Posted: 5/3/2012 6:54:32 AM EDT
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I need help with an antenna for receiving commercial FM/AM radio broadcasts, so this isn't a question about Ham band specific antennas.
A very dear friend of mine lives about 50 miles from Austin on a ranch that is a little lower, but not significantly, than the surrounding terrain. He gets terrible radio reception from stations in Austin while in his large metal building shop, and a mid 90's receiver/stereo/all-in-one. I'm going to replace this (open to suggestion to what), but I am concerned that the reception will still be poor unless I mount an external antenna to the side, or roof of his metal building. I am wondering if I should simply make a dipole, and then figure out how to connect it to the radio (don't think I've seen a head with coax connector), or look for some commercial solution. I am reasonably technically skilled (I can solder, and managed to install a yaesu ft-7900r in in my car), but I am unsure of the best way to proceed. Thanks in advance. |
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You could try making a copper pipe J-Pole and mounting it outside for FM stations. Directions are on the net for how to do so. You will need to change the lengths of copper pipe for the middle of the FM radio band (shoot for 98 MHz).
This URL contains instructions and shows you the math to get it right http://www.alpharubicon.com/elect/jpolejaden.htm Connecting to the radio will likely entail opening it up and figuring out where the existing antenna connects to. You can get 50-ohm coax and PL-259 connectors at radio shack that will fit the bill. You will want to make a pigtail with a piece of coax and connector to come out of the radio. Solder the center conductor of the coax to the spot where the antenna connects to. You will want to solder the braid/shielding to the ground on the radio's circuitry (hopefully close to the antenna spot on the board). Mount the antenna outside, hook up a length of coax between the antenna and radio and hope for the best. 
-Foxxz |
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Quoted:
Just for shits, I'd run a wire to the wall of the metal building itself. It's probably grounded, but I've seen weird stuff like that work before. If it doesn't work, you're not out anytihng but a few minutes. I was going to suggest this 
The other option is run a very long piece of wire in a loop around the shop. If you end up mounting an antenna outside you can use some TV antenna 300ohm twinlead and connect it to the J-Pole or other antenna. You will not be transmitting so it is not critical, it will give less losses than the coax, and will likely not require modifications to the radio. |
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Check the back of the receiver it may have a 300 ohm twin lead connection. This will dictate the type of feed line and antenna. A small TV balun in reverse would provide a coax connection to be run outside to a 75 ohm antenna system. Radio shack used to sell a yagi for the FM broadcast band and was probably a 300 ohm antenna setup for twin lead. By all means, try the metal side of the building. It doesn't matter if it's grounded as long as your a couple of wave lengths up. Generally, an outside antenna will do a much better job. HTH Best, Rob |
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Quoted:
Just for shits, I'd run a wire to the wall of the metal building itself. It's probably grounded, but I've seen weird stuff like that work before. If it doesn't work, you're not out anytihng but a few minutes. I used to work in a guardshack next to a fence that ran around a 16 acre property. Hooking a portable SW all mode receiver did wonders for reception 
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I am concerned that the reception will still be poor unless I mount an external antenna to the side, or roof of his metal building.
A "passive repeater" might work for FM broadcast: perhaps a 1/4-wave ground plane for 98 MHz right side up on the roof, a 1/4-wave ground plane for 98 MHz up side down on the ceiling, and just enough coax to connect the two. Is his roof metal also? |
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First, you must understand that the antenna should more or less match the frequency it needs to receive.
The AM Broadcast band is down at 530 - 1700 khz (0.530 - 1.7 mhz) roughly centered around 1 mhz. A halfwave dipole for 1 mhz would be 468' long. The FM Broadcast band is 88-108 mhz, up in the VHF range, actually, between channels 6 and 7 on your TV. A halfwave dipole for 100 mhz would be 4.68' long. Quite a difference, huh? One antenna will really not serve both. For the AM band, due to the length, a dipole is really not practical. So what is? A loop, tuned or untuned! Simple DIY untuned loop for AM Broadcast: Make a wood frame, a cross, with the vertical and horizontal pieces just over 17". Put brass hooks (like a cup hook), from the hardware store, on the four corners forming a diamond 17" on the diagonal, 12" on the side. Wind 4 turns of 22 ga stranded insulated hookup wire around this frame. Tie the two ends to 300 ohm twinlead. The other end of the twinlead... and it does not matter which wire goes to which radio connection... attaches to the AM antenna post on the radio and the ground post. This antenna is directional and can be used to null noise, or bring in a weak station by rotating it around some. This is really the old Carver Ultimate AM Antenna, a sketch they supplied with the Carver Ultimate Tuner. Here is a great tuned loop. This type of loop is known as a "box loop" or a "sportsmans loop". Popular for receiving the local ballgame blacked out on the local stations, but this loop will pick up distant stations and bring in the game. It is 24" on the diagonal, 17" on each side, and used 15 turns of 22 ga magnet wire (aka "enameled wire"). The two ends of the loop attach to the frame and one lug of a 365 pf (or 370, 380, 400 pf , etc) tuning capacitor. On the inside of the frame make one turn of a separate "pickup" loop. The two ends of this pickup loop connect to the AM antenna post and ground post of your radio. http://www.mtmscientific.com/loop.html If your radio has a built in ferrite loop stick antenna, then no direct connection needed. Place the radio beside the loop antenna, orient for best reception, dial in the tuning capacitor, and be amazed. This antenna is far superior to any AM antenna you will stretch out outside. I personally own and use this loop for AM reception with my stereo gear. It will also tune the 160 meter band. FM broadcast... the easiest and simplest, again, 22 ga insulated hookup wire, connect to the FM antenna terminal and stretch out 10' to 50' outside. Better, any yagi type VHF TV antenna you may have, that old Channel Master in the garage is fine. Put it outside, up 20'-30'. and run twinlead down to your FM tuner. Best, a dedicated FM band yagi. Look up at Antennacraft. http://www.antennacraft.com/ The Antennacraft FM6 is excellent. You can spend much more and not get much better antenna. Radio Shack used to sell the FM6 under their label for $25. I've got mine on an eves mount, about 6' above the peak of my roof. I think this is the same as the Antennacraft FM6. http://www.radioshack.com/product/index.jsp?productId=11113386&filterName=Category |
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Incredibly easy to homebrew a little dipole for FM reception. Get some RG59 cable with an F connector at one end, and some PVC pipe and fittings. 30" on each side of a T connector, you figure out how to attach it to the building. At the antenna end of the coax, go 30" back from the bare end of the coax, carefully remove a small section of the jacket, and then start pushing the braid apart. Push the braid apart enough to create a hole big enough to fish the center conductor and insulator out through the hole. If the insulator has a foil shield, you'll need to remove that. Voila, FM band dipole. Maximum reception is broadside to the dipole.
For AM reception, just run a long wire on insulated standoffs down the roofline or from roof to another support. Alternatively you could get or make a little multi turn loop antenna. If you do the long wire I would suggest a static drain type lightning protector. |