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Feeding an antenna with RF power - RF Cafe Forums

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 Post subject: Feeding an antenna with RF power
Posted: Tue Dec 27, 2005 3:09 pm 

Joined: Tue Dec 27, 2005 1:51 pm
Posts: 8
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?

 Post subject:
Posted: Tue Feb 14, 2006 2:44 am 

Joined: Tue Dec 13, 2005 2:42 am
Posts: 5
Location: Czech Rep.
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) 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.

 Post subject:
Posted: Sun Mar 05, 2006 4:18 pm 

Joined: Tue Dec 27, 2005 1:51 pm
Posts: 8
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.

 Post subject: Short antenna
Posted: Thu Mar 23, 2006 8:39 pm 
User avatar

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104

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 for information on the subject.

Good Luck!


Posted  11/12/2012