Antena question

[Claude]

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.
Montreal Canada

 

[Bob Eld]

"Claude" <claudec@cae.com> wrote in message news:fu047a$89b$1@dns3.cae.ca...

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.
Montreal Canada

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.

 

[Claude]

"Bob Eld" <nsmontassoc@yahoo.com> wrote in message
news:Nh5Nj.2575$%V7.768@newssvr14.news.prodigy.net...

"Claude" <claudec@cae.com> wrote in message
news:fu047a$89b$1@dns3.cae.ca...
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.
Montreal Canada


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 )

 

[Tim Wescott]

Claude wrote:

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.
Montreal Canada


Radio wavelengths are about 300 meter/MHz; 150m/MHz is right for a

half-wave.

Google on 'dipole antenna'. It's easy (you can make it from lamp cord
or speaker wire), it works well, and it's not critical to tune. If it
didn't blow up your transmitter final amplifier, you could probably use
a coat hanger, so a dipole should be more than enough.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html