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Author Topic: 2m VHF Duplexer for portable repeater  (Read 2499 times)

KG9ZTX

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2m VHF Duplexer for portable repeater
« on: December 04, 2018, 03:58:54 PM »

Currently I have a UHF repeater and UHF Duplexer setup in a trailer for off roading/camping groups in remote locations where there are few to no repeaters or cell service. 50watts

The repeater can be setup to repeat on 2m VHF bands. Also 50 watts, However, I would need to get a new Duplexer. The cost of a Duplexer with a 6khz separation is they run $1,000 or more.

The reason for the desire for 2m band is many of our participants only have vhf radios. We can’t require them to get dual band radios, even though they aren’t that expensive.

So two questions for two possible options...

Option 1.... I add a second antenna. I can get the same antenna I am currently using. The question is, if they are on the same horizontal plane, what distance should the two antennas be away from each other. The first antenna is a dual band antenna on a 12, telescoping pole. The antenna is 6’ long, so putting it in the same vertical plane but different horizontal plane I don’t think would work because their would not be enough height to get above trees etc...

Option 2. Get two 2m  Duplexers, with one tuned for recieve the other tuned for transmit. These two duplexers are cheaper ($200 each). Ie antenna feed line feeds into a splitter, with one going to the receive Duplexer, the other too the transmit Duplexer. Then a line from the transmit Duplexer to the transmit radio and a line from the receive Duplexer to the receive radio. Ie remember two separate duplexers.

Or are there other cheaper alternatives. Seems for $1,000 one can get two really awesome antennas, poles and lines and get better quality transmit receive the. One antenna with a expensive Duplexer.

Remember this is also for portable trailer use, so space for equipment is at a minimum.

Yes I know about frequency assignments, the legalities etc etc etc... lets keep this technical please.

 Thanks
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N8EKT

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RE: 2m VHF Duplexer for portable repeater
« Reply #1 on: December 04, 2018, 05:25:31 PM »

Vertical separation of 50 feet between TX and RX
antennas will give nearly 60db of isolation and you will need at least 80 even with a GE master 2.
Modern repeaters need far more isolation than that.
Horizontal spacing needed for 60db would be about 800ft.
With 60db of isolation you might get by running 1 watt
or so without too much desense.
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K5LXP

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RE: 2m VHF Duplexer for portable repeater
« Reply #2 on: December 04, 2018, 08:24:32 PM »


There's physical separation and frequency separation.  If you run something greater than a 600kHz split then the level of required isolation is reduced.  How much reduced depends a lot on your equipment.  Being a portable/temporary setup you could make the split whatever you want, and program the field units accordingly.  If your input was 144 something and your output 147 something, you could have upwards of 3MHz of separation.  Other than being mindful of not stepping on other users like satellite or weak signal subbands, as long as you're not interfering with anyone you can pick your inputs and outputs.  Sounds like where you're going there wouldn't be much frequency congestion.

Another option is split site, with a UHF link in between.  You end up with a bit of coverage disparity (TX coverage a little different than RX) but I've done it successfully.

Mark K5LXP
Albuquerque, NM
 
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KG9ZTX

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RE: 2m VHF Duplexer for portable repeater
« Reply #3 on: December 04, 2018, 08:46:41 PM »

Vertical separation of 50 feet between TX and RX
antennas will give nearly 60db of isolation and you will need at least 80 even with a GE master 2.
Modern repeaters need far more isolation than that.
Horizontal spacing needed for 60db would be about 800ft.
With 60db of isolation you might get by running 1 watt
or so without too much desense.

Well Darn, that’s not feasible for what I need.

Could a radio even handle 800’ of feedline and still have any valuable power left over to transmit?

And antenna height for vertical separation be unfeasable from a portable/mobile point of view. At least not without a dedicated antenna/mast trailer.
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KG9ZTX

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RE: 2m VHF Duplexer for portable repeater
« Reply #4 on: December 04, 2018, 08:52:54 PM »


There's physical separation and frequency separation.  If you run something greater than a 600kHz split then the level of required isolation is reduced.  How much reduced depends a lot on your equipment.  Being a portable/temporary setup you could make the split whatever you want, and program the field units accordingly.  If your input was 144 something and your output 147 something, you could have upwards of 3MHz of separation.  Other than being mindful of not stepping on other users like satellite or weak signal subbands, as long as you're not interfering with anyone you can pick your inputs and outputs.  Sounds like where you're going there wouldn't be much frequency congestion.

Another option is split site, with a UHF link in between.  You end up with a bit of coverage disparity (TX coverage a little different than RX) but I've done it successfully.

Mark K5LXP
Albuquerque, NM
 

Well I have to maintain the standard 600 kHz differenc, frequencies for the areas that we frequent need to be assigned. So the 5mhz split that mobile 2m duplexers have for $200 obviously won’t suffice.

That’s why I asked about using two duplexers tuned to the transmit receive frequencies.

I know two antennas can be used in place of using a Duplexer, so just wondered the amount of physical separation of the antennas either vertically or horizontal would be needed for the 2m band.

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KG9ZTX

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RE: 2m VHF Duplexer for portable repeater
« Reply #5 on: December 04, 2018, 08:53:41 PM »

This is just simple analog voice repeater, no digital or aprs or anything else.
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KA5IPF

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RE: 2m VHF Duplexer for portable repeater
« Reply #6 on: December 05, 2018, 03:45:34 AM »

You might try a simplex repeater. One antenna and no duplexers. It receives and records incoming audio. When the incoming station unkeys it plays it back and waits for the next incoming signal. Otherwise you need a set of 2m duplexers made for 600kHz split. Be much cheaper to buy a bunch of dual band radios and loan them to those who don't have one.
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W9IQ

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RE: 2m VHF Duplexer for portable repeater
« Reply #7 on: December 05, 2018, 03:56:03 AM »

You might try a simplex repeater. One antenna and no duplexers. It receives and records incoming audio. When the incoming station unkeys it plays it back and waits for the next incoming signal. Otherwise you need a set of 2m duplexers made for 600kHz split. Be much cheaper to buy a bunch of dual band radios and loan them to those who don't have one.

One of the disadvantages of this type of repeater, beside being annoyingly slow, is that "doubles" happen very easily and it can take minutes to untangle the stations.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K5LXP

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RE: 2m VHF Duplexer for portable repeater
« Reply #8 on: December 05, 2018, 03:47:01 PM »

Well I have to maintain the standard 600 kHz differenc, frequencies for the areas that we frequent need to be assigned.

That's not true.  The only rule you need to observe is that you don't interfere with a coordinated user.  I've put up both coordinated and uncoordinated repeaters over the years.  Never a problem.

Quote
just wondered the amount of physical separation of the antennas either vertically or horizontal would be needed for the 2m band.

Further than practical...


Mark K5LXP
Albuquerque, NM
 
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KA5IPF

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RE: 2m VHF Duplexer for portable repeater
« Reply #9 on: December 06, 2018, 08:03:38 AM »

Here is a decent paper on separation of antennas. They don't go into VHF-VHF that I saw in a brief perusal but will give you some ideas.

http://service.content.sydney/standards/Antenna%20Placement%20Guideline.pdf
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KA4GFY

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RE: 2m VHF Duplexer for portable repeater
« Reply #10 on: December 06, 2018, 11:58:54 AM »

Have you looked at a mobile duplexer? 

Bridgecom makes one for a little over $300.  That's less than the cost of option #2 and it will work better.  Not only that, it will be far less complicated and nowhere near the amount of loss in the cabling with 2 duplexers.   

Our radio club bought one for a repeater in a fixed location a little over a year ago and it works pretty well.

73,
Rich, KA4GFY
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KG9ZTX

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RE: 2m VHF Duplexer for portable repeater
« Reply #11 on: December 06, 2018, 11:31:20 PM »

It looks as though A 2m mobile repeater is unfeasible without using a Duplexer.
With maximum antenna height being only about 30’ getting enough vertical separation isn’t feasible. And horizontal separation at 800’ even when your out in the middle of knowhere isn’t feasible as 800’ of feed line would be needed. And even then that would outweigh the cost of a Duplexer.

Someone might ask why not use an existing Duplexer? Two reasons. In some of the ares our group goes. There are no repeaters.

In some areas while there are repeaters on the mountaintops, there aren’t any in the deep canyons and thus the height of the mountain is pointless. Can’t reach it anyways.

When we are in a canyon, the idea is to setup the repeater on the canyon rim.

Which then leads to my next question. How close can you setup two crossband repeaters?
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WB0DZX

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RE: 2m VHF Duplexer for portable repeater
« Reply #12 on: December 10, 2018, 04:53:18 PM »

You have considered and asked much! It would be helpful if you would indicate the depth of the canyon, the distance of the forays, and whether any forays will be on top.
 
Canyons are usually too irregular to enable acceptable radio coverage from the top or the bottom. They are like caves without roofs. The material (rock) is resistant to the coverage filling multipath that cities have. Even the water surface is not helpful. The reflections and refractions are not there. You would need lots of power and/or height to achieve radio line of sight and significantly counteract the losses. The most enhancement (however little) will be from fixed equipment on top. The BEST enhancement (however more so) may be from MOBILE units on top travelling along with the pack down below (if possible), even considering not using the tall masts.
 
Regardless, ALL users must be licensed amateur radio operators. Stay away from weak signal, satellite, simplex, etc, frequencies. "Coordinate" all frequencies with the affected ham radio frequency coordinator(s) because you never know when someone coordinates a permanent system within range which may or may not be on the air. They'll want to know where skip is coming from if there's nuisance interference and the callsign of the repeater when it is being identified.
 
If the treks are short, use only enough power to reliably maintain communications. Using lower gain, even unity (0dB) "gain," antennas on top will help coverage in the canyon at the expense of range on top. Those two rules will also reduce interference within the system.
 
Everything is SO much simpler and less expensive at the fixed site if everything takes place on UHF. Since not all field radios have UHF, you desire VHF, AND 600 kHz offset (hopefully NOT the 6 kHz in your post!) at that. Not only that, but you desire the cost to be far less than $1000USD.
 
From a KISS standpoint, you might consider using two radios capable of crossband repeat as you considered -- but with the following differences:
 
Radio #1: VHF receive on the system input frequency F1. Crossband UHF transmit link F3 to the system output site. Distance separated with the output transmitter power on F2 kept low enough to keep the output transmitter from desensing the receiver too much. There will be an acceptable amount of desense. Preferably the VHF receive frequency F1 should require Tone A. The UHF link on F3 will avoid the need for 800+ feet of transmission line. It only needs to transmit in one direction! A crossband mobile diplexer (receive and/or transmit simultaneously on two different bands - not a duplexer which is single band only) is smaller than the radio and is inexpensive or comes with the radio. Single-band or dual-band mobile antennas with a ground plane are inexpensive. Diplexers are installed at the radio and usually are not dust and moisture resistant as the radio may be.
 
NOTE: For best results, with different height masts, Radio #1's antenna (the F1 input) should be at the highest height. Radio #2's F2 output power would help counteract the lower height difference. Most of the isolation will be from horizontal separation. Vertical separation isolation generally occurs when one antenna is above the other for the distance required to achieve the required isolation.
   
Although the link F3 could easily be miles in radio line of sight without beams, the F1/F2 coverage and/or range disparity will probably be unacceptable at that distance.
 
Radio #2: Receives the UHF link F3 signal from the other site and crossband repeats it on the VHF system output frequency F2, preferably with Tone B. VHF receive at the other site will still suffer some desense, but noises won't show up on the link because the tones are different. Tones are categorized in groups. Research so you do not pick tones in the same group. Normally this is done for reducing interaction between co-sited public-safety/commercial systems. Tones from different groups, in your case, will further reduce system noises. Note: Radio #2 will not interfere on the link frequency F3 with Radio #1 IF people using simplex on F2 use Tone C. Therefore Radio #2 will not and should not ever transmit on F3 - if the system is configured properly.
 
Hopefully, I've remembered how to do this much less expensively than I normally did decades ago (not necessarily for ham radio).

Or you can get fancy for a little over $400USD:

https://qrznow.com/portable-repeater-controller/amp/

Note that filtering and radios are also needed for the fancy system.

Mike WB0DZX
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LA9XNA

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RE: 2m VHF Duplexer for portable repeater
« Reply #13 on: December 12, 2018, 10:24:25 AM »

One option is to use two separate radios on your repeater.
By setting up a TX site and a RX site a few 100 m apart you will get some of the isolation required by the filter.
Put in two link radios in UHF and some small beam (2 or 3 element).
By doing this you can get away with cheaper filters.
Turn down the power on the radio so that you dont run more power than required. 50W is a lot of power.
Another option is to go for 10m radios and get away form the whole repeater.
The TYT TH-9800 got all bands from 10M to 70cm ecxept the US 1,25m.
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W1BR

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RE: 2m VHF Duplexer for portable repeater
« Reply #14 on: December 13, 2018, 08:22:16 PM »

Vertical separation of 50 feet between TX and RX
antennas will give nearly 60db of isolation and you will need at least 80 even with a GE master 2.
Modern repeaters need far more isolation than that.
Horizontal spacing needed for 60db would be about 800ft.
With 60db of isolation you might get by running 1 watt
or so without too much desense.

Well Darn, that’s not feasible for what I need.

Could a radio even handle 800’ of feedline and still have any valuable power left over to transmit?

And antenna height for vertical separation be unfeasable from a portable/mobile point of view. At least not without a dedicated antenna/mast trailer.

Why would you even consider 800 feet of feedline???????   The only option is to keep the transmitter and receiver at their respective antenna locations, and just run a control and audio cable link between them.  I'd even wonder if some sort of short range WiFi or BlueTooth connectivity could  be used for voice and control?
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