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Repeater, duplexer and SWR


Flameout
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I'm sure many of you are getting tired of my questions, but I have one more, for now 🙂

I was attempting to put together a portable repeater system, using two radios and two antennas. That part worked out fine, and the repeater was working.

I decided to try adding a duplexer into the mix and just go with one antenna. Again, it seemed to be working ok, other than the duplexer absorbing more than half of the power from the TX radio.

I was curious about the SWR between the TX radio (462.650) and the LOW side of the duplexer so I checked it and was surprised by the results. It was well over 20:1 (but between ANT output of the duplexer and the antenna, it was normal - about 1.2)

Is that normal swr to have between a TX radio and duplexer? I disconnected everything for now, until I  know for sure. Google was no help at all as I couldn't find any info on that specific configuration of TX to duplexer. Maybe I somehow fried the duplexer?

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I can't help with this, however, the important measurements would be power before the duplexer and power after the duplexer. Don't get obsessed with SWR, it is just a function of forward and reflected power. In the end, power into the antenna is what's important.

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1 hour ago, Flameout said:

I'm sure many of you are getting tired of my questions, but I have one more, for now 🙂

I was attempting to put together a portable repeater system, using two radios and two antennas. That part worked out fine, and the repeater was working.

I decided to try adding a duplexer into the mix and just go with one antenna. Again, it seemed to be working ok, other than the duplexer absorbing more than half of the power from the TX radio.

I was curious about the SWR between the TX radio (462.650) and the LOW side of the duplexer so I checked it and was surprised by the results. It was well over 20:1 (but between ANT output of the duplexer and the antenna, it was normal - about 1.2)

Is that normal swr to have between a TX radio and duplexer? I disconnected everything for now, until I  know for sure. Google was no help at all as I couldn't find any info on that specific configuration of TX to duplexer. Maybe I somehow fried the duplexer?

Information regarding the duplexer make/model might be useful. Difficult to "fry" a cavity duplexer, there isn't really any circuitry in one of those (they basically consist of a collection of cans that can be adjusted to tune them to frequencies is some scheme to produce band-pass filters in each direction while blocking the two frequencies from each other). Smaller units may be coil/capacitance filters and could be fried by too much power. The duplexer in the Retevis RT97 also sucks up half the power -- the repeater module is rated 10W, but the output power is only 5W. Not sure I'd really trust the smaller hand-held models (my only exposure is in "diplexers" which don't work within one band -- a 2m/70cm model to combine the output of a Kenwood TS-2000 to feed one antenna, and an HF&6m/2m&70cm on an FT-100 to feed a single tunable screwdriver antenna [ATAS-100]).

 

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7 minutes ago, axorlov said:

I can't help with this, however, the important measurements would be power before the duplexer and power after the duplexer. Don't get obsessed with SWR, it is just a function of forward and reflected power. In the end, power into the antenna is what's important.

Many radios will reduce output power if they see a high reflected power level to save the output final transistors which may not be rated for the combined power forward/reflected levels. Granted, HTs may not do this, but anything with a decent power level should.

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Agree here w/Gortex.

SWR at the radio before the duplexer should be the same or slightly higher than the SWR looking at the antenna after the duplexer. Your losses "should" be ~25% depending on quality of the filter. (ie) 25w in ~19W out roughly.

stay away from that CCC.

JE

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18 hours ago, gortex2 said:

SWR before duplexer should be very low if the duplexer is tuned properly. What model duplexer, tester and radios are we dealing with ? Who tuned the duplexer. 

I was initially using two Kenwood 880 radios. Now using one 880 as the RX radio and using an Icom F6011 as the TX radio. The SWR from the 6011 going directly to the antenna is very good. Around 1.2 and about 44 watts. When I put the meter between the 6011 and the LOW input of the duplexer, it is crazy high swr. With the meter between the ANT output of the duplexer and the antenna, swr isn't bad, but power drops to about 8 watts. A big difference from the 40+ going in. The duplexer is a chinese Fumei, tuned to 462-467. Never was tunned to a specific frequency. I just went back to using two antennas for now and working good. If I can ever figure out this duplexer, maybe I'll give it another shot

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Duplexer needs tuned. No way around it. Its not on the frequency you are trying to use.

Agreed. From what I have seen they are tuned 462/467 flat there in lies your problem. It's a half Meg out at a minimum and that's not gonna cut it.

I have seen some screen shots of properly tuned ones and they seem to be ok but have some power handling issues just an FYI

JE

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40 minutes ago, gortex2 said:

Duplexer needs tuned. No way around it. Its not on the frequency you are trying to use. 

I was hoping I'd get lucky, but I guess not. I have no idea where to even get it tuned properly so for now, it will be sitting on a shelf. Just for the heck of it, I attached a dummy load to ANT and connected my comet CAA-500 Mk II analyzer. I'm sure this is in no way an accurate test, and probably completely irrelevant, but the best SWR, which was about 1:1 at 50 ohms was 454.600 on the LOW port and HIGH was 468.200

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21 minutes ago, Flameout said:

I was hoping I'd get lucky, but I guess not. I have no idea where to even get it tuned properly so for now, it will be sitting on a shelf. Just for the heck of it, I attached a dummy load to ANT and connected my comet CAA-500 Mk II analyzer. I'm sure this is in no way an accurate test, and probably completely irrelevant, but the best SWR, which was about 1:1 at 50 ohms was 454.600 on the LOW port and HIGH was 468.200

You can get pretty darn close (much closer than it is now) with a NanoVNA, a few jumper cables and two dummy loads.

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29 minutes ago, Flameout said:

I was hoping I'd get lucky, but I guess not. I have no idea where to even get it tuned properly so for now, it will be sitting on a shelf. Just for the heck of it, I attached a dummy load to ANT and connected my comet CAA-500 Mk II analyzer. I'm sure this is in no way an accurate test, and probably completely irrelevant, but the best SWR, which was about 1:1 at 50 ohms was 454.600 on the LOW port and HIGH was 468.200

Find a local 2 way radio shop. They should be able to do it on a VNA in less than an hours labor. Yes you will spend about the same as you paid for it but it will be done correctly.

 

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9 hours ago, Flameout said:

but the best SWR, which was about 1:1 at 50 ohms was 454.600 on the LOW port and HIGH was 468.200

Definitely an untuned unit... That's a 13.6MHz offset when GMRS uses 5MHz. The "high" port is almost in the range of repeater inputs (467.550-467.725) -- about 600kHz off... But the "low" side is 8MHz off. I'm not going to look for an FCC channel assignment chart, but I'm fairly certain there is no service class that jumps that far between input and output sides of a repeater.

 

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I tried something today and now I'm not sure what is what and what is even accurate. As I said earlier, I put my Comet analyzer on the duplexer and seemed to get the best readings around the 454.xxx MHz area. Well, today, I tuned the radio to different frequencies as I tested the SWR (on low power)going INTO the LOW side of the duplexer with a dummy load on the ANT port. At the frequency of 454.200, I was getting perfect SWR and at the GMRS frequency of 462.650, it was awful. Do these reading even mean anything or is using an antenna analyzer way off base?

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First of all you cannot transmit on any UHF channel you want. 454.200 is a licensed channel. Secondly as we have said multiple times the duplexer MUST be tuned to the working frequency. Until it is tuned to a GMRS channel you will see bad VSWR and power issues. Take the duplexer to a shop and have it tuned. I'm not sure what your trying to accomplish but multiple folks have told you the duplexer is tuned wrong. 

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28 minutes ago, gortex2 said:

First of all you cannot transmit on any UHF channel you want. 454.200 is a licensed channel. Secondly as we have said multiple times the duplexer MUST be tuned to the working frequency. Until it is tuned to a GMRS channel you will see bad VSWR and power issues. Take the duplexer to a shop and have it tuned. I'm not sure what your trying to accomplish but multiple folks have told you the duplexer is tuned wrong. 

I have been known to break a rule now and again. I sometimes go over the speed limit, I have drifted through a stop sign or two and I have transmitted out of my allowed frequencies (I'm only a general licensed ham operator but have made a qso or two in the extras bands) However, this time I was transmitting into a dummy load.

I know the duplexer needs tuned, and I attempted to do that using that antenna analyzer. It probably doesn't make sense, but it is now working much better since I attempted the tuning. Granted, it certainly isn't perfect. The SWR was awful on the gmrs bands, but I set the analyzer to 462.550 and 467.550 and was able to tune (using all six adjustments) to a very good swr and much higher power output. The two photos are after some duplexer adjustments. Reading went from off the charts to near perfect. I know I can't be this easy, but it's working better now

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