Making a homebrew 800/900 so... some advice is appreciated

[br0adband]

I'm planning to use this most basic design:

http://www.pbase.com/dickh/image/27104369

and cut the leads to the middle of the bands I'm hoping to monitor - I plan to make several of these over time, actually, each tuned to a more specific frequency. The question I have is what length should provide the best signal: a 1/4 wave based on the frequency or a 1/2 wave, or one of the others listed below? I understand the basic principle that the more radiator you have the better but I'm curious as to what someone else thinks would be the best of the choices available, all easily created and cut.

Might even be able to manage 5/8 wave since we're talking about 800/900 MHz monitoring, and I plan to cut the leads to the length to tune it for ~900 MHz as here in Vegas we have a lot of 900 MHz band activity nowadays with more on the way I'm guessing. Several casinos have 900 MHz trunked systems in the 935 area, and with me monitoring so much around 855 or so I figured cutting the leads to ~900 centered would give me great coverage on either side. If that's incorrect I'd sure love to know it and know precisely why.

So, if anyone has any tips as to which length might be best, I'd love the advice. Currently lengths would be:

1/4: 3.12 inches
1/2: 6.24 inches
5/8: 7.8 inches
3/4: 9.36 inches (a possibility)
Full: 12.48 inches (a possibility)

Of those, what would most likely result in the best overall signal reception given the ~900 MHz center frequency?

 

[nd5y]

1/4 wave ground plane antennas only work well if all the elements are 1/4 wavelength.
Using more than 3 or 4 radials will not make it work better.
Making the elements longer or shorter will not make it work better at the design frequency.
To calculate 1/4 wavelength in inches 2808/f (MHz)
2808/900 = 3.12

 

[br0adband]

I realize that: the question is, given that all the radiators would be the same length, what's best of those choices from 1/4 wave all the way up to Full wave. Logically I want to instantly say Full wave is obviously the best, but since I don't have the background in antenna design to answer the question, I'm asking others.

 

[rfradioconsult]

A 1/4 wave or an odd multiple thereof will present the best match for your receiver. However, a longer antenna wll tend to have the main power lobes at less than 90 degrees toward the far end with the main power lobes concentrating more toward the far end. A Beverage is such an antenna. In other words a 1/4 wave will perform better for your application than a 3/4, 5/4, 7/4, etc. antenna. A 5/8 will exhibit a small amount of gain but will require an impedence transformation to match your feedline and receiver.

 

[Jim41]

I realize that: the question is, given that all the radiators would be the same length, what's best of those choices from 1/4 wave all the way up to Full wave. Logically I want to instantly say Full wave is obviously the best, but since I don't have the background in antenna design to answer the question, I'm asking others.

Theoretically you want to use odd multiples of 1/4 wavelength: 1/4, 3/4, 5/4, 7/4, etc.

 

[br0adband]

You folks do realize this is just for receiving, right? I did say I was creating these for monitoring purposes, and not transmitting in any way. I'm trying to create frequency tuned ground plane antennas and I'm wondering what's the best choice of those available: 1/4 wave, 1/2 wave, 5/8 wave, 3/4 wave, or Full wave - strictly for reception purposes. Or is there any difference whatsoever for reception purposes. Surely there has to be some differences... and since I don't know what the differences are or which wavelength would work best for monitoring only, I'm asking.

I just ain't looking for someone to say "Stick a coathanger on it and be done with it" or "Just use a piece of wire" so if you can help, please do.

 

[digitalanalog]

You did mention making more then one of these, so how about this.....

Make one set of radials for 1/4,1/2,5/8 and full wave.
Then just run them for a couple day's or what ever, then change the radials to the next wave length and do some comparisions.

Keep tract of the outcome in a notepad so you can view the results without trying to remember each.

Also, try mounting at different heights, because this will affect your reception as well.

Good luck with your project and let us know what works the best.....

 

[Hoofy]

Here's a web site that shows how yo build a colinear. I built a variation of one and it works great and has some gain as opposed to the ground plane.


http://www.angelfire.com/extreme/sideband/collinear.html

 

[rescuecomm]

1/4 wavelength antennas are fed between the radial set and the bottom of the element. For a vertical halfwave (two 1/4 waves), you don't need the radials, and feed the antenna between the two 1/4 wavelength sections (a vertical dipole). Anything else requires phasing coils to prevent the RF from cancelling itself. A 3/4 wavelength antenna functions as a 1/4 wave. The out of phase signals in the bottom half wavelength cancel each other IIRC. I use a 19.5 inch 1/4 wave mag mount for 146 mhz, and it is also a 3/4 wave antenna at 446 mhz for my dual band amateur radio.

Bob