Laird FG4607 UHF GMRS Repeater Antenna

[kayn1n32008]

So the issue with very high SWR readings at the antenna has come back up. Today when visiting the site I noticed that the SWR readings on the feed line and at the antenna were consistently around 6.0. However when I hooked up my Bird Watt Meter to the TX port of the repeater with a dummy load I also noticed that my repeater had not been putting out any power.

6:1 is not good. Not good at all. Likely the cause of your repeater failing.

Now my repeater is putting out 40watts right out of the back and about 20-22watts after the Duplexer (seems like a fair amount of loss?).

That does seem like rather excessive loss. Especially for a 4 cavity bandpass 5MHz split duplexer.

I have a UHF repeater with a 5MHz split, that is outputting 12.5w into a Sinclair BpBr 4 cavity ResLoc, and there is 11w coming out. Easily seeing 50-60Mile radius of coverage.



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[RFI-EMI-GUY]

This document is helpful. The appendix has troubleshooting tips.


https://www.google.com/url?sa=t&rct..._catalog.pdf&usg=AOvVaw2wSTi4KCHBaDNWD4C3s2AV

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[jeepsandradios]

I know I have asked but what are you measuring SWR with ?

Also that duplexer is not the best. I am guessing you are loosing around 3db. Hence 3db is about 50% loss of power so your numbers match.

 

[RFI-EMI-GUY]

I know I have asked but what are you measuring SWR with ?

Also that duplexer is not the best. I am guessing you are loosing around 3db. Hence 3db is about 50% loss of power so your numbers match.

The spec on that duplexer is like 1.2 dB insertion loss, so I would be looking carefully at the cabling and then the tuning. Still I wonder about that antenna, it could have had a hard trip in a UPS truck or assembled wrong.

 

[BlueDevil]

I have a Bird 4304 Wattmeter. I also have a Times Technology VHF/UHF Antenna Impedance Analyzer.

What is a good duplexer that I can get without breaking the bank. I don’t need anything real fancy. This is just a GMRS Repeater System.


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[kayn1n32008]

What is a good duplexer that I can get without breaking the bank. I don’t need anything real fancy. This is just a GMRS Repeater System.


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The duplexer May be fine, but 3dB is far more loss than spec. You should be able to get it tuned better than the specs. The one thing to make sure is that the inter connecting cables are the correct length for the frequency you are operating on. There lengths are critical and are based on frequency and velocity factor.

If you are at a site with little adjacent channel/out of band noise, a Chinese mobile duplexer may work fine. The ones I have used and tunes will meet and beat the specs of the Sinclair duplexer they are copies of. They Canberra found online for under US$200. Get the cavities that are about 8” long or so.

They are a pain to tune, require patience, but they work just fine. Make sure they are tuned properly, do not rely on the manufacturer tuning.

Ideally you want a Sinclair ResLoc BpBr 6 cavity duplxer(Q3330e)or equivalent from another company, but you will pay for brand new ones. The Sinclair 6 cavity is probably in the range of $1500-2000 new.


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[RFI-EMI-GUY]

I have a Bird 4304 Wattmeter. I also have a Times Technology VHF/UHF Antenna Impedance Analyzer.

What is a good duplexer that I can get without breaking the bank. I don’t need anything real fancy. This is just a GMRS Repeater System.


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You cannot go wrong with a 4 can T1504 Motorola duplexer. I think the manual is on repeaterbuilder.

These have both passband and reject filtering. They are actually better than the TX RX pseudo bandpass filters in that the entire isolation is better from TX to RX.

Make sure you get a 450 to 470 MHz version with the proper loops and cables. They are color coded. Avoid anything corroded or damaged.

Dusty and dirty you can fix.

They are tricky to tune as far as the notch. You tune the notch by tigtning the hold down nuts finger tight and using a couple of nut drivers, lightly tapping the handles together like you would a hammer and chisle, to move the nuts ever so slightly the direction needed.

You tune in this order. Passband for best insertion loss, Notch for maximum rejection, Then Tighten Notch, then walk the passband knob slightly just about 1/8 turn either way to increase the depth of notch.

There are some more modern versions of this filter that were used in MSR2000 stations.


Expect to pay $200 to $350 for one. Pick the nicest you can find.

I have no idea why duplexer are so pricey on the used market.


While you are at it, if you trip over an isolator panel with two or more circulators , snatch it up. Be sure it is within 5 or 10 MHz of your transmitter freq. Some can be field tuned, but require patience. This device will prevent future intermodulation interference between your station and any others nearby.

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[BlueDevil]

The duplexer I have, I bought used. It was suppose to be already tuned to the GMRS frequencies. It is labeled that it is tuned for the GMRS frequencies. I haven't changed the interconnecting coax between cavities however I do not know if they are the correct length.

The environment that this repeater stored doesn't have much noise. So is the duplexer I have worth keeping and taking to a shop to have it gone through or do I need to find a different duplexer all together?

 

[kayn1n32008]

The environment that this repeater stored doesn't have much noise. So is the duplexer I have worth keeping and taking to a shop to have it gone through or do I need to find a different duplexer all together?

It is worth keeping IF it will meet or exceed the max 1.2dB loss spec. If it will not meet that spec, look at replacing it. I would take it to a shop and get a competent tech to tune it. Pass is if it has no more than 1.2dB loss on TX. Fail if it has more than 1.2dB loss on TX.



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[RFI-EMI-GUY]

It is worth keeping IF it will meet or exceed the max 1.2dB loss spec. If it will not meet that spec, look at replacing it. I would take it to a shop and get a competent tech to tune it. Pass is if it has no more than 1.2dB loss on TX. Fail if it has more than 1.2dB loss on TX.



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And the notches must be about 80 dB in depth! 80 is the spec, yet I see the factory "passed" one at 78 dB.

That duplexer might have been tuned on a different GMRS channel and the notch depth is in the wrong place.

Not sure I would trash it if it measured 1.8 dB IL on the bench as I doubt anyone has calibration that tight unless you go to a very serious RF lab. But do check those connectors.

Now back to that antenna.....

 

[kayn1n32008]

And the notches must be about 80 dB in depth! 80 is the spec, yet I see the factory "passed" one at 78 dB.



That duplexer might have been tuned on a different GMRS channel and the notch depth is in the wrong place.



Not sure I would trash it if it measured 1.8 dB IL on the bench as I doubt anyone has calibration that tight unless you go to a very serious RF lab. But do check those connectors.



Now back to that antenna.....

Likely the duplexer could be tuned to pass all the GMRS repeater pairs with ease. Probably be able to get 78dB isolation if not more on all the GMRS pairs.

I did that with a portable repeater once on VHF. 13 repeater pairs, all repeater transmit frequencies were 153.xxxMHz all receive frequencies were 5.260MHz higher. This was with a Sinclair mobile duplexer. You could select any of the 13 pairs. 5w out of the duplexer. Worked like a charm.

 

[BlueDevil]

All the connectors appear to be in good condition. The entire duplexer actually appears to be in good condition. No signs of corrosion or oxidation or worn connectors or cable.

What about the antenna?

 

[RFI-EMI-GUY]

All the connectors appear to be in good condition. The entire duplexer actually appears to be in good condition. No signs of corrosion or oxidation or worn connectors or cable.

What about the antenna?

See post #40.

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[BlueDevil]

Ok, that is a notch type duplexer. It has reject tuning only. It is a common duplexer, but not as ideal as a band pass/reject type filter duplexer.

Tuning is critical to get the notch depth as good as possible on each section and leg. The jumpers are critical length. Make sure they are the ones for that duplexer. and are for or near the frequency of operation. Make sure they are not damaged.

The BNC connectors are problematic, if silver, they may be tarnished needing clean up. If nickel plated watch out for green corrosion. The connectors may need to be tightened up so that the shield and center conductors mate tightly.

Who tuned the duplexer? Is it tuned properly?

But before that, let's go back to the high VSWR which was the original complaint. And try these things.

Connect the wattmeter directly to the transmitter and feedline. Send an assistant to the roof with a 10 foot section of PVC pipe. From that distance have the assistant lightly thump and push the antenna radome as well as the feedline connection and watch for any abrupt VSWR changes.

If the antenna checks out, reinstall the duplexer, send the assistant out to town to transmit while you enable and disable the transmitter in a crude desense test. While doing this, manipulate the duplexer cabling and note any changes.

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I will give this a shot when I have the resources I need to make this happen. Right now the duplexer it out of the loop but I could test it out just as a base station and see how it does. I did put my Wattmeter on there and was getting essentially no reflected power.

 

[RFI-EMI-GUY]

I will give this a shot when I have the resources I need to make this happen. Right now the duplexer it out of the loop but I could test it out just as a base station and see how it does. I did put my Wattmeter on there and was getting essentially no reflected power.

You need to shake and bake things. Problems like high VSWR don't just go away.

I was working on a new system one time where a particular receiver site would frequently drop out only during working hours. The equipment room was newly constructed mezzanine (made of lumber) within a penthouse area. Whenever the Tech went into the site the microwave link (2Ghz) would go away. The Tech started taking some blame. Turned out that the coaxial cable connector wasn't properly mated and the floor would flex when he walked into the room and the rack would move just a couple millimeters.

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[BlueDevil]

I took the duplexer in to be tuned after suspicion that it might be out of alignment. The service tech said it was and returned the duplexer with the above results.


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[RFI-EMI-GUY]

I took the duplexer in to be tuned after suspicion that it might be out of alignment. The service tech said it was and returned the duplexer with the above results.


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Did he retune it for you? If he was set up to take those measurements, he could easily tune to spec.

Otherwise it would be like having the tire pressure in your tires checked without adding air. Yup, your front tires are indeed low on air.

If those are the results after tuning, the low side TX, rejection is bad, 65 vs 80 target.

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[n1das]

I took the duplexer in to be tuned after suspicion that it might be out of alignment. The service tech said it was and returned the duplexer with the above results.


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65 dB vs a target of 80+ dB is bad.

What type of duplexer is it? Combination pass/notch or notch-only type? How many cans on each side??

 

[BlueDevil]

This is the duplexer that I have been using. The specifications that I posted above are after the duplexer was tuned.

The repeater is now operating much better however I think that I could get more out of it with a better duplexer.

Here are some pictures of the duplexer I am using...

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[N4GIX]

65 dB vs a target of 80+ dB is bad.

What type of duplexer is it? Combination pass/notch or notch-only type? How many cans on each side??

Transmit Target is -81 dBm -> Tuned is -79 dBm (pretty darn close!)

Receive Target is -76 dBm -> Tuned is -65 dBm (not so good)