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1-4 of 4 messages
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Unity Gain versus High Gain Repeater antenna
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by K2AAU on November 25, 2005
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This maybe a stupid question perhaps, someone could shed light on this for me.
I have a UHF repeater antenna about 40' in the air here on Long Island. Not that high considering height is might. The antenna's manufacturer claims a 11.7db gain. The antenna is a Diamond 510NA.
Because I don't have the height, people tell me that I would be better off using a unity gain or 3db gain antenna with no radials (down tilt)to radiate the signal towards the horizon. I am no expert about antenna's and was always the believer that the higher the gain the antenna produced the better the receive would be in detecting the signal.
Can someone out there help me.
73
Artie
k2aau
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RE: Unity Gain versus High Gain Repeater antenna
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by K5LXP on November 26, 2005
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Well, it kinda doesn't matter what gain you have, or what direction it's headed. Here's why. Your range is limted by antenna height, not gain. You can put up a 25dB gain antenna, and you're still not going to get any further out. All you'll do with antenna and power gain is increase the signal strength within the area the antenna covers. 'Line of sight' is what dictates your range. If you want greater coverage, going higher is the only way to make that happen.
Mark K5LXP
Albuquerque, NM
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RE: Unity Gain versus High Gain Repeater antenna
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by N2FDU on November 27, 2005
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More signal will be radiated to the horizon if you use a high gain antenna. The only reason you would select a low or unity gain antenna would be if you had coverage problems close in to your site. Generally this happens only when your antenna is mounted on a very tall building/tower; it's called the umbrella effect - coverage to and from locations right under the antenna become problematic. Unless your antenna is at the Empire State Building, Chrysler Building, or E.H. Armstrong Tower, I don't think you need to be concerned.
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RE: Unity Gain versus High Gain Repeater antenna
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by WB2WIK on November 28, 2005
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Artie,
What your UHF repeater can reach and hear is determined much more by antenna height than any other factor. However, a higher gain antenna will help extend your horizon a bit compared with a lower gain one -- for certain.
A downtilt antenna has no application when it's only up 40' above ground. Downtilt antennas are for very tall towers or mountaintops, to help cover terrain in the "cone of darkness" beneath the repeater. With an antenna up only 40', your "cone of darkness" only extends a block or two, and I'm sure your repeater can hear that.
Having said all that, though:
1. Don't believe amateur antenna manufacturer's gain claims. The X510NA hasn't anything like 11 dB gain at 440 MHz.
2. You can believe commercial/industrial antenna manufacturer's gain claims, usually, since they are EIA certified by verifiable methods and test results. A commercial antenna about the size of the X510NA built for 440 MHz usually claims about 7 dB gain.
3. A difference between a real "commercial" UHF omni antenna and the Diamond or similar is that the commercial antennas don't use a skinny little brass wire as the radiator (inside the fibreglas tubing radome), they use something much larger in diameter with lower loss and higher power handling ability. They also use heavy duty, low-loss coaxial cable as the phasing sections (phase shift networks) between the colinear arranged radiators -- instead of little tiny coils of wire. They also use better (thicker, stronger) fibreglas, always a single section and not 2-3 sections in series with couplings to hold them together. And they use internally soldered or welded construction, not little wire couplings with setscrews and such.
The reason all that stuff is relevant is because the "commercial" antennas claiming far less gain than the Diamond usually actually work better and will extend repeater range farther. The problem is, they also cost a lot more, and are heavier and more difficult to support.
At one local site, we went from an X510NA to a Stationmaster (commercial) antenna about the same size and the difference in "repeater range" was rather astonishing -- without any other changes. The only downside was cost and weight: $500+ vs. $150, and 20 lbs vs. 2-3 lbs.
WB2WIK/6
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