Feeding an antenna
Feeding an antenna
(OP)
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?
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?





RE: Feeding an antenna
But if you're building a lambda/4 antenna, doesn't a null at the distal end mean that it's working exactly as it should?
Mike Halloran
Pembroke Pines, FL, USA
RE: Feeding an antenna
Make your wire longer and make sure there is a RF ground plane under your wire. If you just used a wire connected to an oscillator, the wire inherently needs a large RF ground plane underneath it or those formulas don't work. You probably used a tiny DC ground. The ground plane could be 4 wires, but they should be at least 1/4 wavelength long (8 feet each), then make your antenna wire grow in size (up to 8 feet long) until you get your results.
kch
RE: Feeding an antenna
With respect to the original posting, it sounds like he's found the so-called 'magic' of RF (measurements that don't make sense...). I don't think it can be diagnosed remotely.
RE: Feeding an antenna