"Mark Keith" wrote in message
m...


Q of the circuit? Not really sure. I don't build AM radios. But in
judging from my loops, I'd be surprised to see one tune the whole band
with a single 365pf cap. I'm fairly sure neither of mine would. If I
dumped one of the gangs of my dual 365 cap on my 16 inch loop, it
WOULD NOT cover the whole BC band. When you build a loop, you build it
around the cap. IE: adjust the number of turns to give the desired
range with the cap at hand. If you can get a loop to cover the whole
band with a single gang cap, more power to you. But I'd have to see it
to believe it. It ain't happening here. MK

You're exactly right when you say you build the loop to work with the
variable cap.

I made a loop with an am broadcast oscillator cap, rather than the more
common antenna cap. The oscillator caps usually max out at something like
250 pf. I needed more turns on the loop antenna than usual, something like
14 turns on a form about 2 feet across. Limiting the distributed
capacitance with the extra turns is the biggest problem. I had to evenly
space the windings about 1/4" apart. I was able to get coverage from a
little more than 530 to 1700 kHz.

Basically, the frequency range with a fixed inductance will be proportional
to the square root of the ratio of the max capacitance to the minimum
capacitance. The distributed capacitance adds to the maximum capacitance
and the minimum capacitance of the tuning cap in the circuit. A 9:1 total
capacitance ratio will give a 3:1 tuning ratio. A 16:1 total capacitance
ratio will give a 4:1 tuning ratio.

A 10 to 365 pf tuning capacitor will tune from 530 to 1700 kHz if the
distributed capacitance can be held to below about 28 pf.

An 8 to 250 pf tuning capacitor will tune the same range if the distributed
capacitance can be held to below about 18 pf. This is more difficult
because a 250 pf tuning cap takes more turns to resonate in the AM BC band.
Wide, even turn spacing with thin wire does the job.

Anyway, those are the numbers I calculate. Hope I got it right! They seem
to be in the right ballpark. I won't try to make any attempt to measure the
distributed capacitance of a loop antenna.

I'll recommend Reg Edward's programs. These are small, ready to run
programs for all sorts of radio design problems. Among them are is the
RJELOOP3 programs for multiturn loop antennas.

http://www.btinternet.com/~g4fgq.regp/

http://www.btinternet.com/~g4fgq.regp/page3.html#S301"

Frank Dresser