Feeding an antenna

[Whitebird001]

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?

 

[MikeHalloran]

No expertise claimed here.

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

 

[Higgler]

Why did you pick such a tiny antenna made of wire. When you make electrically small antennas, even though you compensate then with a matching inductor, they are such weak antennas that their fairly useless. A one ohm antenna isn't common unless you're doing some simple detection device with a range of a few inches.

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

 

[VEBill]

Cheap R/C cars (from Radio Shack, etc.) often have controllers at about 27 MHz and have similarily short antennas (maybe 6 inches). Even with a super-cheap receivers in the R/C car, they still achieve control ranges out to several hundred feet while emitting less than 100mW.

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.

 

[logbook]

"a ground plane under the wire" is a bit ambiguous. If the wire is run parallel to the ground plane and close to it, for example a pcb track on one side and a ground plane on the other side, that is a bad antenna. The ground plane wants to be near the feed-end of the wire and perpendicular to the wire.