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Small yagi for 700

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ryradio

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#1
I would like to try a Yagi on the local P25II system. I monitor 775.000 and would like a general range thereabouts. I hoped to use it indoors in the same room as the scanner so I was hoping to experiment with a smaller sized, perhaps 3 element and not a monster 4 footer. If it solves my issues, then perhaps I can move it to the attic or build another suitable for the outdoors.

So the two calculators I used give some numbers that don't appear right, for example one site is saying it goes on a 10" boom with element spacings of only 2 7/8".

A 10" yagi would be right on for where I intend to put it... but 2 " spacing doesn't look like any yagi I have seen.

So I was hoping someone can let me know if a small yagi is possible for 700/800 and maybe give me some feedback.
 
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#2
1/4-wavelength for 775MHz is 3.8". Your element length and the space between them should be about that size.
 
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#4
1/4 wavelength for 775 MHz precisely would be 3.625 inches (92 mm) so your driven element would be 7.25 inches precisely (184 mm) to fulfill the 1/2 wave usage of a Yagi driven element. If you use this page:

Martin E. Meserve - K7MEM - VHF/UHF Yagi Antenna Design

and punch in 775 MHz and a 10" boom it'll provide you the rest of the calculations and based on the numbers the Yagi should produce about 7.5 dB of gain using 1 reflector, 1 driven element, and 3 director elements, something that should be pretty easy to construct without a lot of hassles. If you scroll down the page a bit after punching in the necessary numbers it'll even provide you with a rough graphical image of what the resulting Yagi should look like as well.
 

adcockfred

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#5
Thanks I will give that a try. My old one works so good on 859 I hate to disturb it, so I will just build another one. will just have to hunt some more material.
 
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#6
all UHF TV antennas work well at 700 MHz and they are very inexpensive and easily fed with rg6

antennas like the scantenna are really just normal TV antennas mounted vertical
 

adcockfred

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#7
Yeah this hard system has a freq spread of a 100, so I assuming I have to increase the diameter, but scanner hates gain on antenna. I had a large bay antenna on it and it worked some what, but looking for smaller, I can mount on headliner.
 
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