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Repeater antenna query

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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bellersley
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Repeater antenna query

Postby bellersley » Tue Oct 04, 2005 3:47 am

Hi gang, quick repeater/antenna question.

The answer to this is probably yes, but. If I were to set up a UHF repeater with an omni antenna on the TX but a Yagi type antenna on the RX, assuming both the TX and the RX lines went through duplexers like usual, would there be any issues I need to worry about?

Reason being, the site in question has very good TX range and can be heard over a large area, but reception to the repeater site isn't very good in the downtown core, but if I use a Yagi antenna, I can bring up very weak signals to something very useable, while still maintaining good coverage over the rest of the required area.

Thanks!

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chartofmaryland
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Postby chartofmaryland » Tue Oct 04, 2005 5:44 am

Yes, and no, your better bet is to go with a high gain, stick or dipole antenna with a nice -35db pre-amp on the receiver.

Unless you don't mind losing a good amount of your receive coverage, then go all the way and put that ear right into the center of the coverage problem area.


Good luck, and the 800 board will be out at the end of this week.

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Cowboy
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Postby Cowboy » Tue Oct 04, 2005 5:52 pm

I generally stray away from using highly directional antennas for primary RX on a repeater. When running in to a problem like that, we will usually use a standard 4 pole dipole array for primary rx with a couple additional receivers fed off of 7 element yagis through a voting controller.

Dan562
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Postby Dan562 » Tue Oct 04, 2005 9:12 pm

Your Tx Site sounds like it's very high in the air with a +9 dB vertical antenna. If you should find the manufacture's antenna radiation pattern, it should provide a diagram that radiates far over the horizon rather than downward into city itself.

This is known as the "Umbrella Effect" for the Repeater's receiver's capture area or lack of it. The person can often visually see the repeater site but can't access it with the Subscriber unit. This is where you should consider a vertical antenna with a mechanical "Downtilt" adjustment in the base of the antenna to provide better signal penetration down into the city.

Depending on how you have mounted the present antenna and if it's an open face dipole arrangement mounted to a tower, have the antenna inverted so the top is the bottom and bottom is now the top. This will force more signal down into the city with little or no effect to the out lying Tx coverage areas. If the antenna is a Fiberglass version, this same mounting can be accomplished but you must waterproof the aluminum base so no moisture can enter into the Fiberglass Radome electrically shorting out the radiating elements.

If you are determined to use another antenna with the High Gain Vertical antenna such as a Yagi on the same Duplexer output port, you will be automatically be changing the antenna's 50 Ohm impedance to 25 Ohms which poses a 2:1 VSWR Ratio / Mismatch to the Repeater's transmit RF power amplifier. Having this antenna mismatch can cause unwanted RF Spurious emission radiated across the entire UHF Band plus everywhere else through the spectrum.

The FCC and other users in the UHF bands plus 1st, 3rd, 5th & 7th harmonics coupled with mixing to other nearby transmitters can cause devasting communication problems and heavy FCC government fines when the other commercial users find your offending Transmitter and render documentation to the commission on your repeater's improper installation.

Do NOT even consider combining antennas using this method!

WB5ITT
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Postby WB5ITT » Wed Oct 05, 2005 2:08 pm

Running different antennas will hurt your overall TX/RX
"balance"..you really dont want to do this!!! UNLESS you only need coverage in ONE directions toward downtown and not in other areas away from the rptr site..THEN just stick a directional on the rptr inplace of the omni....BTW in your proposed scenario, you would not need duplexers if you run split antennas....thats not needed....just enough isolation because of the antenna sep and the use of GOOD quality feedline like Heliax (not RS coax here!) will do just fine with most 3-5MHz split UHF rptrs...

You may just want to stick a second rcvr downtown and link it back to the main site and use a voter (or with just TWO rcvrs, set the downtown for tight squelch, and I assume you are running PL?...then you dont need a voter)...OR you can consider a keyhole pattern antenna which COMBINES a omni and directional together...gives you a circle with a LONG slot in the direction of the directional antenna...making a keyhole pattern is not easy...contact DB Products for their assistance (You could use a DB420 for the omni side and a DB436 or highergain for the directional but then a DB420 has 10db gain and the 436 is about the same...but only on one direction!) Keyholes have been used for years on VHF HI and LOW band systems...but are rare on UHF...but the principle is the same and will work ok..


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