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Author Topic: Interesting FM (broadcast / receive) Antenna ...  (Read 8977 times)

Posts: 40


« on: October 01, 2012, 04:22:11 PM »

I'm thinking about putting up another FM (receive) antenna.

This is an interesting FM (broadcast / receive) Antenna ... seems ok, but I'm a bit
puzzled over the longer lengths that could be used.

"Calculate the length of the antenna - The formula for a 5/8 wave antenna using typical 300 ohm "twin lead" is L = 300/f x 5/8 x 1/2; where "L" is the length in meters of the antenna and "f" is the frequency in Mhz of the station to be tuned. This can be simplified to L = 93.75/f."

Improve the antenna design. The antenna improvement in this article is a design for a simple 5/8 wave "folded dipole" or "T" shaped antenna. This design will outperform any internal or telescopic rod antenna that may have been supplied with the receiver. It is also similar to those provided with some more expensive home stereo receivers.

"To improve upon this simple design, simply double, triple, quadruple, etc. this value as such: 37.66" x 2 = 75.32", or 37.66" x 3 = 112.98", and so forth.

The 112.98 inch antenna will outperform the 75.32 inch antenna, which outperforms the 37.66 inch antenna.

Of course, there is a "point of no return" when the multiple is so great that the signal at the ends of the antenna cannot travel the entire length due to the electrical resistance of the wire. This limit is around 100 meters (a little more than the length of a football field)."

The impedance transformation for my receiver (Grundig) would have to be - 6:1.
(50 ohm input / connector.) Anyway, they use a 75/300 ohm transformer. 
Right now I'm using a loop - 1005 / 101  - 9' 11-1/4" I (trimmed) Tuned the loop with mfj-259b. It's up about 20 feet. Works ok, but not great.

"To improve upon this simple design, simply double, triple, quadruple, etc. this value as such: 37.66" x 2 = 75.32", or 37.66" x 3 = 112.98", and so forth." Hmmm? So we could make this like 37.66 x 24 and have a length of 903.84". This would be a high gain (receive) antenna for 98 Mhz? 

Has anyone tried this antenna?

Posts: 17476

« Reply #1 on: October 03, 2012, 12:27:03 PM »

Quote from: VE9TS

This is an interesting FM (broadcast / receive) Antenna ... seems ok, but I'm a bit
puzzled over the longer lengths that could be used.

I'm more than a bit puzzled over ANY of the lengths he used.

His antenna is NOT 5/8 wave, and even a 5/8 wave folded dipole isn't going
to work particularly well.  Yes, I'm sure it will pick up some RF, and perhaps
more so that a standard whip, but you might as well choose any other random
length in that case.

A folded dipole should be a half wavelength.  Since 100 MHz = 3m wavelength,
this will be very close to 1.5m, and this value is probably adequate to cover the
band without any further adjustment.

Doubling the length and using a folded dipole on its second harmonics is NOT
a good idea:  the currents are out of phase in the two wires, so it takes a
lot of total current to get any effective radiation.

If you want a longer antenna, then an Extended Double Zepp might be a good
choice:  use regular wire for the radiator (not a folded dipole).  The
total length is about 1.25 * the wavelength, so 3.75m, or say 1.85m on
either side of the feedpoint.  This will have more directivity, so it should
be installed broadside (at right angles to) the incoming signal.

Your current loop (assuming it is in the vertical plane rather than horizontal)
will outperform his recommended antenna.  And probably more so for his
longer versions. 

If you need more gain that that, there are several possibilities depending on
the space you have available:  a 2-element quad, a yagi, various sorts of
colinear, broadside or end-fire arrays, or even a vee-beam or rhombic.

Posts: 40


« Reply #2 on: October 03, 2012, 05:55:40 PM »

Thank You OM ..

OK .. I thought it kind of fantastic Smiley

I've heard about people picking up TV stations (years ago) many hundreds of miles away, using wire Vee-beams and Rhombics. 

vhf yagi beam antennas, boosters, rotators and all that is a bit much for now, so I think I'll try a Vee Beam and see how that goes.



Posts: 681

« Reply #3 on: October 05, 2012, 05:48:04 AM »

That WikiHow article may have been entered into a competition for maximum number of technical errors. The author can't seem to make up his mind whether he is making a 5/8 wave or a folded dipole, and the stuff about lengthening it is pure drivel. That is the nice thing about the internet: everyone has the same democratic right to share their ignorance of any subject with the whole world.

Posts: 17476

« Reply #4 on: October 05, 2012, 08:06:54 AM »

There are a lot of myths among the less-technically informed regarding the magic
5/8 wave antenna, and I've seen them used in many places where it makes no
sense at all (such as the radiator on on a J-pole.)  In this case the best I can
figure out is that the author assumed a total wire length of 5/8 wavelength would
make the best folded dipole for some reason, which makes it 5/16 wavelength

I modeled it, and while the radiation pattern isn't that different from a normal
dipole (which is true of any short dipole) the feedpoint SWR is about 50 : 1
on 300 ohm twinlead.

The regular half wave folded dipole is a much better choice.

Posts: 159

« Reply #5 on: October 05, 2012, 10:31:21 AM »

Not be seem negative or anything, but in my experience, categorically, anything you see in Wikihow is wrong or seriously flawed, especially when it comes to anything technical. The site is essentially a click generating vehicle for ads.
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