To Duplex or not to Duplex

[WRYW415]

As the title says, I'm trying to outweigh the pros and cons of using a duplexer or not for a planned repeater. I see through research that there is a power loss going through a duplexer in one form of another and that's to be expected when trying to tx and rx at the same time on one antenna. Space isn't an issue to run 2 antennas, and I would think that I'd have less loss in both directions doing it this way even with twice the amount of coax. I've read somewhere that the rx and tx antennas should not be on the same horizontal plane. Is this a myth or fact? I can go to 20 different sites and get both answers. How do I know how far apart to put them, or is "as far away from each other as you can possibly get" the theory here? I'd like input from someone who has actually tried this both ways (duplexed or not) and their real world results, and not just opinions. Thanks!

[gortex2]

You need vertical separation for a dual antenna setup. Unless your doing a combiner and receive multi-coupler I don't see the value in the dual antenna setup. A good duplexer will have a small amount of loss and in the end work much better than the dual antenna setup. There is alot more to this discussion than just antenna. The repeater is another big item that needs to be figured out. A repeater made from cheap mobiles will be worse in a dual antenna setup than a purpose built LMR repeater. 

In the LMR world a lot of sites use dual antenna. My one SAR site uses a receive multi-coupler with a DB408 at 35' up a tower. The TX antenna is thru a TX combiner at 15' off the ground. With the filtering on both TX and RX I see no desense at all on 75 watt Quantars on GMRS and our SAR channels. Another site we have an ICOM FR4000 with a BPR duplexer. Out of the duplexer is a 30 watts into 1/2" LDF 150 up the tower. Again no desense at all and there is multiple LMR repeaters at this site. Both sites have great coverage for the area and both serve specific purposes. If I didn't have other SAR LMR frequencies at the first site we would have a duplexer. 

Normally the cost of feedline and antenna will be the same cost of the duplexer. 

 

 

[WRUE951]

i've heard of repeater installs placing the radio at each antenna by way of of running the CAT 6 interface cable between them.  Of course, interfacing a controller in this scenario would be a PITA .. 

[marcspaz]

I can 100% tell you through experience, if you don't use some serious filtering, like found in a duplexer, vertical separation isn't enough unless you plan on using very low power... like 5 watts.

 

I have a portable repeater system the doesn't use a duplexer. At 5 watts, it can have both antennas about 100 feet from each other and it works locally with limited issues. However, if I want to run 50w, or even 200w, I have to separate the two antennas by about 400 to 500 yards to avoid desense issues.

 

I also have a fixed repeater with a duplexer sharing 1 antenna. The duplexer cooks off about half the power, both transmitted and receive. However, antenna placement is going to impact performance more than transmitted and receive losses in the duplexer. You would have to cut your power 4 times to see a single s-unit of loss. 

 

Most people who don't run a duplexer, don't do it for performance benefits, mostly because it's not a noticeable benefit. It's done to save money or to provide rapid diversity in deployment and changing frequency or even bands without having to re-tune or replace the duplexer every time. Such as my portable field unit.

[WRYW415]

17 minutes ago, marcspaz said:

I can 100% tell you through experience, if you don't use some serious filtering, like found in a duplexer, vertical separation isn't enough unless you plan on using very low power... like 5 watts.

 

I have a portable repeater system the doesn't use a duplexer. At 5 watts, it can have both antennas about 100 yards of each other and it works locally with limited issues. However, if I want to run 50w, or even 200w, I have to separate the two antennas by about 400 to 500 yards to avoid desense issues.

 

I also have a fixed repeater with a duplexer sharing 1 antenna. The duplexer cooks off about half the power both transmitted and receive. However, antenna placement is going to impact performance more that transmitted and receive losses in the duplexer. You would have to cut your power 4 times to see a single s-unit of loss. 

 

Most people who don't run a duplexer, don't do it for performance benefits, mostly because it's not a noticeable benefit. It's done to save money or to provide rapid diversity in deployment and changing frequency or even bands without having to returned or replace the duplexer every time. Such as my portable field unit.

Thank you. These are the kinds of things I NEEDED to know!

[quarterwave]

It's not a real question if you intend to run a real, reliable repeater. Some loss is part of the game. Good equipment reduces that issue.

I setup a split repeater once, antennae were separated by 150' horizonally, and about 30 feet vertically. Just a pain in the ass to maintain, and still needed a can on the receive side. 

You don't have to buy a $10k repeater setup, but going cheap will result in cheap operation.  

[Radioguy7268]

The main reason 90% of repeaters use a duplexer is because of ISOLATION, not because of power loss. As others here have said, yes you can work it with 2 antennas, but you will likely still need additional filtering. If all you can measure is wattage & SWR, you are never going to appreciate what true isolation and improved receive sensitivity (and selectivity) can do for your repeater.

How's that saying, when your only tool is a hammer, every problem looks like a nail?  I guess in the radio world, when all you have is a watt meter, every problem looks like a lack of power.

If you have enough RF power to reach out to whoever you want to talk with, but they can't reach you back because your repeater's receiver is being drowned out due to poor isolation (likely from your own transmitter)  - then what's the point?  You've now got a bunch of time and money invested in a one way paging system. The point of having mobile transmitters is to allow them to talk back.

I'd gladly give up 3 dB of transmit power in a duplexer if it offered 100dB of isolation. Most of the affordable compact notch duplexers will only give you about 65dB of isolation with a 5 MHz split. Add a tuned bandpass cavity or two to the receive side, you'll pick up even more isolation - and you'll still be ahead of the 45 or 55 dB isolation of a split antenna system with 20 or 25 ft. vertical spacing.

Oh yeah, and if you do a split antenna system, don't use braided cable for your coax. You'll leak somewhere around 6-10% of the signal with most decent brands - even worse with the cheap knockoffs that use a loose weave for the coaxial braiding. Think about where that 6-10% of your signal is going as it travels up the tower. Even the real Heliax hardline has some RF signal leakage, but it's about as good as you can get in an imperfect world.

[gortex2]

^^^^^ Is 100% accurate. It baffles my mind when folks say they need a 50 watt repeater and all they have are 4 wat handhelds. We used to deal with this in the LMR world all the time. Balance of a system is not only good practice but can help reuse frequencies also. We had multiple factories in one city all using the same frequency. All of them were far enough apart the 5 watt TX repeaters did not interfere with the other sites. 

Our C-C SAR repeater on UHF is set for 10 watts at our antenna on TX. We only have UHF portables and no mobiles. Rarely does one who can talk to the site not hear the site. Even my testing with a mobile was great. When you can see the antenna from miles away no need to have a ton of power. 

In the GMRS world with limited repeater frequencies this is something all should remember. 

[WRYV757]

My opinion is go with the duplexer, its simpler solution. yes there will be loss, a well tuned Q circuit 6 cavity duplexer will be about 2.2 db ish loss at 450-470 mhz. Lets say a 7.4 db gain antenna - 2.2 db = 5.2  so still considerable antenna gain and higher the antenna the better. Its just UHF ....loss is inevitable..you just minimize it as much as reasonably possible.