Tuning a duplexer

[Flameout]

I'm sure this has been answered, but I can't find it. (Sure are lots of results when searching duplexer)

If a duplexer says it has been tuned for 462-467 does that mean it will now work on all of the GMRS repeater frequencies, or does it need to be tuned for a specific frequency, like 462.650 - 467.650? 

I asked a seller on ebay and they had no idea. 

[WRAM370]

While I understand the negative comments aimed at the quality of Chinese duplexers that are sold on Amazon and eBay, I have built several inexpensive GMRS and digital voice amateur 440 repeaters using the Fumei and SGQ duplexers, and have achieved results comparable to using Phelps-Dodge and Combilent duplexers, which are usually a bit more money. These notch-type duplexers are a compromise, but for budget builds, they work ok. Like you, I had to start to digest the info that is out there, and I couldn’t afford the quality band pass-band reject duplexers, but we all have to start somewhere in the learning process, so these inexpensive Chinese duplexers serve a need. 

I ended up buying this cheap spectrum analyzer with tracking generator off eBay, so I could tune these duplexers…Tracking generator

I also have a NanoVNA, but I found this SA w/tracking generator easier to deal with, although you have to search out the instructions, since it doesn’t come with any. Ok, I’ll save you the search…SA w/ tracking generator instructions

Looking at the photos you posted, I would guess you don’t have things set up correctly. You will also need a 50 ohm dummy load connected to the duplexer port you are not adjusting, and with the duplexer you have, you should be able to get to -85dB or greater.

Use this site to figure your loss through the duplexer …dB calculator

Using this calculator, and measuring the output from the transmitter radio, and measuring the output of the duplexer, I generally achieve about a 1.3-1.5 db loss through the Fumei/SGQ/Combilent duplexers, which is average for these types of duplexers.

I am not a professional. I am not someone who lives and breathes this stuff. I am just a hobbyist with decades of experience in blowing things up, shorting things out, and otherwise screwing the pooch with electronics. I am just giving you some info that works for me, after building 4 or 5 GMRS repeaters out of old commercial mobile radios, and 3 multimode (DMR/C4FM/D-Star) amateur repeaters.  Are there better ways to do it than I did ?  yeah…do I care? no. The repeaters I built with these types of duplexers still work, don’t cause issues because they are not in RF-intensive locations (well, actually, one is at a county EOC and works great), and serve the users’ needs at an affordable cost.

 

[BoxCar]

Tuning to a range implies the duplexer is tuned to the center frequency 2.5 MHz above the lower frequency or 465.15. That means it will work across the entire band, just not well. A duplexer needs both sets of cavities tuned to the frequencies you will use, one side to the 467 channel, the other to the 462 channel to work at its best efficiency and lowest insertion loss.

[JohnE]

It also depends on the duplexer itself. They however have to be tuned to a specific frequency.

a br/br(notch/notch) will be more "forgiving" than a bp/br(pass/notch) and even that varies manufacture to manufacture. most can ~500kC's, the GMRS "band" is only 175kC's

W/that being said a good br/br filter tuned to 462/467. 6375 "should" be able to handle all the GMRS channels w/o any real issues.

[Flameout]

Ok, thanks. I certainly don't have the equipment to tune one of these things. Seems I have (lots) more reading to do. I have no idea if they are br/br or bp/br but one was a Fumie the other I didn't see a name. Both from China of course so certainly not high end duplexers. I just made a portable repeater out of two Kenwood 880's and it's working ok with separate rx and tx antennas but thought it might be easier with just the one antenna.

This isn't something I planned on operating 24/7. Just traveling, camping etc. And I imagine a duplexer would take away some of the power from the TX radio. It's putting out 24 w right now so a duplexer would probably take half of that. Been looking for an 880H, for that little bit more power, but the ones I've found must have gold bars stashed inside

[JeepCrawler98]

7 hours ago, JohnE said:

It also depends on the duplexer itself. They however have to be tuned to a specific frequency.

a br/br(notch/notch) will be more "forgiving" than a bp/br(pass/notch) and even that varies manufacture to manufacture. most can ~500kC's, the GMRS "band" is only 175kC's

W/that being said a good br/br filter tuned to 462/467. 6375 "should" be able to handle all the GMRS channels w/o any real issues.

This - thanks for shedding light on the "it's only good on one frequency" myth.

Duplexers, filters cans, etc. are all rather 'pointy' devices, however GMRS is narrow enough to where if you shoot for the middle your repeater can be frequency agile so long as you remain within the service. You will have an optimum frequency of course, and if it's a fixed install I'd absolutely tune and optimize it it for that specific frequency, but changing the channel will only cost you a fraction of a dB on most duplexers (both br/br and bp/br).

[JohnE]

Quote

And I imagine a duplexer would take away some of the power from the TX radio. It's putting out 24 w right now so a duplexer would probably take half of that.

stay away from those CC duplexers , they are made for a 10mC split not 5. find a celwave or a micro magic
here is a pc of 2 bp/br and a br/br duplexers as well as an Isolator and a receive preamp

as to losses the br/br small duplexer has an insertion loss of ~1.5 dB max typical when tuned correctly is ~1.1 that would be 20%. In your case that wold be ~5W loss

Jeep crawler you should have seen some of the sketchy stuff I had to make work for one reason or another.

JE

[Radioguy7268]

A typical "notch" style compact duplexer (the ones you usually see advertised at prices around $100) can have a really wide and lousy tune job applied - and will "cover" the whole GMRS band, but they won't do it very well.

If you're OK with having 3-5 dB of desense - then yeah, go for it. It will halfway kinda work as long as you've got a strong signal into the repeater.

 

[Flameout]

Is there a way to check a duplexers high and low preset using a nanovna? I realize you can tune one this way, but thought maybe there might be a way to find this info. You tube wasn't much help

[WRKC935]

5 hours ago, Flameout said:

Is there a way to check a duplexers high and low preset using a nanovna? I realize you can tune one this way, but thought maybe there might be a way to find this info. You tube wasn't much help

Yes, there is.

You are going to connect it up as if you are tuning it and look at the notch that shows up on the VNA. 

With one that's correctly tuned for a single frequency, the notch should be fairly tight, but you will be able to see the notch go down, and then come back up.  The deepest part of the notch is the primary tuned frequency.  You can also put markers on the close repeater frequencies and see what the difference in the notch value is.  Obviously it will NOT be as deep as the primary tuned frequency but you can see what the numbers are.  If the numbers are 3 dB or less between the center tuned frequency and the frequency of operation then you are gonna be safe to use it on that frequency.  Now if you don't understand dB measurement then 3 dB doesn't sound like a lot.  But it's 50% of the specific signal level.  Lets say you are running full legal power of 50 watts.  That is 47dBm.  If you loose 3 dB of power it's now 44dBm.  Thing is 44dBm is 25 watts.  So a 3 dB change is NOT insignificant.

 

[SteveShannon]

This guy looks at the characteristics of a duplexer using two different NanoVNAs in this video.

In this next one he tunes a duplexer:

 

 

[Flameout]

Ok, thanks. I'm gonna give this another try. I got these numbers but I just had the nanovna connected to low then high. 463.860 and 464.280. It said it was tuned to 462 -467  so I guess I should expect something closer to that

[JeepCrawler98]

Can you post a pic of your test setup and change your span to 10mhz centered on 465? The dynamic range on a NanoVNA isn't great but you should be able to see down to -70dB somewhat OK. A notch at -22dB is barely anything and indicates something's amiss.

[SteveShannon]

30 minutes ago, Flameout said:

Ok, thanks. I'm gonna give this another try. I got these numbers but I just had the nanovna connected to low then high. 463.860 and 464.280. It said it was tuned to 462 -467  so I guess I should expect something closer to that

The best advice I can give you is to download and use the free software: https://nanovna.com/?page_id=90

[Flameout]

55 minutes ago, JeepCrawler98 said:

Can you post a pic of your test setup and change your span to 10mhz centered on 465? The dynamic range on a NanoVNA isn't great but you should be able to see down to -70dB somewhat OK. A notch at -22dB is barely anything and indicates something's amiss.

I'm going to have to learn this VNA a little better first. I'm kind of lost at what you are saying. I bought this thing over a year ago, but never really got the hang of using it

[Flameout]

Thanks for that info. There was a video on you tube where someone used that same spectrum analyzer that you linked to. For that price, I may have no choice to to buy one, or something similar haha. The ebay link was also showing a few others. I initially was just building a portable repeater that I could take camping, use in emergency etc but it's easy to get carried away. Although the repeater was working ok using two antennas, adding the duplexer and going with one antenna I thought would be easier.

So I now have the cheap Fumie duplexer which I didn't have tuned but says 462-467. I was just curious as to what they actually tuned it to.. I haven't tired it yet as I needed two more N connectors. Really appreciate the help and advice from these forums

[K7lon]

I would like to thank the those who provide the technical information and humble advice in this thread.

It's good to know I am not  the only one to "short out things, and screw the pooch on occasions. I have that same ebay S.A./ tracking generator. I have had  success with celwave .8 db loss ( dumb luck), total failure with a fumie 98% loss, shorted out two radios, and various small things a long the way. It's hobby, otherwise I would starve to death.

I am glad to hear about the .6375 center tune. I built two 5 watt portables to avoid interference with establish systems. And you just cut 40% of the weight out of my portable system and that's cool.

Thank you
WRJA397

[Flameout]

On 8/12/2022 at 10:38 AM, Sshannon said:

The best advice I can give you is to download and use the free software: https://nanovna.com/?page_id=90

I was able to get the software up and running on my laptop (win10) but unfortunately can't get it running on my shack computer (win7) First things first, I need to work on updating my older nanovna. Running version 0.4.0-3-ge04d3af from Nov 25, 2019 and as soon as I can figure that out, I'm going to give it a shot at seeing if I can figure out where this duplexer is tuned. Right now I have it up and running on .650 and seems to be working ok. Can this cause interference on say .625 and .675 if it wasn't specifically tuned for .650?

[SteveShannon]

4 minutes ago, Flameout said:

I was able to get the software up and running on my laptop (win10) but unfortunately can't get it running on my shack computer (win7) First things first, I need to work on updating my older nanovna. Running version 0.4.0-3-ge04d3af from Nov 25, 2019 and as soon as I can figure that out, I'm going to give it a shot at seeing if I can figure out where this duplexer is tuned. Right now I have it up and running on .650 and seems to be working ok. Can this cause interference on say .625 and .675 if it wasn't specifically tuned for .650?

I had to use the older version of the software on my Win7 computer. The update didn’t work. 

The duplexer doesn’t change the frequency of your signals if it’s not tuned correctly.  You just don’t get enough filtering to prevent the very strong transmitter signal from entering the receiver section, so I wouldn’t classify it as interference. 

[Flameout]

I was able to update the firmware of my nanovna (from 0.4.0 to 0.8.0 build date nov 25, 2019 to jun 19, 2020) so not sure if it's even the latest, now to work on checking the duplexer, but I do have another question.

The two TK-880's that I'm using as my portable repeater, can I still use the Tx radio of the repeater to transmit on? Of course it takes it out of the 'repeater' configuration. For example, I still have it programmed with the GMRS channels, but when using it as a repeater, I have it set on channel 19. Can I just plug a mic into that Tx radio and use it, transmitting while it's still connected to the duplexer? Reason I ask is  that it is receiving much better on the antenna connected to the duplexer than a small junk antenna connected to my other GMRS radio. The photo is how I have it set up, and the radio on the right is the Rx radio, which is still programmed to the GMRS channels

[gortex2]

Yes if the radio is set for RX/TX on the 462.xxx you can talk and receive on it. The GR series repeaters used to ship with a mic for that reason.  It would allow a "dispatcher" to talk back to his vehicles without needing another radio on site. 

[Flameout]

29 minutes ago, gortex2 said:

Yes if the radio is set for RX/TX on the 462.xxx you can talk and receive on it. The GR series repeaters used to ship with a mic for that reason.  It would allow a "dispatcher" to talk back to his vehicles without needing another radio on site. 

Could I have that radio set on a repeater channel, say R21, and transmit 467.700 to a repeater and Rx back on 462.700? I didn't explain myself very well in the previous post,sorry. Just didn't want to screw up the duplexer if I were to TX into the low side which is meant for 462.xxx. Maybe I should just get another antenna

[gortex2]

I guess Im confused what you want to do. Is not the TX radio programmed for 462.700 ? If so you can talk on your portable to 467.700 and hear it on your RX radio. You can then answer on your TX radio and your portable will hear it. Also if your portable is on simplex (462.700) you can transmit and receive on the TX repeater radio (462.700). Thats how the repeater works. Your RX radio should have 467.700 RX Only and your TX radio would have 462.700 TX/RX as you can't make a TX only personality.

 

[KAF6045]

2 hours ago, Flameout said:

Could I have that radio set on a repeater channel, say R21, and transmit 467.700 to a repeater and Rx back on 462.700? I didn't explain myself very well in the previous post,sorry. Just didn't want to screw up the duplexer if I were to TX into the low side which is meant for 462.xxx. Maybe I should just get another antenna

Only handhelds and mobiles are permitted to transmit on 467MHz frequencies. Handhelds, mobiles, base, and repeaters are permitted to transmit on 462MHz.

Quote

§ 95.1763 GMRS channels.

The GMRS is allotted 30 channels - 16 main channels and 14 interstitial channels. GMRS stations may transmit on any of the channels as indicated below.

(a) 462 MHz main channels. Only mobile, hand-held portable, repeater, base and fixed stations may transmit on these 8 channels. The channel center frequencies are: 462.5500, 462.5750, 462.6000, 462.6250, 462.6500, 462.6750, 462.7000, and 462.7250 MHz.

(b) 462 MHz interstitial channels. Only mobile, hand-held portable and base stations may transmit on these 7 channels. The channel center frequencies are: 462.5625, 462.5875, 462.6125, 462.6375, 462.6625, 462.6875, and 462.7125 MHz.

(c) 467 MHz main channels. Only mobile, hand-held portable, control and fixed stations may transmit on these 8 channels. Mobile, hand-held portable and control stations may transmit on these channels only when communicating through a repeater station or making brief test transmissions in accordance with § 95.319(c). The channel center frequencies are: 467.5500, 467.5750, 467.6000, 467.6250, 467.6500, 467.6750, 467.7000, and 467.7250 MHz.

(d) 467 MHz interstitial channels. Only hand-held portable units may transmit on these 7 channels. The channel center frequencies are: 467.5625, 467.5875, 467.6125, 467.6375, 467.6625, 467.6875, and 467.7125 MHz.

Granted, you are using a pair of mobile/base units to make up the repeater, so on one aspect there is nothing to prevent you from setting either one into normal "mobile/base" mode transmitting on 467 and receiving 462 -- other than having power on the receive side blowing out the receive circuit of one when it gets hit with more than a few milliwatts of input signal leaking via the duplexer. I'm using simple antenna switches to share one antenna between two HF rigs -- and have to worry about leakage feeding back down the "disconnected" side of the switch into the other radio.