GMRS repeater without duplexer, antenna distance

[m4f1050]

I have setup a repeater but it’s 25 watts.  I don’t want to use a duplexer so I can get the most out of the TX power.  Question is, how far apart and what height distance should I place my antennas in order for them not to interfere with one another?  25 watts TX.

[gortex2]

What repeater ? What Radios ? What antenna ? What feedline ? 

Normally by the time you buy feed line to get the distance you spent more than a cheap duplexer. If you are doing sperate antennas you will need as much vertical separation as you can get. Most dual antenna systems use filtering on the RX side of the system to notch out the TX frequency. 

[tweiss3]

You can use this to get a good estimate https://extapps.commscope.com/calculators/qvisolation.aspx

 

Keep in mind, even a cheaper mobile duplexer can achieve 70db isolation, better on decent mobile duplexers, and you will likely need greater than 60 feet vertical, one directly above the other, to get it working anywhere close to the performance of a duplexer.

[Radioguy7268]

If you're focused on getting 'max power' out for Transmit, then I need to ask, what are you talking to on the other end?  If you've got 4 or 5 watt portables, transmitting at 25 watts (and setting aside any antenna system gain/loss for a moment) creates an imbalanced system at best - and generates a bunch of unnecessary interference to your own receiver at worst.

In other words, if you can transmit out 15 miles, but your portables can only talk back in from 5 miles, what have you accomplished? Balance to the system is the key.

As gortex mentioned, filtering the receive side is probably more important, but most advertising will focus on the 'horsepower' number, because that's what sells. Any rookie will pick a 50 watt transmit over 25, because more MUST be better. It's just one number, and transmit power rarely tells the whole story, especially when it comes to repeater system performance. Losing 1.5 or 2 dB to gain 60 or 70 dB of isolation is a pretty good trade off, and if you put a bandpass cavity tuned to the receive frequency on the receive side of that cheap duplexer, you're going to pick up even more isolation without increasing any losses on the transmit side.

I can tell you that my basic rule of thumb for a generic repeater is to shoot for double the power of the portables being used. You're well above that at 25 watts, and I'm not aware of too many low cost repeaters that will do an actual 100% duty cycle at 25 watts. There are plenty that will approach 100% duty at 10 watts with a good cooling fan.

Until you've got a quality receiver with decent filtering on the receive side of the system, it really doesn't matter how much power your transmitter has. Isolation, selectivity, and sensitivity are all more important than raw transmit power when you're looking at a repeater system.

[Lscott]

Forgot to mention that more TX power is going to require more isolation on the RX input side. A few people have experienced RX desense with the power being too high. They did what would at first appear to be counter intuitive, they decreased the TX power.

[Lscott]

The attached files give some info on isolation requirements and using cavity filters to get it.

Calculating Required Isolation.pdf Duplexers and Repeaters.pdf Understanding Maintaining and Re-Tuning Antenna Duplexers.pdf

[m4f1050]

Thank you for the clarification.  I will be buying the duplexer then.  Makes total sense, that way I can go higher on the antenna and achieve more reach at, let’s say, 40ft @ 15 watts notched vs dual antenna, having one lower to isolate and having to run expensive coax to not even be close to “acceptable loss”

 

My Vertex VXR-7000 came with its own duplexer and I now understand why.  Cable, antenna and wire expense (distance in wiring) vs the duplexer, it’s a no-brainer to go with duplexer.

[Lscott]

9 hours ago, m4f1050 said:

Thank you for the clarification.  I will be buying the duplexer then.  Makes total sense, that way I can go higher on the antenna and achieve more reach at, let’s say, 40ft @ 15 watts notched vs dual antenna, having one lower to isolate and having to run expensive coax to not even be close to “acceptable loss”

 

My Vertex VXR-7000 came with its own duplexer and I now understand why.  Cable, antenna and wire expense (distance in wiring) vs the duplexer, it’s a no-brainer to go with duplexer.

Disclaimer, I haven't purchased anything from this company but they have been around for a long time and seem to have reasonable prices.

They have a verity of cheap notch type duplexers. Might be worth buying one to experiment with. Then later go with some notch/band-pass types for more isolation.

https://www.409shop.com/409shop_shopcat.php?&usercat=4942

http://www.repeater-builder.com/antenna/pdf/chinese-mobile-duplexer-measurements.pdf

SGQ-450D-N.pdf

[SteveShannon]

18 hours ago, Lscott said:

The attached files give some info on isolation requirements and using cavity filters to get it.

Calculating Required Isolation.pdf 136.11 kB · 1 download Duplexers and Repeaters.pdf 250.6 kB · 1 download Understanding Maintaining and Re-Tuning Antenna Duplexers.pdf 141.62 kB · 0 downloads

So far I’ve only read the article “Duplexers and Repeaters” but it was a good article, written at a level that anyone hoping to establish a repeater should be able to understand.

[KAF6045]

11 hours ago, gortex2 said:

What repeater ? What Radios ? What antenna ? What feedline ? 

Normally by the time you buy feed line to get the distance you spent more than a cheap duplexer. If you are doing sperate antennas you will need as much vertical separation as you can get. Most dual antenna systems use filtering on the RX side of the system to notch out the TX frequency. 

Heh...

Might be cheaper to obtain some sort of water-tight housings and mount the radios only a foot or so from each antenna... And put together a LONG wiring/power harness with which to connect/control the radios.