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  #1 (permalink)  
Old 04-02-2013, 9:05 AM
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Default How-to make homemade FM Antenna - for listening purposes

New here. If this is the incorrect section I apologize. I've got an old AM/FM radio for my shop and there's no antenna. There are hookups in the back, and I can switch the radio from 75 (ohms?) to 25. There are multiple connections for me to hook an antenna up to which you don't have to educate me on here. I don't want to flood you with a million questions. I am mostly wanting an idea of the best antenna I can build for this radio from the materials I have on hand in my shop:

3-5' x 1/2" of copper pipe and various copper fittings.

1000' of cable tv coax

About 100' of heavier gauge copper stereo wire (think ground wire)

I can solder a little and I have thought of some designs, but don't know if they'll be any better than simply hooking up a long stretch of coax to the radio and fixing it to the roof of my shop.

Any help is greatly appreciated and illustrations would be a dream. If I can see it, I can build it.
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Old 04-03-2013, 4:06 AM
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You definitely want to see the FM antenna projects of K6STI:

88108 MHz

Great stuff even if you aren't into FM dx-ing..
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Old 04-03-2013, 4:47 AM
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How 'fancy' do you want to get? A simple five foot vertical antenna fed from one end mounted on your roof would work well. Or, feed the thing in the center to lower the impedance. The size (diameter) of the conductor used for the antenna doesn't make a lot of difference. It would have to be large enough so that it doesn't 'wilt' or fall over, but that's also a matter of how you mount the thing, or hang it. If your roof isn't metal, a 10 foot long wire made into a loop antenna laying on that roof should work too. What antenna would work best? Beats me, depends on how reception is where you are and I have no idea.
You can't hurt anything by 'playing' with different types of antennas so give it a try.
- 'Doc
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Old 04-03-2013, 6:19 AM
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WDE86- From what you wrote I have the impression you'd actually like to construct an FM broadcast antenna (88-108 mHz) using some of the materials you described that you have on hand. But to be one step ahead of the critics who will chime in on this thread I will first tell you the reasons NOT to make the antenna I'm about to suggest:
- It's not specifically tuned for the FM broadcast band
- It's not horizontally polarized
- It has no gain, with performance similar to any half-wave antenna

The antenna I'm referring to is the Off-Center Fed Dipole (OCFD) described with construction details here in the RR Antenna Wiki: Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

Here are a few reasons you might want to consider the OCFD for FM broadcast:
- It is very broad banded, even though its center frequency is in the AM aircraft band (which is not far from FM broadcast)
- My experience with it on FM broadcast is very good. Some people report that it is so good on FM broadcast that they need a trap to keep out FM stations when they want to use it for aircraft and general scanning
- It is very simple to build
- It can be installed indoors in an attic or high location without becoming a lightning magnet, nor requiring special grounding considerations
- It can be built with the stuff you describe you already have on hand
- It's extraordinarily inexpensive in both $$ and effort, so if it doesn't work the way you expect you're not out much of either

If you make one, please post back here with your experience. Thanks for reading!
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Old 04-03-2013, 7:12 AM
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Quote:
Originally Posted by LtDoc View Post
How 'fancy' do you want to get? A simple five foot vertical antenna fed from one end mounted on your roof would work well. Or, feed the thing in the center to lower the impedance. The size (diameter) of the conductor used for the antenna doesn't make a lot of difference. It would have to be large enough so that it doesn't 'wilt' or fall over, but that's also a matter of how you mount the thing, or hang it. If your roof isn't metal, a 10 foot long wire made into a loop antenna laying on that roof should work too. What antenna would work best? Beats me, depends on how reception is where you are and I have no idea.
You can't hurt anything by 'playing' with different types of antennas so give it a try.
- 'Doc
Fanciness or lack thereof doesn't bother me one way or another. I can sweat pipe a little and I'm fairly handy, so I don't mind getting a little "involved" with this. It seems like it could be a cool little project and CB/Ham radio stuff has always interested me a little. So maybe this will light the fire. I don't know for sure.

Quote:
Originally Posted by popnokick View Post
WDE86- From what you wrote I have the impression you'd actually like to construct an FM broadcast antenna (88-108 mHz) using some of the materials you described that you have on hand. But to be one step ahead of the critics who will chime in on this thread I will first tell you the reasons NOT to make the antenna I'm about to suggest:
- It's not specifically tuned for the FM broadcast band
- It's not horizontally polarized
- It has no gain, with performance similar to any half-wave antenna

The antenna I'm referring to is the Off-Center Fed Dipole (OCFD) described with construction details here in the RR Antenna Wiki: Homebrewed Off-Center Fed Dipole - The RadioReference Wiki

Here are a few reasons you might want to consider the OCFD for FM broadcast:
- It is very broad banded, even though its center frequency is in the AM aircraft band (which is not far from FM broadcast)
- My experience with it on FM broadcast is very good. Some people report that it is so good on FM broadcast that they need a trap to keep out FM stations when they want to use it for aircraft and general scanning
- It is very simple to build
- It can be installed indoors in an attic or high location without becoming a lightning magnet, nor requiring special grounding considerations
- It can be built with the stuff you describe you already have on hand
- It's extraordinarily inexpensive in both $$ and effort, so if it doesn't work the way you expect you're not out much of either

If you make one, please post back here with your experience. Thanks for reading!
Thank you for your informative post. I will definitely post pictures of whatever apparatus I come up with. It may not be pretty to you all, but if it works I'll be happy.
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Old 04-03-2013, 8:08 AM
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IF the copper stereo wire is the zip cord type speaker wire try this:

Cut 25 feet.
Tie a knot 5 feet from one end.
unzip the wire from the knot to the short end.
Separate the long end a couple inches and strip the insulation about a half inch.
connect the bare wires to the antenna terminals.
Stretch out the unzipped part and try different locations until you get the strongest signal.
Hang in between two convenient objects.
Enjoy the music.

Quick, cheap, effective.
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Old 04-03-2013, 8:49 AM
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Or, if you want to do it by the book:
1. Cut a piece of stranded wire to 4 feet 9.5 inches.
2. Cut the piece of wire in half.
3. Attach a piece of 300-ohm twin lead or, (really makes no difference) a piece of RG59 coax
to the two pieces of wire (center conductor to one wire, shield to the other)
4. Terminate the other end of the lead-in wire with proper connector plug into the radio and start
listening. The 4 feet 9.5 inches dimension is for 98 mHz, which is the center of the band. You may
have to move the antenna around a bit to get best signal on some stations. Hang the antenna itself
in a horizontal position because that's the polarity of most FM broadcast signals.
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Old 04-03-2013, 11:33 AM
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Quote:
Originally Posted by W2NJS View Post
Hang the antenna itself
in a horizontal position because that's the polarity of most FM broadcast signals.
Then why do vehicles come with a vertical?
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Old 04-03-2013, 11:45 AM
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Most of the FM broadcast transmitter antennas are either dual or circular polarity. Which makes sense when you think of the wide variety of receiver antennas (vehicles, portables, console radios, etc.).
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Old 04-03-2013, 1:28 PM
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Yep. Out of curiosity, did you know that or have to look it up?
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Old 04-03-2013, 2:54 PM
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In the 70's I worked in commercial radio broadcasting as well as AFRS while in the Army. So I knew the circular polarization part but when looking up to confirm I learned the cross-polarization info.
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Old 04-03-2013, 4:44 PM
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When FM broadcasting started on the 88-108MHz band (originally 88-98 only) they used vertical or horizontal polarisation the same as TV stations as they thought that the broadcasts would be received on fixed non-portable receivers with external antennae. When portables and car radios came along, they found that they suffered from blind spots caused by re-radiation and cancellation of the signal, a phenomenon called 'picket-fencing' especially noticed on car radios as the signal came and went.

So they switched to circular or cross polarization so that re-radiation was not in or out of phase with the main signal and the problem disappeared.
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