vhf loop antenna

I have a short loop antenna--not matched to 50 ohms and very much smaller than a wavelength running at 20 Mhz. How do I drive it to get a maximum current AND be able to filter out harmonics from radiating. I was thinking that some sort of emitter follower bipolar would work fine to drive a big current into the antenna. However, it might be non-linear and generate harmonics. As the antenna has a very low (almost short circuit) input impedance, how can I place a lowpass filter between the current driver and the antenna? All the lowpass filter designs I know of start with asking what the source and load impedance is!

These were popular in ham work in the past. The loop is series resonated with a capacitor. The feed is by having the capacitor be two in series and using the center and one end as the terminals to the outside world.

These proved to be poor radiators and faded quickly in popularity.

Hi,

Have a look at these two links:

http://www.ee.surrey.ac.uk/Personal/...les/cloops.htm

http://ww1.microchip.com/downloads/e...ceDoc/prt5.pdf

These links suggest some tricks to augment the very small input impedance by making the one turn loop into a few turns one and some more tricks (if you can
physically do it by considering your design circumstances of course).

Alternatively, from these links you could calculate the input impedance pretty close to reality and use it as the load impedance data at the output of a low-pass filter.

I think you could simply solve the matching problem {narrow band case of course and I assume coax (assymetrical) feeding} by using a parallel resonant LC circuit for 20MHz. Use around 0.5-1 uH (or less) high Q inductance (on toroidal RF core like Amidon T-56) as the coil and a max 150- 200pF variable (air) capacitor. Connect any one end of the coil and the stator end of the variable cap together. The other (rotor) end of the variable cap goes to the coax shield. Now you connect the remaining end of the coil to any one end of the 1 turn loop and you connect the other end of the loop to also the coax shield to close the parallel circuit (the loop is in series with the coil). The coax inside wire goes to the common point of the coil and the stator end of the cap.
The idea is to insert the 1 turn loop into a parallel resonant LC circuit of 20MHz, this way the series loss resistance of the coil will include that of the loop and the two impedances get uptransformed to higher impedance manifesting between the two legs of the parallel circuit where the coax connects to.

Hope this is understandable? :D

Regards
unkarc