Which antenna should I get?

Ok I've been dealing with this issue off and on for a while now. I run a feed that carries a P25 system. It is locked on the system, no analog, and it pulls off a single site that is about 10 miles away LOS. There are a few other sites for this system (on different frequencies) that are between 30 and 40 miles away (it's basically one per county). The scanner is connected to a regular FM rooftop "cross loop" antenna where it normally pulls in 5 bars solid. But every now and then (more often during spring and summer, it's getting bad now) it'll have fits for days where the signal fades in and out, sometimes down to 1 or no bars and the T disappears..This causes it to miss tons of traffic. Changing to a mag mount ham whip pointed out the window (the scanner is basically in a faraday cage so anything indoor is out of the question) during these episodes does help to an extent, it doesn't go down to 0 but the control channel is still scratchy.

When I monitor the control channel during these periods, I hear the normal "diesel engine" sound, but there is static that fades in and out on top of the diesel as the signal meter goes up and down. I'm pretty sure that this has something to do with it not only being an omnidirectional antenna, but it's oriented horizontally. I think it's picking up reflections and maybe interference from something else. And for the record it performs much better going through a 6dB attenuator than it does connected directly to the feedline.

So I have a $150 gift certificate from ScannerMaster that is expiring soon. I had hoped to use it toward another scanner but damned if my money tree didn't die on me. They have 800MHz Yagis ranging from $42 to hundreds of dollars. Anyone care to recommend the best one for my situation? I have no idea what the gain on the antenna I'm using is, since it's not for 800MHz, but due to the fact that the transmitter is so close and I have to use an attenuator right now, I doubt gain is the issue. So what's the difference between the $42 10dB one, the $140 6dB one, and the $175 9dB one?

Rooftop Single-Band Antenna for Police Scanner Radios | ScannerMaster.com

Summarizing your situation:

1 - You are using a directional antenna now.
2 - You are monitoring the KSICS system.
3 - You have enough signal from the system to have to use attenuation.
4 - You notice signal fade is a seasonal problem.
5 - You haven't told us anything about the feedline.

Symptoms 1-3 tell me it's likely not the antenna and 4-5 tell me you ought to consider your feedline portion of the system.

It's an omnidirectional FM cross loop, not a directional.




Feedline is RG6 coax, and I have already replaced it, the ground block, splitter, and the balun at the antenna with no improvement. The problems don't line up with the wet weather to an extent that it could be related either. The most recent episode started about 4 days before that monsoon came through (prior to that it hadn't rained in weeks), and ended while it was still raining - now it's been fine for a week. It just seems to happen more in the spring and summer.

what frequency range (in the 800mhz) is the single tower want to reach.

The control channel is 852.4625, so give or take a couple megahertz

I would personally get this one if I needed to use up $150 gift card ASAP. Just throw in $20 more and you have a top of the line antenna.

Maxrad MFB8967 Yagi Base Antenna | Scanner Master

OK, the picture clarifies your description. Changing to a frequency-specific directional antenna should help. Your report of using attenuation means you have to give consideration to how you'll handle the extra input from the more effective antenna. It appears you can use one of the smaller, less expensive Yagi antennas from your linked page because you don't need a lot of gain for the signal.

It appears you've taken the feedline components out of consideration as factors. If you decide to acquire the new antenna, it will give you an opportunity to really examine those components with an eye toward any potential water issues. Your observation about how the symptoms seem counter to the wet weather certainly adds to the mystery. I wish you the best of luck in solving it.

Ok been doing more research on this.. I can't confirm if it's coincidental to the start of the issue, but there are three cell towers that have gone up within 1 mile of me over the past couple years - one of which is directly in the line of sight to the P25 tower (the other ones are in the other direction). They carry equipment for Verizon, AT&T, NexTech Wireless (our local carrier, uses nTelos CDMA phones), and Viaero Wireless (a regional AT&T SRP). NexTech has recently launched LTE, and I know they're running in 700MHz band 12, but I have learned that all of those carrier's cellular and 3G run on 850. I'm wondering if one or more of the cell transmitters are ramping up power under certain conditions, swamping the receiver (since they're a lot closer than the P25 tower)? I know the cell frequencies actually "straddle" 850-859, but I'm assuming if the power is high enough it can cause problems in the center band, right? I don't see one on Scannermaster but does anyone make "small window" bandpass filters that'll just give me 850-860 and knock down anything else? I mean if it's just one of the providers and it's on one of the towers to the south, then the yagi should fix it, but if it's on the north tower I'd still see the same problem - possibly even make it worse - with the yagi since it's right in line with the P25 tower..

And no, I don't have a spectrum analyzer or know anyone who does..

Good job on the research and gathering more detailed info! I wish I were familiar with filters that might do the job for you. I'll make some inquiries with a friend that may have some knowledge in this area. He certainly seems to be able to come up with solutions for our ham club repeaters.

N5TWB said:

Good job on the research and gathering more detailed info! I wish I were familiar with filters that might do the job for you. I'll make some inquiries with a friend that may have some knowledge in this area. He certainly seems to be able to come up with solutions for our ham club repeaters.

Click to expand...

Ok I lied about the spectrum analyzer lol! I remembered that I have some cheap RTL dongles that I bought a while ago to play with, and fired up an SDR that gives a waterfall display.. Yes, the SDR does also have the same issues missing calls and the control channel fades out (even though the signal on the waterfall remains hot).

So this is centered on the control frequency 852.4650.. When connected to the roof antenna, I see a few constant streams a few kHz off the control. - these are NOT frequencies listed for any other KSICS site that I'd be capable of picking up from here. When I tune to the one closest to the control I can hear the 'dieseling' sound, but it will not decode. On the others I hear nothing but static.

Then I saw when it was decoding calls on other channels, they would show up on multiple frequencies in the waterfall.. Are these reflections or harmonics? Could this be the cause of my issues? There is a voice channel at 853.865 that has a "reflection" right alongside the control channel at about 852.446.