Building antenna to scan P25

[DickH]

You think that it would help me hear the tower that's out of range by 15-17 miles? Gr8rcall

Doubtful. Do you get any signal at all? Here's how to check. Tune your scanner to the control channel and open the squelch so you hear noise. Then move the scanner around,
SLOWLY. Up, down, left, right, different rooms, etc. If you can get any signal at all, then a Yagi antenna should work for you.
Here are two I found. A search for "800MHz antenna" should yield more. You want the highest gain you can afford. Bear in mind some manufacturers might stretch their specs a bit.

AMAZON - Wilson 13db gain w/ 20' of lo loss cable $71.50
Amazon.com: Wilson 14 dB 800 MHz Yagi Cellular Antenna 301111 and Wilson400 20 Feet Ultra Low Loss Coax Cable 952320: Electronics

The Antenna Farm
800 MHz Yagi Antennas : The Antenna Farm :: , Your Two Way Radio Source!

 

[gr8rcall]

Ouch! No offense, but I really don't have 50$-90$ to spend right now...
Any other suggestions?

 

[DickH]

Ouch! No offense, but I really don't have 50$-90$ to spend right now...
Any other suggestions?

Radio Shack has some a little cheaper - $30-$40?.
The only other alternative is to make your own. Scroll down to post #16 in this thread.
http://forums.radioreference.com/scanner-receiver-antennas/45999-yagi-antennas-under-5-a.html
(I would try 1" square wood. It is easy to drill, etc.)

 

[prcguy]

The dimensions for circular waveguide antenna at 800MHz is about 6" diameter and the probe would be about 3" long and spaced about 3" from the back wall of the can. Looking at the pictures it appears the can is much smaller meaning its not an antenna at 800MHz and just a bunch of random stuff soldered together.

If it picks up anything now imagine how well it might work if you made an actual antenna.
prcguy

 

[DickH]

The dimensions for circular waveguide antenna at 800MHz is about 6" diameter and the probe would be about 3" long and spaced about 3" from the back wall of the can. ...

Are the exact dimensions published someplace we can see them?

 

[prcguy]

Here is some info on designing and building both an open waveguide antenna (cantenna) and horn antenna for 915MHz which is close to whats needed for 800MHz. BTW, the gain of an open ended waveguide is not that much. http://smartech.gatech.edu/jspui/bitstream/1853/34414/1/PG_TR_040813_YL.pdf

Here is a site with a crude calculator and it looks line a can diameter around 9" would be good for the 800-900MHz range if thats what you want to pursue. How to build a tin can waveguide antenna
prcguy

Are the exact dimensions published someplace we can see them?

 

[plusEric26]

Originally I thought I would want to build something better, but I've been using this thing for some time now, and I have to tell you, it's doing exactly what I need it to do.

I'm not saying it's the best waveguide/cantenna a person could build, but that may be exactly the point. Simulcast issues being what they are, I'm thinking my terribly inefficient antenna is probably why I'm no longer suffering multipath distortion where I was before at home. Distortion so bad I couldn't listen to the system at all.

At the end of the day, I get nearly no distortion and my signal meter shows full strength at all times. How much better could it get?

I think my next antenna will be a yagi, and I will be building it myself. I'm interested in pulling in a few distant Ohio Marcs sites that I can't hear with a little ducky. Probably a better way to spend my time instead of chasing marginal gains.

 

[ad5kl]

Your choice of cable is also important. Use LMR-400 or 9913 if the cable run is very far.

Cheaper cable doesn't do well at 800 mhz & all the gains in your new antenna will be lost before they get to your scanner.

 

[kevin2525]

I've been a amateur radio operator for alot of years,
And I have not seen any-thing like that. But if works
my hat is off to you. That is just to cool ! Congrats!

73's: Kevin.

 

[DickH]

... I'm thinking my terribly inefficient antenna is probably why I'm no longer suffering multipath distortion where I was before at home. Distortion so bad I couldn't listen to the system at all. ... At the end of the day, I get nearly no distortion and my signal meter shows full strength at all times. How much better could it get? ...

I think you have done a great job. Never mind all the fancy formulas and theories, it's what works that matters, as you have so aptly demonstrated.
One thing to remember about 800MHz. The wavelength at 850MHz is about 13", so, theoretically, there is a peak and a null every 3-1/4 inches. Moving an antenna around slowly, you will find "sweet spots" where you will get a stronger signal - or perhaps no signal at all a few inches away.

 

[plusEric26]

Yeah, I couldn't be happier, and it's definitely gotten me into the idea of building other antennas as well. Fully intend to build a yagi and maybe something like the "4-bay dipole" antenna in the DIY section of these forums.

I have to say I'm also a bit surprised everyone is saying they've not seen something like this before. I feel like the wifi "cantenna" thing was quite popular at one time and would have thought that maybe others would have picked up on it. I would be interested to see how a properly built antenna of this type would perform, maybe I should try and build something better just for that reason.

As I think I said, I knew at the beginning of the project that it wasn't going to be a properly built antenna and I really didn't have much hope for it. My intentions were to build something just to see if I could get the scanner to decode anything at all.

I'm still shocked it works so well, and clearly has a lot to do with luck LOL. But now I have no reason to move to something else for listening to my county p25 system, "if it ain't broke, don't fix it."