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Feeding an antenna with RF power - RF Cafe Forums
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| Whitebird
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Post subject: Feeding an antenna with RF power
Posted: Tue Dec 27, 2005 3:09 pm
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Joined: Tue Dec 27, 2005
1:51 pm
Posts: 8 |
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Hi all,
I built a 29,5 MHz oscillator for
RF carrier generation. When I connect a 5K resistor
between ground and the output of the oscillator,
amplitude is divided by 2. That means the output
impedance is arround 5K.
I also built a 10
cm long antenna made of copper wire. I will use
this antenna as a Lambda/4 one.
Because at
29,5MHz (lambda/4=2.5m), the antenna which is 10cm
long has a capacitive impedance, I have to compensate
it with a 33uH coil. (this value was computed by
the use of this formula: L=(63/F)*cot(360*(h/lambda))
where L is the inductance in H, h:heigth of the
antenna in meters)
Antenna and coil are serially
connected, the other end of the self is connected
directly to the output of my oscillator.
Coil+antenna should have a very small impedance
(arround a few ohms) so when connected to my oscillator
which has 5K of output impedance, coil and antenna
act as a short, and oscillation may stop.
But when I watch the signal it is like nothing
was connected (amplitude is unchanged) , but at
the end of the antenna there is just 1% of the amplitude.
Where am I wrong?? Is it my interpretation that
is wrong or the circuit?
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Honza |
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Post subject:
Posted: Tue Feb 14, 2006 2:44 am
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Joined: Tue Dec 13, 2005
2:42 am
Posts: 5
Location: Czech Rep.
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Everything you describe here make sense. But I guess
the problem is with the antenna and presumptions
you made about it.
You are using 10cm long
antenna for 30MHz (lambda = 10m) ...it means that
your antenna is 1/100 lambda long. And that's pretty
small antenna. Effectively radiated power will be
very small in comparison with the power the oscillator
is able to produce (even if your matching is good).
I guess only a few percents of input power will
be radiated into the space.
But back to the
matching and your oscillator. As I said everything
you write make sense and I can't tell you where
is the mistake. But I guess it is in the length
of the antenna. I guess your antenna do not behaves
like real antenna but more like a common peace of
wire. And what is the influence of 1/100 lambda
long wire on the rest of the circuitry? Almost none!
1/100 lambda long wire placed in the space has very
small capacity against ground.. .and with the coil
in series... it makes quite perfect open circuit.
Isn't this the reason why your antenna has no influence
on the amplitude of oscillations?
If I am
wrong, correct me...
Good Luck, Honza.
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Whitebird |
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Post subject:
Posted: Sun Mar 05, 2006 4:18 pm
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Joined: Tue Dec 27, 2005
1:51 pm
Posts: 8 |
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Thank you for your answer,
I think what you
wrote is correct.
An antenna cannot be shorten
so much. In some cases it is possible to use a lambda/10
antenna and tune it with a coil, but in my case,
as you wrote it, the wire is only Lambda/100 long
and that is not enough.
So it cannot work
properly.
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fred47 |
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Post subject: Short antenna
Posted: Thu Mar 23, 2006 8:39 pm
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| General |
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Joined: Wed Feb 22, 2006
3:51 pm
Posts: 104 |
HI! Electrically short antennas are never
as efficient as half-wave dipoles. But they
can still be used effectively - NIST runs WWVB at
60 kHz (!), which corresponds to a wavelength of
5 kilometers. Their antenna is vertical, and certainly
not 1.25 km tall! See
http://tf.nist.gov/stations/wwvb.htm for information
on the subject. Good Luck! Fred |
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Posted 11/12/2012
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