2 Repeaters & One Antenna

[taco6513]

I have reached out to Bridge Comm. They have a video using 3 duplexers to combine 2 repeaters to one antenna. I have failed to get a response from them at this time.

Who would be able to help with this type of setup?

I do have an application: One GMRS Repeater on Channel 21 and One 70cm Repeater on a yet to assigned pair.

The split should be about 18 Mhz between pairs. 

Bridge Comm is using 2 4 cavity duplexers and one 8 cavity duplexer.

I have been researching this and found RX TX Systems has much information on this subject. However they use a combiner set up.

I plan to reach out to Bridge Comm again. Any help would be great. Also don't think this matters the 70cm repeater will be DMR.

Thanks

KI5GXD

WRCW870

  

[tweiss3]

Do you have a link to the video? 

Most transmit combiners are not much different from duplexer cans. I'm not 100% sure, but I would think that for a two repeater system, you would need a 4-channel https://www.sinctech.com/collections/cavity-combiner-1/products/combiner-4-ch-8-5-cavities-dual-stage-7-16-din-406-512-mhz. 

[gortex2]

It can be done but not cheap. We had a similar setup at a tower site. EMS Med system (462.xxx) with a 451.xxx channel. It was a mix of duplexer and combiner. No the cheap flatpacks. Basically 462/467 was on one duplexer (TX/RX 6" cavity) and 451/456 on another duplexer. Both antenna leads came into another TX/RX duplexer with notch/pass filters. It was very tight and 451-456  was about 80db out of the 462-467 channels. This was all custom built by TX/RX. No clue what it cost but just the 6" cavities (6 per duplexer), can't be cheap. Although it worked ok, the antenna swept about 460 so worked better on the 462 channel. 

I would think the bigger issue with your idea is getting an antenna to tune up good on 462 and 440. Thats a pretty big split. 

[taco6513]

Link to video: 

The antenna is a DB420B. The local ham club is using a DB420B on the 70cm machine. Frequency is 444.525 and it has been working fine for years.

[tweiss3]

It's on their website: https://www.bridgecomsystems.com/collections/uhf-duplexers/products/bridgecom-bcd-combiner-duplexer

Quote

In this video we show a system built for a customer who required two #repeaters to operate on one antenna.

This system was built using two BCR-40U, 2 BCD-440250 Notch Style duplexers and one BCD-2x440150 Band Pass/Band Reject duplexer.

The system requires a 2.5 MHz split between combining frequency pairs.

(TX Freq 1) +- 2.5MHz (TX Freq 2)

(RX Freq 1) +- 2.5MHZ (RX Freq 2)

You must use  BCD-440250 duplexers with this combiner! This will not work with the BCD-440 Duplexers! 

So it appears they used two duplexers then added in the wide notch "combiner" which would balance the system and maintain 50 ohms.

Here is their diagram: 

 

This group of hardware will not work for a ham repeater + gmrs repeater. This way only worked because the two repeaters were close enough to each other the wide notch duplexer could pass two TX on one channel and two RX on the other.

 

[JohnE]

at the very least there should be an isolator on both transmitters for IM , mixing and protecting the PA's from a bad antenna.

Did this in the 80's w/2 remote base's 700kC's apart. Much easier w/frqs that close. I think the total loss was 3dB overall.

[PartsMan]

Would the power loss be horrible on this setup?

[tweiss3]

9 minutes ago, PartsMan said:

Would the power loss be horrible on this setup?

Based on the specs from bridgecom, its 4dB (60%).

[gortex2]

All depends on design. TXRX makes some great systems for filters and combiners. As said the frequencies were close enough but just far enough apart. And one frequency was out of the GMRS section. There are many places as TXRX and Filtronics that build this stuff but its costly to do right. 

[tweiss3]

It's $4k for the Bridgecom, Sinclair makes a nice cavity 4 channel that would work in this situation AND in a GMRS + ham version for $6k. Cost isn't everything, but the insertion loss is better with Sinclair, isolation is better, power handling is better, and its much more simplified.

[WRKC935]

The issue with this design is receive attenuation. 

The design is a window filter and then a split duplexer.  The Duplexer is going to present 2dB of so of loss and the window filter another 3dB of loss.  For transmit, that's significant but will not effect the TX as much as you would think.  But on the receive side, it's gonna make a repeater receive rather deaf.Another option is to use the two antenna's and a RX multicoupler and TX combiner.  OR an RX multicoupler and the transmit side of this setup.  The RX multicoupler is gonna have a window filter to pull out the RX and an amplifier to compensate for the loss in the window filter.  Sure it takes an additional antenna, but you can stick 8 transmitters (at a higher loss figure) on a single antenna.  And the combiner can be used for ham and GMRS and of course any commercial LMR in between.

 

There are some calculations that need to be done so that there is no interference due to mixing in the combiner but those are simple enough calculations to do. 

But it does raise a question.  Can you increase the power output of a repeater past 50 watts to compensate for the losses in a combiner?