Small indoor directional UHF?

[dlnorth]

I'm having what appears to be multipath interference issues on a UHF P25 system. I have tried moving the scanner around to find that sweet spot, where there is minimal interference, to no avail. I have also tried using the attenuator and have tried several different antennas (all omni). So far, I've still not been able to get the reception that I desire. Signal strength bounces around all over the place. It can go from 67-97 during any given transmission.

I'd like to try a small directional antenna and aim it at one of the closest transmitter sites, in hopes of blocking out the other nearby towers. I need something in the 490 MHz range and it has to be fairly small, preferably something that could sit on a desk or on top of a metal filing cabinet.

Any suggestions?

[kruser]

Try searching for Laird antennas. I'm sure they have some Yagi's for UHF that are just three or five elemants. At UHF, they will still be large enough that I doubt you would want it on a desk though.
Also, I dount it will really help at all if used indoors. UHF bounces (reflects) off many surfaces. If you can aim it at the source through a window may work but just sticking it indoors and aiming it at a wall or something is still going to pickup all kinds of reflections from furniture, walls etc.
If you can get it in an attic that is wood construction sometimes does help.
Can you get a magnet mount mobile stuck on something outdoors through a window?
That may be a better choice.
If in an apartment, stick it on a balcony rail or something.

[dlnorth]

Nothing outside to put a mag mount on at all and not much of an attic to work with, just a small crawl space with open areas above the garage and one bedroom (on opposite sides of the house). I had considered attempting to get a Yagi into one of those spaces before, but was trying to avoid having to fish coax through several walls to get to the scanner. There is also no power up there, so I'd have to run that as well. I do have two windows that point in the general direction of two of the towers for the system that I am trying to monitor.

I will look at the Laird models. Thanks for the suggestions.

Originally Posted by kruser

Try searching for Laird antennas. I'm sure they have some Yagi's for UHF that are just three or five elemants. At UHF, they will still be large enough that I doubt you would want it on a desk though.
Also, I dount it will really help at all if used indoors. UHF bounces (reflects) off many surfaces. If you can aim it at the source through a window may work but just sticking it indoors and aiming it at a wall or something is still going to pickup all kinds of reflections from furniture, walls etc.
If you can get it in an attic that is wood construction sometimes does help.
Can you get a magnet mount mobile stuck on something outdoors through a window?
That may be a better choice.
If in an apartment, stick it on a balcony rail or something.

[kruser]

Originally Posted by dlnorth

Nothing outside to put a mag mount on at all and not much of an attic to work with, just a small crawl space with open areas above the garage and one bedroom (on opposite sides of the house). I had considered attempting to get a Yagi into one of those spaces before, but was trying to avoid having to fish coax through several walls to get to the scanner. There is also no power up there, so I'd have to run that as well. I do have two windows that point in the general direction of two of the towers for the system that I am trying to monitor.

I will look at the Laird models. Thanks for the suggestions.

You don't need power unless you are trying to use something amplified.
I strongly suggest not using anything amplified. They usually always make things much worse than they already are for a scanner.

If you really insist on something amplified, most of those things get their power through the coax so you would still not need power at the antenna.

Good luck getting the signal you are after!

[benbenrf]

Originally Posted by dlnorth

I'm having what appears to be multipath interference issues on a UHF P25 system. I have tried moving the scanner around to find that sweet spot, where there is minimal interference, to no avail. I have also tried using the attenuator and have tried several different antennas (all omni). So far, I've still not been able to get the reception that I desire. Signal strength bounces around all over the place. It can go from 67-97 during any given transmission.

I'd like to try a small directional antenna and aim it at one of the closest transmitter sites, in hopes of blocking out the other nearby towers. I need something in the 490 MHz range and it has to be fairly small, preferably something that could sit on a desk or on top of a metal filing cabinet.

Any suggestions?

Lets start by confirming this is a carrier-noise or multi-path problem.

If it is typical reciever response (as far as digital P25 type encoding/modulation goes) will be complete loss of audio recovery and receiver speaker silence. These silences are usually very brief (millisecs in length) and occur repeatedly between patches of otherwise quite good/clear audio. Is this what you are experiencing?

I don't know of a small directional antenna that will solve mutli-path in the average sized room at 490Mhz unless it has something like 30dB plus front-back ratio (and similar isolation on the sides) - even then I suspect it won't work. You'll still land land up loosing all the audio that isn't in phase at the front of the antenna. You are going to have to get the antenna into the attic at least, or even better, mounted outside.

The only other way? Expensive phase shift filtering, in hardware or software - and they cost many more times than what the average consumer type scanner/receiver.

There is no short cut to dealing with multi-path, except improving the carrier to noise ratio and getting the antenna away from so many reflections, or adopting modulation techniques (like COFDM) that are inherently resitant to multi-path.

[dlnorth]

While receiving P25 audio, the audio sounds pretty choppy at times and also drops out completely. I have also missed entire transmissions on this unit. While this is occurring, the signal meter goes from a full 5 bars to 0 and everything in between, and just jumps back and forth, both while the scanner is stopped on an active voice channel and when it is just scanning. If I pick the scanner up and move it around the room, the reception improves, but only temporarily. The next day it could be back to having choppy audio again. I'm constantly moving it around from one spot to another because the sweet spots in my house change all of the time. I have tried several antennas (all indoor) but none of them have solved the interference issues, which I attributed to multi-path. Next step will probably be to buy an antenna to put into the attic. Outside antenna mounting isn't an option.

Originally Posted by benbenrf

Lets start by confirming this is a carrier-noise or multi-path problem.

If it is typical reciever response (as far as digital P25 type encoding/modulation goes) will be complete loss of audio recovery and receiver speaker silence. These silences are usually very brief (millisecs in length) and occur repeatedly between patches of otherwise quite good/clear audio. Is this what you are experiencing?

I don't know of a small directional antenna that will solve mutli-path in the average sized room at 490Mhz unless it has something like 30dB plus front-back ratio (and similar isolation on the sides) - even then I suspect it won't work. You'll still land land up loosing all the audio that isn't in phase at the front of the antenna. You are going to have to get the antenna into the attic at least, or even better, mounted outside.

The only other way? Expensive phase shift filtering, in hardware or software - and they cost many more times than what the average consumer type scanner/receiver.

There is no short cut to dealing with multi-path, except improving the carrier to noise ratio and getting the antenna away from so many reflections, or adopting modulation techniques (like COFDM) that are inherently resitant to multi-path.

[benbenrf]

Okay - I'll go along with you on that: sounds like multi-path and/or poor SNR, or what is more acccurate to describe as poor CNR or Carrier to noise ratio if we are talking about P25.

Your only practical answer - as you guessed: is going to be an attic antenna. Adding a bandpass filter ( a cheap cermaic inline coaxial type) mounted as close to the antenna as possible is going to stop a lot of trash coming down the coax. For the $15 - 20 a 2nd hand one may cost, I go for it - it could make a siginificant difference.

Er ... just out of interest: why is an external antenna not an option in this case?

[dlnorth]

Thanks again for the advice. An external antenna isn't an option because my better half doesn't want an external antenna on the house. They are not aesthetically pleasing and we would be the only house in our neighborhood with an antenna, which means we are not getting an antenna.

We are eventually moving to an area that is not served by cable, and at that time, I will be putting up an antenna for TV, so I'll more than likely get away with throwing a scanner antenna up on the same mast, but for now, it's just not going to work. Apparently, monitoring communications just isn't a good enough reason to put up an antenna. Only when it's needed to watch Dancing with the Starts, the Voice, Dr. Oz, etc., will an antenna on our house be acceptable.

Originally Posted by benbenrf

Okay - I'll go along with you on that: sounds like multi-path and/or poor SNR, or what is more acccurate to describe as poor CNR or Carrier to noise ratio if we are talking about P25.

Your only practical answer - as you guessed: is going to be an attic antenna. Adding a bandpass filter ( a cheap cermaic inline coaxial type) mounted as close to the antenna as possible is going to stop a lot of trash coming down the coax. For the $15 - 20 a 2nd hand one may cost, I go for it - it could make a siginificant difference.

Er ... just out of interest: why is an external antenna not an option in this case?

[benbenrf]

In the case of a P25 antenna it would be no longer than a few inches and could be totaly concealed outside quite easily - and will enhance performance over an internal or attic mounted antenna - but fair enough, I know our other halfs carry great weight when it comes to "matters domestic" ... and we need to obey them, or else!!