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

Yes, this might be good for another thread...

Given the limited LF bandwidth you're interested in, I can't imagine
that making a transformer would be all that difficult. Not trivial,
certainly, but far from impossible. A 5:1 turns ratio will give you a
25:1 impedance ratio. You'll want to use a core material that doesn't
introduce distortion. A transformer like that also gives you a way to
keep the loop loading balanced. But--are you going to put the tuning
capacitor at the loop, or do you have in mind putting it, say, at the
receiver end of the feedline? If it's at the loop, how will you adjust
it? And just what size signals do you expect to get? One of the nice
things about LF/VLF is the predictability of signal strengths. Also,
beware of worrying a lot about feedline impedance. How long will your
feedline be, in wavelengths? If it's, say, 0.05 wavelengths at 100kHz
(and THAT's 150 meters long!), does it really make much difference that
it's quite a different impedance than the antenna? And...what IS the
impedance of the line, at that frequency? It may well be a bit
different from what you calculate for the line at 10MHz. What would
happen if you fed your one turn loop with 100 feet of "300 ohm
twinlead" or "450 ohm ladder line" and just tuned it at the receiver
with a capacitor across that line? Small transmission line wire size
would ding the Q some, but would that be an issue? I'm just
speculating here, and maybe someone with direct experience with that
sort of feed will offer suggestions.

One thing to keep in mind here is that the LOOP construction will
almost certainly be the most challenging and expensive part, for a big
loop. And--you may really not NEED THAT big a loop! More signal also
means more atmospheric noise, and you won't improve signal:noise ratio
just by getting more of both signal and noise. Anyway, once you have a
well-constructed loop, it's relatively easy to play around with
different feed systems--preamp and remote tuning, straight feedline,
whatever.

Cheers,
Tom

Hi Tom,


Given the limited LF bandwidth you're interested in, I can't imagine
that making a transformer would be all that difficult. Not trivial,
certainly, but far from impossible. A 5:1 turns ratio will give you a
25:1 impedance ratio. You'll want to use a core material that doesn't
introduce distortion. A transformer like that also gives you a way to
keep the loop loading balanced. But--are you going to put the tuning
capacitor at the loop, or do you have in mind putting it, say, at the
receiver end of the feedline? If it's at the loop, how will you adjust
it? And just what size signals do you expect to get? One of the nice
things about LF/VLF is the predictability of signal strengths.

OK, I just ran some numbers for a 6000 ohm to 100 ohm toroid
transformer at 190 Khz. I didn't have to deal with the secondary at
all, because the primary has to be so large...I never made it past the
primary! Perhaps I made an error in the calculations? The transformer
has to present about 6000 ohms of inductive reactance, which is 16.6
millihenrys. Even on a large high mu core, I'd have to wind 400
turns!!!!!!!!!!! With that many turns, the losses would be big, and
would still have to wind a secondary (although it would much much
smaller). Did I make a mistake in the calculations?

I don't mind going to the antenna to tune it-lowfer signals don't
change frequency much.


Also,
beware of worrying a lot about feedline impedance. How long will your
feedline be, in wavelengths? If it's, say, 0.05 wavelengths at 100kHz
(and THAT's 150 meters long!), does it really make much difference that
it's quite a different impedance than the antenna? And...what IS the
impedance of the line, at that frequency? It may well be a bit
different from what you calculate for the line at 10MHz. What would
happen if you fed your one turn loop with 100 feet of "300 ohm
twinlead" or "450 ohm ladder line" and just tuned it at the receiver
with a capacitor across that line? Small transmission line wire size
would ding the Q some, but would that be an issue? I'm just
speculating here, and maybe someone with direct experience with that
sort of feed will offer suggestions.


Was hoping to use twisted wire which can be homebrewed or cat 5...it's
cheap and available. I think the twisted wire runs 80 to 90 ohms
impedance. I considered that I might just tolerate the mismatch since
the run was so short....but it's such a big difference, I am not sure
the input filter in the receiver will react as expected.

One thing to keep in mind here is that the LOOP construction will
almost certainly be the most challenging and expensive part, for a big
loop. And--you may really not NEED THAT big a loop! More signal also
means more atmospheric noise, and you won't improve signal:noise ratio
just by getting more of both signal and noise. Anyway, once you have a
well-constructed loop, it's relatively easy to play around with
different feed systems--preamp and remote tuning, straight feedline,
whatever.

Are there any other ways to convert the impedance without big losses
or without resorting to an active preamp? I'd like to avoid the preamp
if possible, especially since I expect a good output voltage from the
big and relatively high Q loop.

T

Well, the 8:1 turns ratio transformer at
http://www.jensen-transformers.com/datashts/110khpc.pdf would almost do
it for you. It falls off a bit at the high end. But with it, you
could go down to less than 10Hz. That might be fun. What core
material were you using in your calcs?

I really think you're being too cautious about preamps. But again, my
point is that you can put the effort into the loop, put it up, and then
play around with different feed methods. If you do use a feed line and
leave the tuning capacitor at the antenna, you won't notice any
significant loss in the line. Reg will try to "sell" you one of his
programs for looking at how the line transforms the impedance, but I'd
just use a Smith chart program myself. Much more educational, to me,
than just seeing numbers. If you want to try tuning at the receiver
end, use heavy wires in the line: their resistance adds to the
resistance of the loop itself, lowering the Q. But with the capacitor
at the loop, the circulating currents stay out there, for the most
part. You could add a small variable at the receiver end, to do fine
tuning. You may well discover that's an advantage...it's a pain to try
to tune the loop without being able to listen to the signal...

Cheers,
Tom
(about to bow out of this...time for you to go try some things.)