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Practical benefit of feedline upgrade to improve receive.

easthill900

Newbie
Joined
Jun 6, 2021
Messages
3
Sorry if this is posted in the wrong category. Just found these wonderful forums.

I have a UHF repeater in a rural area with lots of hills/valleys, etc. I’ve located it on a building as close to center and highest of the area I want covered. The area of desired coverage is not large, just about 5 mile radius from the repeater. There is no need for long distance coverage, the focus is local area coverage.

It’s working fine, except there are definitely some spots where I can hear the repeater from a portable but the repeater can’t hear me. (“talk-in” performance). I’m using good antennas on the handhelds (no stubby ones)

I’m using a unity gain antenna because I’m not trying to squeeze all the energy out to the horizon, in fact most of the coverage area is immediately “below” the horizon of the antenna. My feedline is less than ideal, at 3.3dB per 100’ loss and I can’t run less than 100’ due to the building owner’s requirements for where I can put the antenna & equipment.

The repeater and duplexer came tuned professionally, so I do not believe there is any desense occurring from an improperly tuned duplexer.

Voting receivers and whatnot is not an option, I want to keep this project limited to the one repeater location. I’m just trying to optimize the one location as best I can.

Do you think given the above scenario it’s worth spending the money to upgrade the feedline to ½ Heliax and reduce that feedline loss from 3.3dB (53.2% loss) to 1.45dB (28.4%)? Or any other suggestions for system optimization much appreciated.

Thanks!
 

vagrant

ker-muhj-uhn
Joined
Nov 19, 2005
Messages
1,991
Location
California
I have a low level UHF repeater. I initially used LMR-400 and a dual band antenna, as that is what I had on hand. I switched to LDF4-50A and a mono band antenna I cut/tuned. The TX/RX difference was significant. Outdoors 4 miles away was sort of working. After the switch, I could be indoors at the same 4 mile location with plenty of walls in between. (My dual band antenna stills sweeps fine and was not the issue)

A duplexer re-tuned/checked on site after install would be fine, but yours was not. Also, are there any other transmitters on site? I use a mobile duplexer (notch filter) but a BpBr duplexer would be essential if installed with other transmitters at a site. You may need additional filtering depending on RF at your site.

What unity gain antenna are you using?
What type of duplexer are you using?
What coax type are you using from the repeater to the duplexer?
 

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
15,858
Location
SNCZCA01DS0
Duplexers need to be tuned on site. They can be sensitive to getting bumped around in shipping.

I agree with Vagrant, knowing what kind of coax you were using would be helpful. "Stepping up" to 1/2" Heliax makes me think you are using something like LMR-400 or less. 1/2" would have been the minimum I would have used in an install with 100' of cable. Probably would have done 7/8", but that may be outside your budget.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
11,158
Location
So Cal - Richardson, TX - Tewksbury, MA
I also agree on the coax upgrade and have seen countless improvements like vagrant mentioned from lowering the feedline loss on a repeater. I would also dump the unity gain antenna and go with some gain and a pattern optimized for your location and terrain. I have several repeaters at my house in hilly terrain and just for fun swapped several antennas around to see the results.

On a UHF amateur repeater I went from a 24ft monster similar to a Diamond X700HNA to a copy of a Diamond X510 to a Sirio 1/4 wave ground plane. The 24ft had the best performance at extreme distance and was useable in some areas 75mi away but was very erratic around my local neighborhoods with hills. The 17ft X510 antenna lost a little at extreme distance but improved local flutter in the hills. The 1/4 wave ground plane fixed most of the local flutter but reduced range to maybe 15mi. I didn't have anything on hand to bridge the gap between a 1/4 wave and a 17ft job and I think an 8 or 10ft tall version would have retained some good distance and would also be satisfactory for the hilly areas around me.

Bottom line is a unity gain type antenna is maybe good for lighting up a building to a couple of stories high or a small size property and you will loose precious range with it. In comparison something with 4 to 6dBd gain and some downward tilt will improve your range tremendously and not loos much if any in nearby valleys. Reducing your feedline loss from 3.3dB to less than 1dB will give you an instant 2.3dB "gain" in both transmit and receive. My personal goal for any repeater anywhere is to keep the feedline loss to .75dB maximum, which would be about 75ft of 7/8" Heliax or about 50ft of 1/2" Heliax on a UHF repeater.

+1 on checking and tuning a duplexer on site and there are specific ways to test the entire repeater system for desense, just ask if you want to know how to do that.
 

easthill900

Newbie
Joined
Jun 6, 2021
Messages
3
Thanks for the replies. I'm using an RFS mobile notch duplexer. 1dB insertion loss. I was considering switching it to a BpBr like a Q3220E, but that only nets me .2dB. There are no other nearby transmitters, very rural area with nothing around.

Yes, I'm using an LMR400 flavor. I didn't mention it because I'm aware it's frowned upon. At least I know that now, didn't when I did the install last year.

The current antenna is fiberglass omni pre-tuned uninity gain and had 1.1 SWR on my transmit frequency as checked when installed on the mast.

Good point about the duplexer getting bumped around in shipping.

I would like to know how to check the entire repeater system desense.
 

Ubbe

Member
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Sep 8, 2006
Messages
5,821
Location
Stockholm, Sweden
I would like to know how to check the entire repeater system desense.
Connect a 30 or 40dB attenuator to a T adapter and then a 50 ohm dummy load, so that the TX/RX goes to the dummy load without any loss and the 30-40dB attenuator goes to a signal generator. You also need a way to listen to the direct audio from the receiver.

Block the transmitter so it cannot transmit and set the signal generator to a low level that has some noise just at the point where it starts to crackle and not give a clean white noise, I.E. 12dB Sinad or 20dB S/N. Key the transmitter and the sensitivity should not change or you could have some duplexer problem or an unclean transmitter.

Disconnect the dummy load and connect the coax from the antenna. The sensitivity should not get worse or you have interferencies in the air.

Key the transmitter and the sensitivity should not change or you probably have some coax or antenna issues.

My experiance are that about half of the site installations I've done have required fine tuning at the site, if you do not accept some degradation.

/Ubbe
 

prcguy

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Jun 30, 2006
Messages
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So Cal - Richardson, TX - Tewksbury, MA
Very good info Ubbe. My version of this is to use a directional coupler, preferably high power on the duplexer antenna port but a low power version between the duplexer and receiver will work. You orient the directional coupler to inject a signal into the receiver and a coupling value of 20 to 40dB is fine. Lately I've been using a special directional coupler element for a Bird 43 wattmeter that gives a convenient point to inject or measure signals and you can change the direction of the coupler by rotating it. Plus it can be placed in a high power transmit line.

First test the exact sensitivity of the repeater receiver with a calibrated signal generator right into the repeater receiver and measure for 12dB SINAD. Then connect the duplexer with the signal generator at the antenna port and measure receiver sensitivity again with the transmitter off. It should be exactly worse than the first measurement minus duplexer and associated cable loss.

Next insert the directional coupler and measure sensitivity again, it should be exactly worse than the last measurement minus the coupler loss rating. With the signal generator on at a level that produces 12dB SINAD, turn on the transmitter. You should see no change in receiver sensitivity. If you do the duplexer may be out of alignment or not good enough for your system. If this test passes ok with no receiver desense connect the antenna and measure receiver sensitivity again.

What this does is measure your site noise floor and its possible the sensitivity gets worse by 1dB or as much as 5dB or more depending on how many other transmitters are active at your site. I've been to sites with a noise floor that degrades UHF reception by more than 10dB due to dozens of UHF TV transmitters at the same site running millions of watts. Note the amount of degradation, re-cal for 12dB SINAD and turn the repeater transmitter on. You should have no degradation from the last measurement and if you do it could be from IMD created when your transmitter mixes with others at the site or you have rectification from a non linear junction like a corroded part in the feedline or antenna or rusted tower parts near your antenna or similar.

If all of your equipment is in good shape and properly aligned and no IMD problems you should be operating at peak performance, congratulations! More often than not a new repeater installation will fail somewhere in these tests and you have to find the cause and fix it. If you don't run these specific tests you will have no clue how your repeater is performing and shame on you for not running these tests.

Connect a 30 or 40dB attenuator to a T adapter and then a 50 ohm dummy load, so that the TX/RX goes to the dummy load without any loss and the 30-40dB attenuator goes to a signal generator. You also need a way to listen to the direct audio from the receiver.

Block the transmitter so it cannot transmit and set the signal generator to a low level that has some noise just at the point where it starts to crackle and not give a clean white noise, I.E. 12dB Sinad or 20dB S/N. Key the transmitter and the sensitivity should not change or you could have some duplexer problem or an unclean transmitter.

Disconnect the dummy load and connect the coax from the antenna. The sensitivity should not get worse or you have interferencies in the air.

Key the transmitter and the sensitivity should not change or you probably have some coax or antenna issues.

My experiance are that about half of the site installations I've done have required fine tuning at the site, if you do not accept some degradation.

/Ubbe
 

easthill900

Newbie
Joined
Jun 6, 2021
Messages
3
Thank you Ubbe and prcguy for the detailed instructions. I look forward to running these tests. I'll find a friend with a calibrated signal generator. I'll need to purchase a T adapter and attenuator or directional coupler. I found some threads referencing the Iso-T RFA-4059-A.
 

Ubbe

Member
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Messages
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Location
Stockholm, Sweden
I'll need to purchase a T adapter and attenuator or directional coupler. I found some threads referencing the Iso-T RFA-4059-A.
Directional coupler and the RFA-4059 are too expensive for private use, but I use both at work. Go with a standard T-connector and a 40dB inline attenuator and if your friend can supply a dummy load then you do not need to get one of those and perhaps the signal generator are a duplex communication analyzer that have both power meter/dummy load connectors and a seperate signal generator connector. Remember that a 50 ohm 40dB attenuator will take half of the transmitters power and needs to handle that power level.

/Ubbe
 

prcguy

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Jun 30, 2006
Messages
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Location
So Cal - Richardson, TX - Tewksbury, MA
Too expensive? Ebay has lots of them very cheap. Here is a 20dB model that covers 10 to 1000MHz for $24. You can find high power version for not much more. Merrimac Microwave 20dB Directional Coupler CRM-20-500 RF 10MHz-1GHz SMA | eBay

Directional coupler and the RFA-4059 are too expensive for private use, but I use both at work. Go with a standard T-connector and a 40dB inline attenuator and if your friend can supply a dummy load then you do not need to get one of those and perhaps the signal generator are a duplex communication analyzer that have both power meter/dummy load connectors and a seperate signal generator connector. Remember that a 50 ohm 40dB attenuator will take half of the transmitters power and needs to handle that power level.

/Ubbe
 

12dbsinad

Member
Joined
Mar 15, 2010
Messages
1,438
Why not just do a very basic desence check to start? All you need to do is inject a weak signal into the receiver while listening to a local speaker from the receiver. You can then fire off the transmitter and see if you hear any degradation to the signal. Do this test with everything hooked up as if it was operational. You can keep the transmitter keyed for a few minutes to see if anything changes. Immediately getting signal degradation everytime you key the transmitter means you have system desence and that will for sure affect your range. If you can get to the bottom of the antenna mast bang it with a rubber mallet. If there is anything loose or rubbing, bad jumpers, loose antenna guts, etc you will immediately know.

Personally if it were me, I'd dump the mobile duplexer for a good BpBr duplexer. They typically don't like high power and sometimes if you just tap on them they can go haywire, especially the cheap Chinese ones. Get rid of the LMR coax for hardline and insure all jumpers in between are double shielded or hardline type.
 

Ubbe

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Messages
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Location
Stockholm, Sweden
Too expensive? Ebay has lots of them very cheap. Here is a 20dB model that covers 10 to 1000MHz for $24. You can find high power version for not much more.
I'll need one for personal use but haven't seen any cheap ones but I will look some more. That Merrimac one are just a 5W device and 20dB are only 100 times and I'll need 50W and at least 30dB or 40dB at 25MHz-500MHz to protect the signal generator.

/Ubbe
 

prcguy

Member
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Jun 30, 2006
Messages
11,158
Location
So Cal - Richardson, TX - Tewksbury, MA
The best directional coupler I've found for placing in a high power path is a rotatable coupler slug for Bird 43 wattmeters. There were a few of these on Ebay cheap so I got one and the BNC connector was damaged and the seller sent me a replacement and said to keep the bad one. This is the bad one after I reworked it. I don't remember the coupling value but it was in the 40dB range and covered a wide frequency spectrum.

Bird also sells a similar unit that has fairly small frequency ratings like 100 to 250MHz, etc, but that's only if you want the coupling value calibrated. You can use one 100MHz to 1GHz as an uncalibrated coupler for injecting into the antenna line of a repeater to make desense measurements.

sample slug.JPG

I'll need one for personal use but haven't seen any cheap ones but I will look some more. That Merrimac one are just a 5W device and 20dB are only 100 times and I'll need 50W and at least 30dB or 40dB at 25MHz-500MHz to protect the signal generator.

/Ubbe
 

xmo

Member
Joined
Aug 13, 2009
Messages
192
I recommend that you consider replacing the antenna with something like a DB408. With a gain of over 6dB compared to a unity gain (0dBd) this will give your portable talk-in the equivalent of increasing from 5 watts to 20 watts.

Replace the feedline with Heliax at the same time and you will have even more benefit.

Conducting desense and effective sensitivity tests will give you a more complete understanding of your situation
 
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