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

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.

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

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.

I agree with Roy... Pretty ambitious range you want... I'd concentrate
on the LW, and hope for the best as a "random" antenna for the upper
HF ranges.. There is really no such thing as a "broadband" small loop,
unless you do some drastic designing. On those lower frequencies
the tuning will be very sharp. You will want to retune the cap for
most any large frequency change. Being the self resonant frequency
of a true LW loop using practical value variables is going to be low,
there is not much point in trying to use it for higher frequencies
unless
you can live with it as a random antenna. I'd have a 2nd loop for the
higher bands. Also one turn 2m dia? Good luck on LW... You are going to
need some high pf cap values.. It would probably be more practical to
use a multi turn loop if you want to use the average BC variable.
Depends on the cap. The average dual gang variable for old AM radios
were usually about 365pf x 2 for a total of 730 pf. Some caps in old
AM/FM receivers have more gangs, and a bit higher total..
I've got a diamond loop next to me that is 44 inches per side. It has
5 turns. With the variable cap I have, my low end is 500 kc. To tune
LW,
I must add on extra fixed caps. Using all the gangs, my upper limit on
MW is about 1300 kc?? or so. I forgot exactly, but I think it's below
the
upper end of the MW band. I have to switch to a single small gang to
tune higher frequencies. My upper limit is about 2300 kc or so... And
thats with a very small cap value.. Sure, I can listen on HF, but it
will
never be properly tuned. A small loop is not that great an antenna for
the HF bands anyway.. You would probably be better off to string out a
wire..
BTW, I don't use a balun or a preamp on mine. I realize that the
unbalanced
to balanced connection is not the greatest thing, but in the real
world,
it's pretty much a non issue once the antenna is tuned. I notice very
little skewing of the pattern, and have had no noise issues. Using a
shielded loop, or shielded coupling loop can help balance if it ends up
being a problem, but in comparing both shielded and non shielded
coupling loops, I saw very little difference. My antenna is well
balanced
and symmetrical, so that probably helps.
I've never needed the preamp.. Plenty of signal as is. Will vary to the
radio.
If you tune the loop so it actually will tune HF, you won't be able to

properly tune it for LW using a BC variable cap. And LW and MW is
where you really want the loop. You can use anything for HF...
Reg has a pretty decent program for "what iffing" small loops..
"rjeloop3"
You might check it out. You will be able to see your appx ranges using
the
caps you have, number of turns, etc.. I've compared it with real loops
and
it's close nuff fer gov work.
MK

Le 2 Aug 2006 02:27:33 -0700,
a écrit :


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.

I agree with Roy... Pretty ambitious range you want... I'd concentrate
on the LW, and hope for the best as a "random" antenna for the upper
HF ranges.. There is really no such thing as a "broadband" small loop,
unless you do some drastic designing. On those lower frequencies
the tuning will be very sharp. You will want to retune the cap for
most any large frequency change. Being the self resonant frequency
of a true LW loop using practical value variables is going to be low,
there is not much point in trying to use it for higher frequencies
unless

I was thinking to use an additional balun between the cap and the antenna for the MW band and another one for the LW band, so at the antenna see a capacitance value multiplied by the power of the rate of the balun.

you can live with it as a random antenna. I'd have a 2nd loop for the
higher bands. Also one turn 2m dia? Good luck on LW... You are going to
need some high pf cap values.. It would probably be more practical to
use a multi turn loop if you want to use the average BC variable.
Depends on the cap. The average dual gang variable for old AM radios
were usually about 365pf x 2 for a total of 730 pf. Some caps in old
AM/FM receivers have more gangs, and a bit higher total..

It is such a cap I have.

I've got a diamond loop next to me that is 44 inches per side. It has
5 turns. With the variable cap I have, my low end is 500 kc. To tune
LW,
I must add on extra fixed caps. Using all the gangs, my upper limit on
MW is about 1300 kc?? or so. I forgot exactly, but I think it's below
the
upper end of the MW band. I have to switch to a single small gang to
tune higher frequencies. My upper limit is about 2300 kc or so... And
thats with a very small cap value.. Sure, I can listen on HF, but it
will
never be properly tuned. A small loop is not that great an antenna for
the HF bands anyway.. You would probably be better off to string out a
wire..

I am living in a city in Switzerland, and a long wire is not an option.

BTW, I don't use a balun or a preamp on mine. I realize that the
unbalanced
to balanced connection is not the greatest thing, but in the real
world,
it's pretty much a non issue once the antenna is tuned. I notice very
little skewing of the pattern, and have had no noise issues. Using a
shielded loop, or shielded coupling loop can help balance if it ends up
being a problem, but in comparing both shielded and non shielded
coupling loops, I saw very little difference. My antenna is well
balanced
and symmetrical, so that probably helps.
I've never needed the preamp.. Plenty of signal as is. Will vary to the
radio.
If you tune the loop so it actually will tune HF, you won't be able to

properly tune it for LW using a BC variable cap. And LW and MW is
where you really want the loop. You can use anything for HF...

2 loops or one loop and a vertical antenna can be an option, one for LW and MW, the second one for SW.

I just received the balanis, and it have an extended chapter on the loop. I have just begun to read it. It appear at, with a big loop as my 2 meters diameter on SW, the resistance as the reactance of the loop show a very big and very sharp increase when the circumference of the loop is between 0.4 and 0.6 wavelength. So, in fact, a bigger loop as a 2 meters diameter will be needed, but I cannot use a coaxial cable if I want to be able to tune it (too much parasitic cap). So maybe at a vertical will be better because I don't know if I can manage a so big loop where I am living.

Reg has a pretty decent program for "what iffing" small loops..
"rjeloop3"
You might check it out. You will be able to see your appx ranges using
the
caps you have, number of turns, etc.. I've compared it with real loops
and
it's close nuff fer gov work.
MK


I will check this program. Thank you.
Dominique

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