own potentionmeter to set the voltage levels. I would reduce the power to less than 10 watts and put the controller into a metal enclosure by itself. The best way to check for desense is to put the reciever on carrier squelch mode and set the squelch to quiet the radio. Then key the transmitter. If the squelch opens, you found your problem.

Bob

Hi Bob, New member here ,,, just registered for the forum.. I'm in the process of setting up a local GMRS repeater in the local Greenville area. Have all the components (receiver, transmitter, controller, power supply, and duplexer) and everything assembled ... the only thing lacking in completing the project is getting the duplexer tuned to the repeater frequencies. The duplexer is a motorola/celwave 6 cavity which was original to a Motorola GR300 repeater. I haven't been able to locate anyone in the Greenville area with the equipment and expertise to tune the duplexer. Would you happen to know anyone in the Greenville area that might be able to help ??

Thanks,
Jim

 

[burner50]

Split from this thread: My GMRS Repeater in Ammo Can - The RadioReference.com Forums

Please don't thread jack...

 

[n8zcc]

Jim,

I am not familiar with the duplexer model you have but if it has notch filters your are good to go depending on how much transmit power you are going to have. I wrote an article on how to tune duplexers several years ago and it will show you how to tune without expensive equipment. All you will need that is somewhat unique is a signal tap (you can get one from RF Parts).

Kevin currently hosts my work on his web site which you may find has additional useful information.

Duplexer Tuning

 

[rescuecomm]

KJ4UAW

One of the local hams tuned the duplexers for the Pickens County 2 meter repeaters. In Greenville, I don't know who could do it with sweep/tracking generators and spectrum analyzers. He said you could tune flat pack duplexers with a signal source. This is because the notch spacing is fixed by the coupling/physical construction of the units.

I just looked at N8ZCC's page. Good info.

Bob

 

[WQOZ476]

In the for what it's worth department, I have a similar duplexer in my GR1225 repeater; I sent my duplexer to gmrsoutlet.com to have it tuned. It was a flat fee of $45, which included return shipping/insurance. Turnaround start to finish was a week, and this was over New Year's day. My setup works fine, so they must have done it right. Overall, I was very satisfied.

(I am not affiliated with them in any way, I just found their site on a google search.)

 

[gman4661]

Best wishes for the new repeater setup. Just curious, but how did you go about finding tower space?

 

[n8zcc]

In the for what it's worth department, I have a similar duplexer in my GR1225 repeater; I sent my duplexer to gmrsoutlet.com to have it tuned. It was a flat fee of $45, which included return shipping/insurance. Turnaround start to finish was a week, and this was over New Year's day. My setup works fine, so they must have done it right. Overall, I was very satisfied.

(I am not affiliated with them in any way, I just found their site on a google search.)

Your lucky or you are running lower power, 25 watts or less. It is always best to tune your duplexer into your antenna system. Outside of the antenna system is possible but I betcha if you do some fine tuning, specially on the notches, you will see receiver improvement. What is your DB loss on the pass band? That could reduce some with a fine tune also. There is no sense turning RF power in to heat.

Tell me I am wrong all you want but after taking care of multiple 2 meter and 70 cm amateur repeats for several years, I learned the old fashion way, baptism by fire.

 

[WQOZ476]

Your lucky or you are running lower power, 25 watts or less. It is always best to tune your duplexer into your antenna system. Outside of the antenna system is possible but I betcha if you do some fine tuning, specially on the notches, you will see receiver improvement. What is your DB loss on the pass band? That could reduce some with a fine tune also. There is no sense turning RF power in to heat.

Tell me I am wrong all you want but after taking care of multiple 2 meter and 70 cm amateur repeats for several years, I learned the old fashion way, baptism by fire.

No, I am not about to argue with you, but tuning it in place was not an option for me. I don't have the equipment, and don't know anyone who does. As you surmised, I am running very low power, a tad over 5 watts ERP. My antenna is only 75 feet up on a homemade tower, and works reliably out about 4 miles, which is good enough for me.

I don't know what my losses are, I can say that my setup shows a SWR of 1.2, so maybe not great, but pretty good for a first effort. Overall, I am quite pleased with everything so far....still have a lot to learn, but so far, so good.