Odd Singnals

[gg750]

I recently got an antenna from a friend. He bought a house and it had an antenna on it's roof (came with the house). Had one long vertical element and 3 long elements coming off from the side that angled down. I took it home, got it cleaned up (was very rusted), put it back together, got it wired up with some good 50 ohm coax, and hooked it up. Does a good job up picking up local freqs, but I was expecting better (my RS mag mount on the car picks up more distant signals). Anyway, I noticed one night that I was hearing music in the background on my scanner when someone was transmitting. Listened more carefully, and I pick it up on every frequency the scanner stops on. I started tuning my radio through local stations and turns out that this antenna is picking up a local county music station. Every time the scanner stops on a freq when connected to this antenna, I hear the music quietly in the background. Now, this is prolly not meant to be a scanner antenna (it did have BNC connector on the end though), and I actually have no idea what kind of antenna this is. I tried new cable, different locations, but I still get the country station in the background. Can anyone clue me in to what kind of antenna this is and maybe how to stop the country music in the background?

Thanks for any input,

G

 

[MacombMonitor]

More than likely it's a C.B. antenna, intended for use around 27~28MHz, which is way below what you are probably scanning. Is the vertical element around 108"? This antenna could be cut, (shorten the elements) closer to the frequencies you're trying to monitor.

As for the interference, it sounds like a strong/local FM station. There are in-line filters you can buy to help eliminate this type of interference. Just about any place that sells scanner/radio equipment should be able to help you. Radio Shack even carries them, but they have "F" (T.V. coax) connectors on them, so you would need some adapters.

You didn't mention what type of coax you're using?

 

[gg750]

The vertical element is almost as long as i am tall (6'1"). It's a large diameter rod (1/4" or larger) with a smaller short rod (12", 1/8" diameter) screwed onto the very top of the larger rod. The angled side elements are close to 8 feet, same diameter as the vertical rod with the shorter smaller rod screwed on at the ends. I'm using R8-58 cable. I'll try to take some pics of the antenna and post them here.

 

[MacombMonitor]

That's a C.B. antenna, and far from being "good" for a scanner, although it will work. What's really hurting you more than the antenna is the RG58 coax! The signal loss in that cable, makes it worthless for the 800MHz band, terrible for UHF, and not good at all for VHF/Hi. You're probably better off using the antenna that came with your scanner. Sorry.

 

[gg750]

RG58 is a problem? But thats what every major antenna manufacturer uses with moble antenna installations. I thought RG58 was what you were supposed to use with radio communications. Also, I'm only 10 feet or so from antenna to scanner.

 

[MacombMonitor]

RG58 is a problem? But thats what every major antenna manufacturer uses with moble antenna installations. I thought RG58 was what you were supposed to use with radio communications. Also, I'm only 10 feet or so from antenna to scanner.

Ok, I had no idea you were using such a large base antenna only 10 feet from your scanner. That is another issue. Below is a chart from Belden, a major coax manufacture, showing various frequencies in the first column, and the attenuation resulting per 100/ft of cable. As you can see, at around 800MHz, it would be about 15dB! If you had this antenna mounted on your home, typical cable runs would average between 50~75 feet. So the loss would still be around 7dB at 50 feet, for 800MHz.

Description Frequency (MHz) Start Frequency (MHz) Stop Frequency (MHz) Nom. Attenuation (dB/100 ft.)

1 0.4
10 1.3
50 3.1
100 4.5
200 6.6
400 10.0
700 14.2
900 16.6
1000 18.1

 

[gg750]

I'm not too concerned about the 800 range. There really isn't much to monitor here on 800, except MSP and DNR. Everything here is VHF...for now anyway. I'm not too concerned about the whole thing anyway. The antenna was free, and I have plenty of connectors laying around the house, so It won't upset me to throw the thing in the trash. It was just an experiment.

 

[MacombMonitor]

If you want to experiment, try this antenna calculator, and cut the element to the frequency that is most critical to you.

http://www.csgnetwork.com/antennaevcalc.html

 

[kb2vxa]

Hi guys,

The practical solution to all the problems in one fell swoop is replace the antenna and coax. An antenna made for scanning and low loss cable is in order, everyone has a different opinion so don't bother asking TOO much, do your own homework. You're the one in the best position since you know what your requirements are, we can only guess at the fine details. "Advice" can put you in the ball park but the final decision is yours, when it comes to antennas there is no such thing as "one size fits all", it's always a compromise.

Without getting all techie on you, a probable cause of the interference is corrosion and poor contact between the metal parts of the antenna. Another could be signal overload, a CB groundplane such as you describe is resonant just below the FM broadcast band (81MHz). The cable rather speaks for itself, RG-58U is great for HF but rather useless above 30MHz.

 

[gg750]

what type of cable should I use for VHF (150-160) range?

 

[mgosdin]

what type of cable should I use for VHF (150-160) range?

OK, let me see if I can give you some decent advice.

There are several factors that dictate the kind of cable you will need:

1. The frequencies you are most interested in. ( VHF 150-160 mhz similar to what I am most interested in. )

2. The length of cable that is needed to reach from where the Antenna is to where the Scanner is. ( Don't know in your case, in mine it's less than 30'. )

3. How hard the cable is to work with. ( Some of the better, lower signal loss, cables are very stiff and really really don't like sharp curves. )

4. How much the cable costs. ( $$$$! )

For my setup I have two RG8X lines that run to antennas that are used almost exclusively for VHF 150 - 160 mhz monitoring. This works well for me as it has an acceptable loss over the less-than-30' of cable run, and I was able to get professionally made runs for a very reasonable cost. My third line is RG6QS and is used primarily for UHF 440 - 460 mhz monitoring. I bought a 50' spool of the RG6QS cable and a type F weather "resistant" connector crimping kit for about $45 at Lowes. That and some adapters from Radio Shack and I was good to go.

The thing to keep in mind is that what works for you will most likely be different that what I'm using, and is bound to be different from many of the other people in this Forum. As was said earlier, there truely is no one-size-fits-all solution. Don't be afraid to experiment.

Mark Gosdin

 

[gg750]

The freqs i'm most interested in are in the 150-160 range, and some 800 (there isn't much to listen to on Michigans 800 system, at least not in my county). It's a 10-15 reach from antenna to scanner, and may drop as I'm thinking about moving the antenna. It's VERY close to a power line that runs down the side of the building...I can hear the 60 Hz AC hum when standing near it.

 

[Al42]

The freqs i'm most interested in are in the 150-160 range, and some 800 (there isn't much to listen to on Michigans 800 system, at least not in my county). It's a 10-15 reach from antenna to scanner, and may drop as I'm thinking about moving the antenna. It's VERY close to a power line that runs down the side of the building...I can hear the 60 Hz AC hum when standing near it.

If the antenna falls, is it at all possible for it to fall on the power line? If the answer is yes, don't touch your coax (or connect it to a scanner) until you've moved the antenna.

Death is a long term disability.

 

[gg750]

No, the exposed power lines are a good 30 feet up at the top of the building, then it runs down the side of the building in a large plastic tube to go underground.

 

[mgosdin]

It's a good idea to keep your scanner, or any other sensitive electronic equipment, as far away from that power main as possible. Even if the antenna can't possibly short against the power lines, you will still get some interference from them.

<begin_old_warstory>

We had a warehouse manager whose PC kept dying with Hard Disk corruption. We eventually isolated the problem to the fact that his PC was on the outside wall of his office and there was a power transformer mounted on the ground just the other side of the wall. Whenever the power was interrrupted the transformer would put out a pulse and bye-bye hard drive. The PC was moved to an inner wall and the problems stopped.

</end_old_warstory>

Just be careful and play it safe where those power lines are concerned!

Mark Gosdin