View Full Version : Radio/CB/electronics gurus?
05-03-2004, 09:44 PM
Sorry for the vague title, but I'm don't know the terminology well enough to describe what I'm trying to do in just a few words.
I work at an amusement park during the summer, doing security. The park is built on a hill. Our radios work fine for the most part, except when sender and receiver are on opposite sides of the hill, at which point they usually can't hear each other at all. In that case, we ask someone near the top of the hill to relay the message, but that's hardly an ideal solution. I'd like to buy or (ideally) build a device to be placed at the top of the hill, to retransmit (and perhaps increase the strength of) the signal.
I asked about this at an amateur electronics shop, where I was told it was impossible, due to FAA regulations. I expect the gentleman at the store knows more about it than I do, but I got the idea that perhaps I wasn't communicating my needs accurately. I was told by another source (who has an MS in electrical engineering) that the FAA doesn't govern transmissions under 5W, so I'm not sure what to think. I turn to you for your thoughts and advice.
Possibly relevant information: The radios we use are a mix of Maxon and Radius. I don't know if that's the manufacturer or model name, but I'm told they use a Motorola chipset. I'm also told they put out about 2 watts maximum, and I have the frequency written down somewhere - I believe it was around 156 MHz, though I may be off by a few. I estimate the necessary transmission range of the device to be less than 500 yards.
Thanks for any help you can give. It seems like this should be a relatively simple project, but I know very little about radio. I'd love to learn all about it, but I'm hoping to have this ready by early July, when the park opens, and that's just not enough time to educate myself sufficiently on the subject.
05-03-2004, 11:10 PM
For starters what you are looking for is called a repeater. The repeater is placed at the highest point in the area you wish to communicate.
Next, your existing radios will not work. You need a radio that is capable of transmitting on one frequency and receiving on another. When you push your button to talk the radio transmits on (say) 150mhz. Along with that transmission an inaudible tone is being transmitted with your voice. The repeater listens for this tone and rebroadcast the message on a different frequency. Usually 5 MHz offset (145 MHz) from the original frequency.
I am not sure what your budget is but I think this company has exactly what you are looking for. If they don’t do a search for “business class low power repeaters”
Here is a snippet from the website
"low cost communications solution to the small manufacturer and to such unique markets as golf courses and municipal and county parks, just to name a few." "
05-03-2004, 11:11 PM
Let's start off by clearing the water. The FAA (Federal Aviation Administration) doesn't give a rat's ass what you do with your radios, so long as airline flight operations aren't interfered with, and you're not likely to do so at 2W.
You should be concerned with the FCC (Federal Communications Commission) since these folks deal with radios.
It would seem that your communication system was bought on the cheap, as opposed to consulting a professional in RF technology.
Not having seen the site, any suggestions for correction would be of little value.
05-03-2004, 11:32 PM
Aha! Jim, the fellow at the electronics place mentioned the dual-frequency receiver/transmitter, which I understood, but for some reason, it didn't click that the radios would have to use two frequencies, as well. Thanks for pointing that out. The fact is, we don't have the budget for new equipment, which is why I was hoping to build a repeater, rather than purchase it. Buying all new radios is out of the question. Looks like we're just SOL.
danceswithcats, pardon my brainfart. I DO know the difference between the FAA and FCC, I just didn't catch it on preview.
I can't speculate on whether or not an expert was consulted in purchasing the system, but I can virtually guarantee that they bought what they did because it was what they could afford. For what it's worth, when the radios were purchased, the site was all on one side of the hill, and reception was no problem. In later years, it was expanded, so that now the hilltop is inside the park.
I'm curious - what was lacking from my description of the site that would help to make suggestions for improvement? Since we can't afford to replace the system, it's a moot point; as I said, I'm just curious. I thought I had given all the relevant information.
05-04-2004, 12:29 AM
When you push your button to talk the radio transmits on (say) 150mhz. Along with that transmission an inaudible tone is being transmitted with your voice. The repeater listens for this tone and rebroadcast the message on a different frequency. Usually 5 MHz offset (145 MHz) from the original frequency.
Hey, be careful there. 145 MHz is smack in the middle of the 2-meter Amateur VHF band (144.000 to 148.000). I know you were just giving an example, but I wanted to point out you can't just go broadcasting willy-nilly on any old frequency.
Also there are what are sometimes called "parrot" repeaters, also known as simplex repeaters. These record received audio and retransmit it on the same frequency when the carrier drops. It's not really an ideal solution, but at least the system could use the same radios they use now.
Let's assume you have the park's permission.
I wouldn't be surprised if there was a structure near the top of your hill, suitable for mounting a passive reflector. The shape isn't be very critical. A horizontal sheet metal disk, square, or roof-style canopy 10 ft across 50ft up a tower might be all you need. You might get away with less, but I wouldn't bother, just as I wouldn't bother using aluminum foil instead of sheet metal. (Foil would work as well as sheet metal, but it's not worth a single extra trip to fix. You want something you can set and forget.)
It might not be hard to rig a 20-ft or larger sheet metal reflector halfway up a cable car or other ride tower. If the tower is 10' wide, your reflector would only extend 5' beyond the tower on all sides
It's an inelegant, possibly trial and error, solution, but cheap and uncomplicated. I bet a local Ham club would assess your site and fabricate the reflector as a fun challenge (I think your liability insurance company would want it installed by employees, instead of volunteer labor)
When pitching it to your bosses, don't make it sound sophisticated. Explain why you want it, but call it a canopy, not a reflector or antenna. It's just an object that radio waves will happen to bounce off of
05-04-2004, 01:21 AM
Hrm..You've been misinformed left and right.
Yes, its a repeater you need for coverage. Put it and/or the antenna at the highest possible point in the park for optimum coverage.
You will indeed need a license from the FCC for this type of system...just like you really need one just with the equipment you're using now
I'm a bit of a Motorola radio fan and I'm pretty sure most if not all radios in the Radius line are programmable. This is likely true for the Maxons too. The Radius GP300 and GP350's are particularily good, IMO.
If your quite handy and ambitious, it is possible to make a repeater using surplus mobile (vehicle mount) radios but I'm somewhat handy and very ambitious and I'd never dream of undertaking such a difficult project. If nothing else, you'd need a duplexer to use a conventional repeater. These are not especally easy to find cheap and will undoubtedly require tuning. A duplexer allows the same antenna to be used for tx and rx simultaneously.
There's no standard offset on VHF. UHF is almost always +5MHz but for VHF, you'll need to read your license. For ham use, its almost always +/-600KHz repeater offsets in the VHF band.
If you decide you need this coverage, I'd estimate at least $1500 for a decently reliable used repeater and antenna. The license won't be cheap and not to mention radio re-programming.
Looks like that over-hill relay system might be around for a while, I'm afraid.
That's my business.
E-mail me (in my profile) and I'll send you my work number. I'll walk you through the process.
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