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Author Topic: UHF Broadcast Antenna trouble Barry Floyd Phenomenal Film Handler Posts: 1079 From: Lebanon, Tennessee, USA Registered: Mar 2000 posted 07-21-2010 03:59 PM                        I know a lot of you guys are into radio, so you might be able to help me out. I have a new "Patron Pager System" at our theatre, identical to the ones you would receive while waiting for a table at a restaurant. They operate on the UHF band at a frequency of 467 MHz. The transmitter came with a small "rubber duckie" style antenna that works fine for a restaurant, but not for a drive-in theatre. What we are doing is taking customers orders for dinner - sending them back to their vehicle with a pager, and when their dinner is ready we buzz their pager to let them know to come pick it up. What we're having problems with is the signal getting out to the cars. I've moved the "rubber duckie" antenna to the end of a 25 ft. 50 ohm cable with BNC connectors up into the attic space of the concessions building, but I think our metal roof system is blocking the signals from getting out to the vehicles. Another problem I think we're having is the metal bodies of the vehicles is also blocking the signal. The company that makes the pagers sells a 7 foot long external antenna for about $700, but that seems to me like a lot of money for an antenna. I have found some other antennas on the web that are tuned to the same frequency range that the pager transmitter broadcasts in, but didn't know if they would work. Whatever solution we come up with I know it's gonna have to be an externally mounted antenna. What does the "gain rating" say about the antenna. Some are rated at "unity", others have a 3db to 7db gain. Any clues to make this work? Somebody educate me in antennas...  |  IP: Logged Andy Muirhead Master Film Handler Posts: 323 From: Galashiels, Scotland Registered: Dec 2000 posted 07-25-2010 08:49 PM                     Hi Barry, I do know radio, but unfortunately don't have much time at the moment to help you out. You have a few problems. The first is the rubber duckie antenna antenna you are using is not much use. You are correct that a better one would help, and you could even make one yourself for pennies/cents that would be more efficient. A simple dipole would be more efficient and there are calculators on the net to work this out. But this would not help you in the second problem - the length from the transmitter to the antenna. 25ft is quite a distance and the losses in your 50 ohm cable are a fair bit in UHF - it's far too long. Which leads on to the third problem - the attic is good for an antenna, but not if it has a metal roof. That is attentuating your signal. Basically, get your antenna as high as possible in clear space, preferably outside, and keep the length from your transmitter to the antenna as short as possible. Hope this helps a little, Andy  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 07-25-2010 09:02 PM                     A straight piece of heavy copper wire exactly 2 ft. and 1/16 in. would be the correct length to make an antenna. If you wanted to make a dipole you would use 2 pieces of wire, each 12-1/32 in. long. If you can't find a suitable piece of copper wire, a coat hanger wire would do in a pinch. I don't think you need to get the antenna up high, per se. Head high or ceiling high would probably be good enough as long as it is not placed near anything metal. You should not place an antenna any closer than 2 times its wavelength to anything metal, IRRC.  |  IP: Logged Kenneth Wuepper Phenomenal Film Handler Posts: 1026 From: Saginaw, MI, USA Registered: Feb 2002 posted 07-25-2010 09:30 PM                     Barry, Have you ever tried to receive a weak UHF signal on your TV set during the analog era? The same touchy coverage is present in what you are trying to accomplish. Difference is that the receivers have the little antenna built in so the signal has to be pretty good for it to work. A half wave antenna element would be 12.24 inches long at 467 MHz. A simple antenna can be made by stripping the end of a short ( 3 feet or so) piece of coaxial cable and bringing the shield back on itself. The exposed center wire and the folded back shielding should each be the same calculated length. In this case, each about 12 1/4 inches. This is not unlike the antennas used for receiving wireless microphone transmissions. This dipole could be suspended by fish line from the ceiling as an experiment. You might also try having the antenna parallel to the ceiling. Horizontal polarization might work better to get into the cars. I assume that the dashboard near the windshield is the recommended placement for the receivers. Anywhere inside the car or between the seats would be very poor reception. Devices of this type are restricted in the strength of signal they can produce under FCC regulations. Therefore the manufacturer may be unable to assist you in covering the area you require. KEN  |  IP: Logged Bruce Hansen Jedi Master Film Handler Posts: 847 From: Stone Mountain, GA, USA Registered: Dec 1999 posted 07-28-2010 08:13 PM                     The type of antenna that Kenneth is talking about is called a colinear. I have used it many times. Since the power output of the xmitter may be limited by the FCC, you may not want to use something like the Ramsey LPA1. BTW, this is a KIT, you must build it.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 07-30-2010 09:53 PM                     Hey, Barry! Sorry for not catching your post earlier. It would help to know a few more things about your set-up. What's the power of the transmitter? [Worst case it should be marked with an FCC ID than you or we can look up.] What's the marking on your coax cable? That'll let us figure out how much power you lose in transmission. Maximizing reception is about maximizing power output and using the most appropriately directional antenna possible, within the confines of the law. I assume you've oriented your rubber duck antenna vertically, right? Stick antennas radiate in a circle radially outward from the antenna, not in the direction the stick is pointing. So it should probably be pointed "up." Generally speaking it should be a few wavelengths (1 wavelength is 64cm at 467 MHz) away from anything metal, if you can manage it. Of course, if cars are only to one side of your structure, this might change. Realistically, the best solution is probably to mount the antenna (and transmitter) out in the middle of the field where the cars are. Of course that might be tough. But you could trot it out there as an experiment one night. In radio we measure power stuff in dB, or deciBels. They are a confusing unit. One dB is ten times the base-10 logarithm of a power ratio. So a drop from 100 watts to 10 watts is 10/100=0.1, in dB that's 10*log10(0.1) => or -10 dB. A simple doubling (or halving) is 3 dB. Andy is right that loss in the coax can be significant. Of course you can do better by using better coax. Bigger and thicker is less lossy. The junkiest 50-ohm coax you can find is RG58, which at your frequency loses 10 dB/100 feet, or 2.5 dB for your 25' run, i.e. basically half of your transmit power (ok, fine you drop to 56 percent). If you used something nice like LMR400, you drop to 2.7dB/100ft, or 0.7 dB for your 25' run , or 86% of your power. If you went absolutely crazy (makes no sense in your application) and used 1 5/8" heliax, you could cut it to around 0.1 dB of loss (97%!). Antenna gain: you have to be careful, because antenna gain is all about comparisons, and different people use different things for comparison. This is usually indicated by the type of dB: such as dBd (dB relative to a dipole antenna, which radiates outward in a plane), dBi (dB relative to an isotropic antenna, which radiates in a sphere), etc. "Unity gain" means 0 dB, probably 0dBd. Any dipole antenna will have 2 dB of gain over an isotropic antenna, since instead of sending radio waves into the sky, you send them out parallel to the ground. Anyhow, you can't make up gain without sacrificing something. Usually the answer is you get an antenna that radiates directionally. So it is stronger in those directions than in other directions, hence more gain. This is easy enough when you don't want the people on the moon to get your dinner pager tranmissions, but harder when you're trying to cover a wide area. But for reasons nobody seems to understand the stock "rubber duck" antennas that come with many RF products seem to just stink. They are typically quarter-wavelength antennas. I don't know how much better you'll do making your own antenna, but it's unlikely to hurt much, so feel free to try. (at least, until you start getting into the tens or hundreds of watts of power, at which point the transmitter starts to care.) Randy suggests a 2'1/16" length of wire as an antenna. That's...not a good plan. If you use a single length of wire, it should be one quarter of the wavelength, not an entire wavelength. At 467 MHz, a quarter wavelength is 16cm, or 6.3 inches. Though because radio waves travel slower in copper wire than the atmosphere, the theoretically ideal length is usually 5% shorter (depending on the exact wire), or 5.9". Randy's dipole instructions are fine, though again you ought to account for the velocity factor, so stick with 12.0 inches. If you follow Kenneth's instructions, the velocity factor in coaxial cable is different...sometimes as low as 0.6 instead of 0.95. Though I guess I'm not sure how that works when you start stripping the braid around... Still, it's going to be a foot or less. Probably the best tool for testing this stuff is a receiver that operates on your frequency. If you're lucky you'll be able to find a portable VHF radio or scanner that can tune to your exact frequency and then you can hear your data transmissions. Then you can walk around and see how effective your antenna is and what kind of coverage you have. Try asking a local ham. Which brings up another point -- a lot might depend on what else is on your frequency, or even nearby frequencies. I guess you're in a fairly rural area, so the liklihood of interference from other devices isn't anywhere near as high as it is for us city folk. Still, you never know. Looks like two of your neighbors at 1726 Linwood are hams. Perhaps you could knock on their door with some movie passes and ask if you could borrow one of them with a receiver. Best of luck and let us know how it goes! --jhawk  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 08-01-2010 08:03 PM                        Good advice above. The duckie is vertically polarized. The coasters are probably NOT vertical. See if holding them vertically helps. If it does, then you will need a horizontally polarized antenna. Of course, your could just kill it with power! (See Ramsey above.) Louis  |  IP: Logged Barry Floyd Phenomenal Film Handler Posts: 1079 From: Lebanon, Tennessee, USA Registered: Mar 2000 posted 08-03-2010 09:03 AM                        Great information guys. Thanks to you all.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 08-19-2010 07:54 PM                     So how is it working? --jhawk  |  IP: Logged Barry Floyd Phenomenal Film Handler Posts: 1079 From: Lebanon, Tennessee, USA Registered: Mar 2000 posted 09-14-2010 10:17 AM                        Thanks for all of the replies. I ended up getting a Ham Radio base-station "ground plane" antenna (tuned in the 450-470Mhz range), a 50 foot run of LMR400, and mounted the whole thing on a 4 foot iron pipe mast on the eave of the building - up near the top of the roof. The pager signals now reach all the way out to the street.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 09-14-2010 11:03 AM                     I have seen some brands of pager that receive check-in signals from the base station every few minutes. That way, if they stray too far and they lose contact with home base, they beep and go crazy until the person brings it back. The farther the transmitter/antenna can reach, the farther the customer can stray before the pager goes nuts. If they're too far away the person will be less likely to return the unit and more likely to throw it out the car window or toss it in the trash. So, basically, there can be such a thing as an antenna that works TOO WELL.  |  IP: Logged Barry Floyd Phenomenal Film Handler Posts: 1079 From: Lebanon, Tennessee, USA Registered: Mar 2000 posted 09-14-2010 11:55 AM                        Ours do the same thing. If they get out of range they will start beeping and keep beeping until they get back in range.  |  IP: Logged

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