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DIY FM Antenna Project

screenersam

screenersam

aka Blind Sugar McGee
I know there are several threads about FM antennas, I have a few things to clarify.

1) I plan to use some heavy-gauge copper wire for an element. how long? 38"?

2) do I split it in half and attach one lead wire to each piece?

3) if I keep the element in one piece, how do I attach the lead wire(s)?

I plan to take pix and post here. waiting advisement from expert types.
 
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See the picture below.

upload_2017-7-23_22-16-19.png

A common mode RF choke at the antenna end of the coaxial cable may improve performance.

CHOKE.JPG





Here is a link to a calculator for the element lengths.

upload_2017-7-23_22-18-0.png

You can choose the frequency of operation for your particular stations of interest if needed.
 
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Note that all antennas and antenna calculations are hit and miss if the antenna is in close proximity to anything, especially metal. Thus, an indoor dipole doesn't need to be trimmed to the last 1/64 inch.
 
As Conrad mentions, it is not a bad idea to keep the antenna away from other conductors and metal objects.

Other conductors and metal objects can adversely impact the reception pattern of the antenna and may lead to the pickup of unwanted electrical noise.
 
I made a folded dipole antenna when I was a teenager. I must have read how to do it in my dad's Popular Electronics or Radio Electronics magazines. It was about 18' long (give or take a few feet). It worked really good. I wish I still had it.

Where can you buy 300 ohm twinlead anymore?
 
Should be easy to order from any of the usual electronic suppliers or audio specialty places, but they don't carry it in the home stores anymore. It was getting scarce a couple years ago, so I bought a hundred feet at a hamfest.
 
-- of course, the venerable TV "rabbit ear" antenna is a tuneable dipole. If you have one, or can scare one up at a thrift store, yard sale or junkpile near you, that's not a bad place to start.

106.jpg

source: www.radioshackcatalogs.com (1973)

Aim low (e.g., the buck-twenty-nine model). :)

I suspect that at this late date, it'd be easier/cheaper for your average scrounger (e.g., me) to find an abandoned pair of rabbit ears than a hank of 300 ohm twinlead to build a dipole. Well, OK, I have a fair amount of 300 ohm twinlead in stock -- but, for most folks, I still think sourcing an old pair of rabbit ears is a fairly sensible strategy :)
 
I had several rabbit ear units but pitched them in a senseless cleaning fit a while back.
just using speaker wire for lead (indoor setup)
 
cut the copper wire and the pvc casing/frame
100_3415.JPG

attached lead wire (probly should have soldered but lazy and taped)
100_3416.JPG

final product leaning against wall (junk room so who cares)
100_3417.JPG

not as big an improvement as I'd hoped
 
What are you using for feed line and what terminals are you connecting the antenna to?

And remember it is all about the amount and quality of signal that the antenna sees.
 
If you have lots of space and don't care how it looks, you might try an indoor cubical quad, which can have up to 7.5dB gain over a dipole. It's just two square loops of wire, one a bit larger than the other, on a common axis. The bigger one, called "reflector," would use a total wire length of 1030 / 98MHz = 10.51 feet. The smaller one, called "driven element" needs 998 / 98MHz = 10.18 feet and is cut for attachment of 75R coax at center bottom. These are directional antennas, so position the driven element toward the transmitter of greatest interest. Spacing between the elements should be 128 / 98MHz = 1.306 feet. Fatter wire yields flatter response over the FM band. I used one of these for a while when I was living in an apartment by myself and decided it was well worth the effort.
 
I never built it, but there was an article in the January 1982 issue of Audio magazine called "Try a Rhombic FM Antenna". Do a search for those exact words and Google will find a PDF of the article. I'd be curious as to how well they work.
 
I LIKE MUSIC said:
What are you using for feed line and what terminals are you connecting the antenna to?

And remember it is all about the amount and quality of signal that the antenna sees.
Click to expand...
plain speaker wire to a wire / coax adapter plug into back of receiver (Insignia)

this isn't it but it's like it
mCn4CP4TxmAvLrIq8I0eoSw.jpg
 
Some comments...


What you show in your picture in not just a basic adapter. It is what is called a balun transformer. It changes impedance from 300 Ohms to 75 Ohms and from a balanced to a unbalanced signal.

Your dipole antenna is already a 75 Ohm antenna. There is no need to convert impedances. This will just add to the loss of signal.

Speaker wire is a poor conductor for the FM signal.

Regardless of the type of antenna, using the correct feed line is important. In the case of your antenna it should be 75 Ohm coaxial cable. The same for the full wavelength antenna in the link above. Speaker wire is not the best choice.

For a basic simple FM indoor antenna, the rabbit ear type antennas work well. Use the calculator that I posted as a starting point for the length of the elements. You can experiment with the length of the elements, the angle between the elements and the location/orientation for the best reception.
 
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