On 15 Sep 2005 01:14:37 -0700, "K7ITM" wrote:

You can find a good program to estimate loop inductance and some other
parameters on Reg Edwards' web pages. He has a link in many of his
postings on this group and the r.r.a.antenna one.

Yes, I found it day before yesterday and was stunned to see the loop
impedance change so much as the loop is moved off frequency. I want my
antenna to cover 50 Khz to 200 Khz and the antennas impedance will
vary from 6K ohms to 1 K ohm, quite a LARGE range.

The software was very helpful and enlightening.

I was hoping to feed the antenna to the house over twisted pair line
laying on the ground. This requires a balun to make the low impedance
line balanced.

I think the antenna should be balanced as well, which helps in
elimination of out of band signals that might overload the preamp. I
think, but aren't positive that the balanced antenna is necessary to
eliminate the 'antenna effect' which allows the antenna to pick up
other signals that it wasn't designed for just because it's a piece of
wire hanging in free space.

So, my plan was to build a balanced loop and feed it to the house with
a balanced feedline.


The impedance comes
from the Q and the fact that you are resonating it--or at least it's
presumed that you are resonating it. So if it has a Q of 300 and the
inductive reactance is 50 ohms, the resistance when resonated is
300*50=15000 ohms, for example. That's why folk like to use preamps at
the antenna: transform that high impedance down to a low impedance
that's easy to send along a transmission line. Seems to me that if
they are having trouble with intermod distortion in the preamp, the
preamp isn't designed properly. It's not terribly difficult to get
very low distortion at LF these days. By the way, if you build a
really big loop and have so much signal you can attenuate it, that
gives you a chance to lower the Q and increase the bandwidth: if what
you want to listen to occupies much bandwidth, you don't want your
antenna to filter out the information you want to listen to!


It's not likely that I will ever want to listen to SSB or any other
wider band modes, but I did consider putting in a resistor to kill the
Q if I ever wanted to do this.

I'd suggest you read an antenna book like Johnson and Jasik, or the
antennas chapter of King, Mimno and Wing's "Transmission Lines,
Antennas and Waveguides." They will make it a lot clearer why you
might want a balanced loop. You don't need a grounded center-tap to
make it balanced--just make it very symmetrical.


No, it's pretty clear that I want a balanced loop. Several lowfers
made strong suggestions that I should not waste my time building
anything that wasn't balanced, and I couldn't agree more.

Cheers,
Tom

Tom, I hope to put up a high Q, but relatively large loop. I expect to
have a very large signal output to the receiver. Signal strength of
received signals will probably NOT be an issue.

Because the signal will be relatively high level, I would like to
resist using a preamp at all.

While I was playing with filters last night, I tried to design a
filter that would also convert impedance from 6 K down to 100 ohms. It
became impractical with a balun BUT, a filter that transforms
impedance seems to kill 2 birds with one stone.

However, performance sucks real bad with filters of higher impedance
and with any filter that attempts to make a large or moderate
impedance transformation.

Is there any other means of converting impedance with out an active
amp (using passive components)? Since I have a big signal, I can
sacrifice some signal strength as long as the losses are not to great.

Maybe I should start another thread?

Regards,

T