A P25 Antenna Option (?)

[benbenrf]

Anyone doing this – or thought of doing this, or done it?

3 or 4 x 120degree or 90degree old/used 800Mhz cellular sector antennas
1 x SPDT type antenna switch

The above has the potential to offer a seriously effective P25 receive antenna setup.

The 3 cellular sector antennas could be setup to monitor together, so that as soon as a desired P25 transmission is identified and you wish to monitor it, the 2 or 3 antennas not in beam-width could be switched out instantly & automatically to maintain a good noise ratio (antennas not contributing a modulated signal that you want to listen to are then just contributing background noise and reducing your noise ratio).

How to switch these other antennas in/out?
An SPDT type antenna switch operated manually, or by/way of using the scanner/receiver audio signal (amplified probably to a couple watts at least) to operate a basic circuit which selects which antenna/s to switch in/out based on a coax voltage/modulated signal strength ....... comments/ideas on how else this could be done cheaply?

The above is just one of several ways I can think of to select the "active antenna" – readers may have other ideas ………..?

Cost of a 90degree -120degree cellular sector antenna?
I’ve seen them on eBay for less than $60 on occasion - more often for less than $100!

They can be seriously good as P25 antenna options as not only does the build quality tend to be excellent to satisfy network operators, some come with top-shelf bandwidth filters as well as pre-amp/attenuator circuitry iincluded, which means your scanner/receiver gets presented a P25 signal of consistant Noise Ratio and Gain - no matter the actual transmitted signal strength.Taking this "equalisation" task off your receiver/scanner front-end can enhance signal processing and demodulated audio quality significantly.


Last but by no means least, is that old cellular antennas are optimised for radiation pattern coverage at ground level and close to the horizon.

In short used +/- 800Mhz cellular sector antennas offer seriously good P25 alternatives, for a fraction of what it would otherwise cost to realize equal or similar base station antenna quality.

Ideas, comments …….?

 

[mmckenna]

Only comment would be that P25 is not limited to 800MHz, at least no here in North America. Interesting idea. An expensive option, but good antenna systems usually are.

 

[aps_ak]

It is my understanding that you don't cut antennas for modulation, you make them for the frequency they are on. If your local P25 system is 800mhz, make an antenna for receiving 800mhz, if its on 150, make on for 150.

 

[mmckenna]

Exactly, modulation scheme has nothing to do with the antenna. Polarization and resonate frequency do.

One nice thing about the cellular panel antennas is that most of them have an electrical downtilt option. Using this would allow someone to remotely adjust the antennas from a pattern directed more towards the ground, or up towards the horizon.

It's an interesting design idea. With a lot of 800MHz cellular stuff being changed out to cover 700MHz, there will likely be more and more 800MHz antennas available for cheap.

 

[benbenrf]

Sorry guys – have I been drinking too much coffee – what’s the point been made related to the word “modulation” and/or that antennas are “not cut for modulation”, or the context I used that in – will someone put me in the picture. Thanx.

 

[mmckenna]

P25 is a modulation scheme. Just another way of putting information on a radio carrier.

p25 doesn't equal 800MHz, P25 can be run on other bands, and 800MHz can run any modulation.
The thing that matters at 800MHz is the resonate frequency of the antenna and the proper polarization. Any antenna will work for P25, as long as it is resonate at the transmitted frequency. Any resonate antenna can run p25.

Sorry for taking your post off topic. I like the idea. With a lot of old cellular gear hitting the market, it would be an interesting exercise to try some of it. On the other hand, those panel antennas are big and catch a lot of wind!

 

[benbenrf]

Mmckenna

Thanks - yes your points are valid: it can of course be any freq, I only cited 800Megs because its so common and used cellular sector antennas are avaliable on this freq.

As far as the word “modulation” goes - it has nothing to do with the antenna, its type, its setup, polarisation or anything else about the antenna …….. that I thought was clear.

It is about using an rf signal characteristic to switch an antenna it into or out of the signal chain.

One (but by no means the only) characteristic of an rf signal that would be relatively easy as well as comparatively cheap to implement antenna selection would be: whether or not the carrier was modulated versus un-modulated, or put another way: whether or not the rf signal contained P25 info versus noise or some other type of modulation that was not P25.

But okay - lets forget optimisation if folk are not interested, lets stick with the basic question: does anyone have experience using an old 800Mhz cellular antenna (sector primarily, but omni as well) to monitor P25, and if so what is your experience?