DIY FM Antenna Project

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 built one in my first apartment! Had a fairly square main room so I stapled 300 ohm flat lead in a diamond where the wall met the ceiling and put in the resistors called for. The 4 legs of the diamond were about 10 ft. long each. Local stations came in like flamethrowers, and it turned out it was aimed just about right to get very distant stations from St. Louis. I was picking up FM from 125 miles on my '81 Sherwood receiver. Good times!

There are a lot of rabbit ears of newer vintage than that old ad above - they came with the last generation of CRT TV's and usually didn't have a base, but a plastic stud that fit into a socket on the TV set. You can find em at garage sales etc. I just cut a scrap of 4x4 and drill a hole in the top. Easy to rotate when needed and you can telescope them down when not in use for a neat appearance.
 
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I have built omnidirectional FM antennas with 300 ohm twin lead. 2 dipoles at 4' 8.5" (just happens to be railroad standard gauge, so easy to remember). Mount the 2 dipoles at right angles and connect in parallel. Add a 300 ohm to 75 ohm transformer, 75 ohm coax to the receiver or tuner, another transformer to get back to 300 ohms. Yes, there is a mismatch with the 2 dipoles in parallel (150 ohms) and the 75 ohm transformer.

The result 19 miles south of downtown Minneapolis in the attic of a 2 story house was listenable stations on about 75 of the 100 available channels. The same antenna configuration, now 40 miles northwest of downtown Chicago, also in the attic of a 2 story, brings in 60 stations on a vintage Fisher 600-T receiver.
 
I bought an amplified rabbit ear antenna at the local thrift for $.50. Hooked it up to my computer room setup (Pio SX 1250), and got excellent results. I can get stations from the Gulf Coast and New Orleans quite well. The local stations (20-30 miles away) bang the signal meter all the way to 5+. The Coast and N.O. stations are usually between 3 and 4. These stations are 60 to 80 miles away and are easily listenable. Beats the old dipole hands down,
 
I have built omnidirectional FM antennas with 300 ohm twin lead. 2 dipoles at 4' 8.5" (just happens to be railroad standard gauge, so easy to remember). Mount the 2 dipoles at right angles and connect in parallel. Add a 300 ohm to 75 ohm transformer, 75 ohm coax to the receiver or tuner, another transformer to get back to 300 ohms. Yes, there is a mismatch with the 2 dipoles in parallel (150 ohms) and the 75 ohm transformer.

The result 19 miles south of downtown Minneapolis in the attic of a 2 story house was listenable stations on about 75 of the 100 available channels. The same antenna configuration, now 40 miles northwest of downtown Chicago, also in the attic of a 2 story, brings in 60 stations on a vintage Fisher 600-T receiver.

I know just enough about antennas to be dangerous so take this with a grain of salt. Just curious why you can't just run flat lead to the receiver? Seems like the two Balun transformers would result in some signal loss. Apparently that isn't much of a problem I guess, based on your results.
 
You can run 300 ohm flat lead to the receivers. However, it must be spaced away from everything using J hook standoff, or similar, insulators and should be twisted periodically to cancel signals picked up along the run.

With 2 dipoles in parallel, there would be a similar impedance mismatch (300 parallel with 300 = 150, then mismatch to 75 ohm cable). Same 2:1 or 1:2 mismatch either way.
 
Add a 300 ohm to 75 ohm transformer, 75 ohm coax to the receiver or tuner, another transformer to get back to 300 ohms. Yes, there is a mismatch with the 2 dipoles in parallel (150 ohms) and the 75 ohm transformer.


Adding matching transformers (baluns) is just adding loss, although possibly small.

An antenna with a 150 Ohm feed point impedance (2 folded dipoles antennas) fed directly with 75 Ohm coaxial cable (no balun or matching transformer) will have a signal loss due to the mismatch of about 0.5 dB (this is called the mismatch loss). If we add a balun transformer (300 Ohms to 75 Ohms) that will add another 0.5 dB or more of loss (some balun transformers may have 2 dB or more of loss) because it is still a 1 to 2 impedance mismatch (150 Ohms antenna impedance to 300 Ohms balun impedance).

Since the dipole antenna is a balanced antenna, if connected directly to coaxial cable, one should use a correct common mode RF choke at the feed point.

No balun transformer will be needed at the receiver/tuner unless the only FM antenna input is 300 Ohm.
 
I was going to build one of those to boost the output on my solid state kit-built AM transmitter for the old radios. Didn't know they were good for FM too.

Slightly off topic, but are you still thinking about doing this? A J-Pole antenna is not necessarily the best choice for this application.

If tuned to the middle of the AM broadcast band it would need to be over 700 feet long, with a matching section about 235 feet long.

A tuned loop antenna would likely work much better. The ground losses and mismatches when using a monopole antenna become significant.



BTW, antennas do not care if the modulation is AM, FM, CDMA, TDMA, quadrature and so one. All antennas care about is the frequency of operation. This determines their needed electrical length.

Below is a picture of a typical monopole low power AM transmitter antenna. The key here for maximum signal output is the correct tuning of the base loading coil. It is not a plug and play operation. With correct tuning, and a low loss RF ground system, even an indoor antenna might be expected to cover a city block or so with a quality type 15 AM broadcast band transmitter and even greater distance if the antenna is mounted outside.

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It should not be difficult to produce a strong signal for your radios, even with a shorter piece of wire used as the antenna element, as long as the transmitter output stage is/can be tuned to the antenna. The antenna will appear as a very reactive load (capacitive reactance) and the output stage of the AM transmitter needs to be able to tune this reactance with a comparable inductive reactance. This along with a good low loss RF ground should provide more than enough signal.

Again, a simple tuned loop antenna may be an effective way to go.

Now, back to your regularly scheduled FM broadcast...


























 
Thanks. It has been some years since I built the SSTRAN AM transmitter kit. It came with a 3-ft single wire and a ground wire, because that would limit the output to 100mW or whatever the FCC limit was. My memory may have failed me, but I thought they had drawings for a J-pole IF you wanted to increase the output to 1W [which would violate FCC regs so they weren't recommending that...]. Possible it was just a vertical copper pipe. I'll make a note of your post and diagrams. Obvious question is how you 'tune' it and how do you design the 'loading coil'? Antennas are a bit of a mystery to me.
 
There are instructions in the manual for selecting a combination of the 4 internal inductors and the internal trimmer capacitor to tune a random length of wire antenna, such as you are using, and tune the transmitter for maximum output. Again, a good RF ground will improve performance.

Here is a link from their web page. It explains how to modify your transmitter to use an external base loaded antenna.

Using an external 3 meter radiator, the amount of inductance for the base loading coil will vary from about 250 microhenries at the high end of the band to about 2600 microhenries at the low end of the band. The exact inductance will depend on the frequency of operation, antenna location, height above ground, length of ground lead, size of ground plane, proximity to other objects, and other environmental factors.

To be clear, the above is just an example. The FCC sets specific rules regarding the total length of the radiating element and the ground lead (3 meters total) and transmitter power. This limits the maximum RF signal strength. This is a method to limit possible unwanted spurious signals and limit possible interference to licensed stations and others wanting to use part 15 AM transmitters.

Properly set up, that transmitter, even using a directly connected piece of wire for the antenna, should provide a rock solid signal just about anywhere in your house if the antenna is in the clear, you have a clear frequency to use, your RF noise floor is not excessive and you have a reasonable RF ground.
 
Funny I was not able to find anything on antenna construction when I looked at the SSTRAN website earlier today. There do not seem to be any links to that page from their home page (sstran.com). Thanks for the link.

I was thinking of the larger one for something like a vintage car show, to cover a couple blocks downtown or a parking lot.
 
The plan specifies 16 ga magnet wire for the loading coil; seems rather heavy duty for such a small amount of power. They are very specific about the number of turns in the coil. Could it be wound using a smaller gauge and would that change the required number of turns?
 
-- 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.

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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 :)
Home Depot sells a GE brandedhttps://www.amazon.com/GE-33676-Indoor-Antenna-High-Definition/dp/B000EIMKYC rabbit ears antenna.These feature a male coax output. ~ $6 IIRC. I have a couple. I also have a yard sale adjustable.
 
thanks to all for input

ended up getting a prefab FM antenna through internet, connected wi coax.

That sounds good. It is working well for you?



The plan specifies 16 ga magnet wire for the loading coil; seems rather heavy duty for such a small amount of power. They are very specific about the number of turns in the coil. Could it be wound using a smaller gauge and would that change the required number of turns?


Changing the gauge of the wire will change the turns per inch and that will change the inductance. The actual amount of inductance that you might need is dependent on the factors previously mentioned.

I would build something similar to the picture below. It is a very simple adjustable inductor. This makes it easy to to tune the antenna for differing situations.

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You can use a basic inductance calculator...

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Your total inductance will depend on your operating frequency, the length of your antenna element and other factors that I have mentioned.

It is not uncommon for a basic 102 inch CB whip antenna to be used. It makes mounting easy.

to cover a couple blocks downtown or a parking lot.

Covering a couple of blocks in the open (a parking lot) should not be difficult.

Downtown...I am not sure what you mean. That could open a can of worms in terms of coverage. If your are talking about a couple of blocks of a street, if you could set up in the middle you would likely be able to cover it, but there are no guarantees because there are so many variables.

Depending on the variables, you might be able to use the built in tuning network of your transmitter, just with a longer antenna element. It might be worth experimenting. Using a 102 inch CB whip would allow you to keep the antenna element vertical and straight.

Again, remember that a good RF ground is important.

Sorry for the drift to AM RF from FM RF, but like we used to say when I was growing up on the farm, designing and building antennas by the light of a flickering kerosene lamp because we did not have electricity, RF is RF.:D:D:D
 
BTW, here is the output stage of your transmitter. Note the series connected, switch/jumper selected inductors to tune (resonate) the antenna and the trimmer capacitor for fine tuning.

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I like music said
That sounds good. It is working well for you?

so far
despite being 'directionless' I'm finding that turning it affects reception!
right now I've got some Spanish-language station ranchero. something different!

thanks again to all for feedback.

wonder how big a diy AM antenna has to be?
 
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