# Antena question

C

#### Claude

Jan 1, 1970
0
Greetings dudes.

I am building a 3 volt FM transmitter, just strong enough to scare the
bejesus out of the wife when she is listening to her crappy music )

The project suggests using a half wave antenna instead of a single wire. I
understand the use of a 50 Ohm coax , it is the wire they hook up in a loop
between the signal and the ground ( respectively the centre of the coax and
the shield) every site that I have checked states that this loop should be
150/Mhz. Here comes the stupid question, what are the units? Do they mean
150 mm per Mhz? I will be attempting transmissions between 99.0 and 100 Mhz
providing I can properly tune the Cap. does this mean my antenna would have
a 15 meter antenna in a loop shape? This sounds wickedly big?

Mathematically challenged old geezer.

B

#### Bob Eld

Jan 1, 1970
0
Claude said:
Greetings dudes.

I am building a 3 volt FM transmitter, just strong enough to scare the
bejesus out of the wife when she is listening to her crappy music )

The project suggests using a half wave antenna instead of a single wire. I
understand the use of a 50 Ohm coax , it is the wire they hook up in a loop
between the signal and the ground ( respectively the centre of the coax and
the shield) every site that I have checked states that this loop should be
150/Mhz. Here comes the stupid question, what are the units? Do they mean
150 mm per Mhz? I will be attempting transmissions between 99.0 and 100 Mhz
providing I can properly tune the Cap. does this mean my antenna would have
a 15 meter antenna in a loop shape? This sounds wickedly big?

Mathematically challenged old geezer.

The units are Meters. The wavelength of 100 MHz is 3 Meters. Therefore a
half wave length is 1.5 meters or 59 inches.

W.L. = c/f where WL = wavelength in meters. C = speed of light, 3 X 10^8
meters/sec. and, f = frequency in Hertz.

The width of the loop should be 1.5 meters, not the length of the wire. The
loop is usually squashed flat and is called a folded dipole. BTW, the
impedance of such a loop is 300 ohms not 50 ohms so you'll need a balun or
other impedance matching device.

C

#### Claude

Jan 1, 1970
0
Bob Eld said:
The units are Meters. The wavelength of 100 MHz is 3 Meters. Therefore a
half wave length is 1.5 meters or 59 inches.

W.L. = c/f where WL = wavelength in meters. C = speed of light, 3 X 10^8
meters/sec. and, f = frequency in Hertz.

The width of the loop should be 1.5 meters, not the length of the wire.
The
loop is usually squashed flat and is called a folded dipole. BTW, the
impedance of such a loop is 300 ohms not 50 ohms so you'll need a balun or
other impedance matching device.

Thanks a lot. Wave lenght, this is all starting to make sense ( I stress the
word starting )
RF seems to be akin to black magic.
So far every little wire in my projects involving oscillators have caused
havoc by causing spurious transmissions ( feed backs that shouldn't be there
etc. Seems like the atmosphere in my shop is laughing at my capacitors )

J
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