Thank you.

If I want to model a such shielded loop for spice, do you know if it is just to model a normal loop and add the capacity between the core and the shield?

In order to be effective, a balun's common mode impedance has to be at
least several times greater than the common mode impedance of the
antenna. The common mode impedance of a very small loop like that will
be very high, so it won't be possible to make an effective balun, except
perhaps at the high end of your frequency range.

I suggest that you use a "shielded" loop. This is a technique to improve
the balance. Also take as much care as you can to keep the loop
physically symmetrical.

2 loops can be an option, or a loop for the LW and MW bands and a vertical for the SW band.

Bset,
Dominique


A frequency range of more than 150:1 seems ambitious to me, but I'll
defer to folks who have more experience with loops than I do.

Roy Lewallen, W7EL

Dominique Michel wrote:
I want to made a broadband receiving loop. It will be a one turn
shielded loop of around 2 meters in diameter. The frequencies I want to
be able to receive are the broadcasting band in Europa, LW (153 - 279
kHz), MW (531 - 1620 kHz) and SW (2310 - 25820 kHz). The antenna will have enough gain for the
SW, but less gain on the 2 other bands.

At the base will be a little box with the connection to the loop and a first balun 1:1 symmetrical to asymmetrical. The signal will just do a few centimeters to another box (a big one), and to an adjustable capacitor (a tuning cap from an old vacuum tube radio, so I can have a little motor to tune remotely the frequency of the loop).

I plan to have a preamp, but only for LW and MW. It will have a higher maximum gain at low frequency, and this gain will be adjustable remotely with a voltage.

Is it possible to do a such 1:1 balun with a bandwidth from 150 KHz to around 25 MHz and if yes, how?

Another question is the size of the loop. I try a few coaxial cables to do the loop, and get a self resonance frequency of the loop between 8 to 11 MHz. So, the tuning cap work very well under this frequency, but its effect is hard to define above this frequency. If I decrease the size of the loop, I will get less signal, but the cap will work in a linear fashion on all the SW band. If I keep the same size, it work well, but the effect of the cap is hard to define above the self resonance frequency of the loop. And what about if I increase the size of the loop?


Cheers,
Dominique