Ham Radio & GMRS on same antenna?

[taco6513]

I have been thinking about this for some time now. The ham radio club has an antenna on a 500ft tower. Nice coverage from this location.

The antenna is a DB420 (460-470Mhz) model. They have been using this for some years with no issues. I have been told this is possible.

I have reached out to several places with no answer or just no response.

Ham repeater is 444.250 and the GMRS would be 462.?. This is a 18Mhz spread.

Both repeaters to use there currently tuned duplexers. Combine them with another duplexer that simply blocks the other transmitter.

Is this proper thinking? Just need to protect from transmitter power from each other. I have a DB4076 that I could use for the combiner 

duplexer.

Thank you for your time.

WRCW870

KI5GXD

[SteveShannon]

9 minutes ago, taco6513 said:

I have been thinking about this for some time now. The ham radio club has an antenna on a 500ft tower. Nice coverage from this location.

The antenna is a DB420 (460-470Mhz) model. They have been using this for some years with no issues. I have been told this is possible.

I have reached out to several places with no answer or just no response.

Ham repeater is 444.250 and the GMRS would be 462.?. This is a 18Mhz spread.

Both repeaters to use there currently tuned duplexers. Combine them with another duplexer that simply blocks the other transmitter.

Is this proper thinking? Just need to protect from transmitter power from each other. I have a DB4076 that I could use for the combiner 

duplexer.

Thank you for your time.

WRCW870

KI5GXD

Two separate antennas on the same tower? It’s done all the time, but I don’t believe you’d need a third duplexer. Just be sure to separate them vertically.

[marcspaz]

It is technically possible, but I sure wouldn't do it. There is so much loss running duplexer that the idea of adding a diplexer to share the antenna sounds painful. Like, depending on the quality of parts, your talking about reducing your transmitter and receive signals by 24dB to 30dB. 

 

Real world, one of my duplexers has 10dB of loss and one of my diplexers has 14dB of loss to provide 60dB of separation. With a station that only has about 20w to start would give you 0.3w... one third of a watt is nothing. 

[SteveShannon]

2 hours ago, Sshannon said:

Two separate antennas on the same tower? It’s done all the time, but I don’t believe you’d need a third duplexer. Just be sure to separate them vertically.

Oops - never mind.  I misunderstood the question, even though it was plainly stated.

But really, just do two separate antennas separated vertically. 

[PartsMan]

Bridgcom has done it. Does not look simple to me.

Here is the video. No mention of loss or price.

https://www.bridgecomsystems.com/blogs/bridgecom-tx-rx-blog/two-repeaters-one-antenna-feed-line

[JeepCrawler98]

6 hours ago, taco6513 said:

I have been thinking about this for some time now. The ham radio club has an antenna on a 500ft tower. Nice coverage from this location.

The antenna is a DB420 (460-470Mhz) model. They have been using this for some years with no issues. I have been told this is possible.

I have reached out to several places with no answer or just no response.

Ham repeater is 444.250 and the GMRS would be 462.?. This is a 18Mhz spread.

Both repeaters to use there currently tuned duplexers. Combine them with another duplexer that simply blocks the other transmitter.

Is this proper thinking? Just need to protect from transmitter power from each other. I have a DB4076 that I could use for the combiner 

duplexer.

Thank you for your time.

WRCW870

KI5GXD

It can absolutely work - it's a poor man's way of combining transmitters, but you will need isolators (that you should have anyways) on both transmitters (to prevent spurs) and adequate frequency separation (70cm and GMRS is more than fine) to notch the other transmitters out. Seen it, done it, and would do it again in the right circumstances.

Basically you have two complete, normal, and perfectly conventional BpBr filtered repeater setups, one on 70cm, one on GMRS, you can then use a flatpack (notch/reject-only) as a splitter to notch the GMRS transmit out of the 70cm pass on the flatpack, and vice versa for the GMRS side (Notch out 70cm transmit). You will have extra loss from the flat pack (about a dB, if not less since the separation is large), but it's not massive.

Feedline becomes even more important as you now have twice the power making noise on the coax, I'm assuming a 500 ft run has heliax already. Also keep in mind your reject duplexer will need to be rated for the sum of transmitter power. There will also be receiver losses, but the band pass filtering on the BpBr duplexers should be minimizing that to be almost negligable.

Nothing complicated about it, just an extra bit of math and an extra component to tune.

Edited January 23, 2023 by JeepCrawler98