Antenna length for 860 Mhz?

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Slyster

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I have an antenna... for '800mhz'... it is 10.25" long... that seems a bit long to me but I really don't know.

What is the PERFECT length for 860 Mhz P25 systems. Should I trim it? Or is that length ok?
 

rescuecomm

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What type of antenna is it? If it is a gain type antenna with the curley coil near the center, don't cut it. A 1/4 wave vertical at 860 mhz is about 3.25 inches long.

Bob
 
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Each operating frequency has a proper wavelength. Receive isn't as critical as transmit.
A coil increases the length without increasing the height.
 

zz0468

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WolfWalksSoftly said:
Each operating frequency has a proper wavelength. Receive isn't as critical as transmit.
A coil increases the length without increasing the height.

That's certainly a misleading statement. I hope people don't go off thinking that the coil is just the "missing" length wrapped up and tucked out of the way. That ain't how it works at all...

A coil is an inductor. In an electrically short antenna, it will be non-resonant and have capacitive reactance. The inductive reactance of the coil tunes out the capacitive reactance of the short antenna and makes it a pure resistive load, i.e. resonant.

An antenna like the OP's 10" 800 MHz antenna is actually a multielement gain antenna called a collinear array. The inductor is used to phase the two elements properly.
 
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B52outpost

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zz0468, I'm impressed with your antenna knowledge. Therefore, I would like to ask "da-man" a question :)
I was wanting to construct a simple 860 Mhz antenna for placement in my attic. I would like to use solid 12 gauge insulated wire connected to a RS connector.
Do you know what length I would need to make a 5/8 wave, or one of those antennas with the curl into it (use a pencil/ pipe to make the curl)? This might be an option for Slyster in the 860 mhz range.
 
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zz0468

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B52 - I don't consider myself the penultimate antenna expert by any means. But I have been in the business a long long time, and have a few clues as to what works... and what doesn't. At 860 MHz, 5/8 wave is about 8.5 inches. But that would require a matching coil at the base, not the center. I don't know the value required off the top of my head.

The typical 800/cellular antenna that you're thinking about is usually a quarter wave section, a phasing coil, and a 5/8 wave section. I'd suggest that you locate a cheap used cellular antenna rather than roll your own. But if you really want to do that, you could duplicate the dimensions, and for a receive antenna, it would work just fine.

If you're listening to a strong signal, perhaps just a ground plane would do. 3.4" radiator, and ground plane elements slightly longer. It's easier to make, and you'll probably never notice that 3db loss in comparison to the gain antenna. At 800, all that does is help counter the feedline loss.
 

B52outpost

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Thanks for the response... I was/am clueless on the matching coil at the base. I saw a link in the antenna Wiki for a 1/4 wave ground plane for the 2 meter band and thought that it could be duplicated @ 860 mhz (234/freq mhz) then double/triple the length for more gusto. Don't want to hijack Slyster thread, just thought it might work in with his/her question. Thanks again.
 

OpSec

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Slyster said:
I have an antenna... for '800mhz'... it is 10.25" long... that seems a bit long to me but I really don't know.

What is the PERFECT length for 860 Mhz P25 systems. Should I trim it? Or is that length ok?

Yours has a coil, so let it alone....but it's about this high for a quarterwave :)

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