Is a duplexer required for a repeater

[WRWE744]

Can I set up a repeater using two radios and a control box, or do I need a duplexer?

[SteveShannon]

2 minutes ago, WRWE744 said:

Can I set up a repeater using two radios and a control box, or do I need a duplexer?

You either need a duplexer or two antennas with lots of distance between them. 
The signal transmitted by a repeater must be isolated from the receiver in order for the receiver to simultaneously receive. 

[Lscott]

3 hours ago, WRWE744 said:

Can I set up a repeater using two radios and a control box, or do I need a duplexer?

Short answer most likely yes, with a duplexer. For more info on repeater building go to the link below.

https://www.repeater-builder.com/rbtip/index.html

[Radioguy7268]

Can you?  Yes

Will it work well without spending time, money, and having some advanced radio knowledge?  Probably not.

[marcspaz]

The short answer is, yes... you can use two antennas instead of a duplexer.

 

The long answer, you can, but you need a separate transmitter and receiver that are directly coupled (linked) with some type of cable to manage keying, audio transfer, etc.  (See KG-1000G, for a less than perfect example). Transmitter power levels, antenna types, cable types, etc., will dictate how far apart the antennas need to be, what the elevation differences need to be, and what the gain/sensitivity of the transmit and receive sides should be.

 

If the antennas are not spaced properly, the transmitter can make it so the receiver doesn't hear well, it could cause a transmit loop, and could also damage the receiver. If you make the antennas to far apart, now you're dealing with transmission line losses causing issues instead. 

 

There is some art to it, as well as the science. If you are at a stage in learning where you have to ask this question, chances are you don't want to try setting up a repeater right now.  This is not ment as an insult, either. If you don't know what you're doing, you can waste a lot of time and money before you learn enough to get a good system stood up.

 

I would recommend finding someone local to you who has a good working repeater and is willing to spend some time teaching you about the details and why things are the way they are.

[WRKC935]

Gonna get into the weeds here with an explanation.

First is antenna separation and attenuation.  And an understanding that a duplexer will provide about 90dB of attenuation.

Two UHF antenna's that are mounted VERTICALLY apart on the same tower leg, will provide about 20 dB of attenuation. 

Two UHF antenna's separated by 80 feet horizontally will provide the same 20 dB of attenuation. So if you had a 150 foot tower, and put the receive antenna at the top and the transmit antenna at about 10 feet you MIGHT get 60 dB of attenuation.  Still not as good as a duplexer. 

Now, lets look at the dollars and cents of this.  First off you will need some sort of hardline for an install like this.  And it's gonna be two runs.  Lets say the tower is 30 feet from the building and it's another 20 feet to the radio.  So you will need an extra 50 feet of hard line at 4 bucks a foot.  Four additional connectors at 25 bucks a piece, two for the ends and two for the additional 90 dollar surge suppressor for the feed line.  Then you need two antenna's.  A USED DB-408 is about 150.  So lets do some math.  50 feet of cable and four 25 dollar connectors comes up to 300 bucks.  Now the additional 150 for the antenna, that's 450 bucks plus the surge suppressor and the labor, so 600 bucks for that additional LOW MOUNTED antenna that is gonna talk like crap because it's 10 feet off the ground.  To save you HOW MUCH for a duplexer?  Hell a new pass notch set is 1300 and used they are around 500 or less.  So it's actually CHEAPER to run the duplexer and do it right and have it perform in a manner that would be acceptable.  Yeah, gonna go the route of the duplexer every time.

 

Oh, and you say you are gonna cheap out and run RG-8 or 9913 and not hardline.  Sure... you are gonna run the RX cable down past the TX antenna in the near field of said antenna and expect the shielding from a BRAIDED CABLE to block 100% of the signal in the near field of a transmit antenna.  Sure... have fun with that too.

 

 

[JB007Rules]

I only use folded dipoles on my repeater systems and I find that 50' of vertical separation is good.  So for my Rugged system, I have RX at the top at 300' extending to 320' the TX antenna is at 240' extending up to 250'

 

My my 2 cents

[WRKC935]

On 4/17/2023 at 8:50 PM, JB007Rules said:

I only use folded dipoles on my repeater systems and I find that 50' of vertical separation is good.  So for my Rugged system, I have RX at the top at 300' extending to 320' the TX antenna is at 240' extending up to 250'

 

My my 2 cents

ANd NO window filter on the RX or running the TX into a combiner or other filtering of any kind?

 

[JB007Rules]

For the RX, the receiver multi-coupler is tuned to accept 460 - 470MHz.

The repeater is a Quantar and has a preselector built in to accept 467.575.

Thanks!

[WRKC935]

On 5/22/2023 at 8:30 PM, JB007Rules said:

For the RX, the receiver multi-coupler is tuned to accept 460 - 470MHz.

The repeater is a Quantar and has a preselector built in to accept 467.575.

Thanks!

Seems an odd way to tune a window filter for a RX multicoupler.  And damn wide to boot.

Mine are 466 to 470 for 461 to 465 inputs.  And there isn't any 465 around these parts.  We top out at 464 with any license I have dealt with.

The other window filter (yes I have two) is tuned to 446 to 450.5 for ham repeaters. 

 

And I assume you are on a TX combiner?

 

[gortex2]

Actually 10mhz window is pretty common. Our Trunking system has both 458 and 465 mhz inputs so need the 10mhz window in the TTA and multi coupler to get both splits. When I was with the shop our PMR trunking system had 456, 457 and 465 frequencies. There we had 2 bandpass windows (455-460 and 464-470)

[PACNWComms]

Thank you WRKC935 for explaining vertical separation as well. Many get caught up on horizontal separation of antennas, with zero mention of vertical being an option and often used by commercial users on radio towers. 

For the OP, just get a duplexer, they are fairly cheap now and work better than two antennas, coax, and figuring out where to place them. Keep it simple and you will get better coverage/service with less maintenance and complexity as a result. 

[DirtyCitizen]

if you need something quick and dirty and you're using handhelds, that have a dual channel and vox, you can just use the attachable speaker/mic trucker handles, tape that speake/mic attachment to the Transmit unit with its vox on, separate them by hanging the receive below the transmit unit(I'll put a cheep Abrrreee 42" on the receiver folded up, usually use the molle pouches and some paracord((tied to the molle pouch with a loop in the cord so the antenna stands upright)) Now fill a dirty sock with some sand and get that bad boy up a tree 50-100ft an in an ammo can with some sound insulation and a rubber ducky on the transmit unit in the ammo can with the speaker. make sure everything is secure before the erection....... for a hunt on the mountain, driving cattle or any contact over an obstruction THIS IS THE WAY...lol

[SteveShannon]

43 minutes ago, DirtyCitizen said:

if you need something quick and dirty and you're using handhelds, that have a dual channel and vox, you can just use the attachable speaker/mic trucker handles, tape that speake/mic attachment to the Transmit unit with its vox on, separate them by hanging the receive below the transmit unit(I'll put a cheep Abrrreee 42" on the receiver folded up, usually use the molle pouches and some paracord((tied to the molle pouch with a loop in the cord so the antenna stands upright)) Now fill a dirty sock with some sand and get that bad boy up a tree 50-100ft an in an ammo can with some sound insulation and a rubber ducky on the transmit unit in the ammo can with the speaker. make sure everything is secure before the erection....... for a hunt on the mountain, driving cattle or any contact over an obstruction THIS IS THE WAY...lol

Have you done this before?

[DirtyCitizen]

Saw it done out in West Virginia to coordinate the push on a hunt with everyone because we would loose signal in the valleys.. worked fine they had cheap Tenways and only  one or two were reaching the guy at the cliffs. I made something like this on an antelope hunt west of the bull mountains just to get back to the camp while I was chasing a herd but the area I was heading into was gonna be a pain to get out of if I had actually bagged one that day, but I didn't. I did get a bit lost after dusk and ended up having to call for a pick up about 20mns north of Billings MT I hit the highway after 2-3 miles of hiking.

[JB007Rules]

On 5/25/2023 at 5:08 PM, WRKC935 said:

Seems an odd way to tune a window filter for a RX multicoupler.  And damn wide to boot.

Mine are 466 to 470 for 461 to 465 inputs.  And there isn't any 465 around these parts.  We top out at 464 with any license I have dealt with.

The other window filter (yes I have two) is tuned to 446 to 450.5 for ham repeaters. 

 

And I assume you are on a TX combiner?

 

Negative.  I have my own dedicated TX antenna.  Up here, there are people using T-Band!  (470+ Mhz)... 478.8625 and 481.8625.