Caps to consider: as you've already figured, polypropylenes can do
very well. Mylars/polyesters are not very stable, and don't have as
high a Q as polyprops. They also show greater distortion, which may or
may not be an issue to you. C0G ceramics should work fine. Be aware
that the loop inductance will change with temperature, too, as the loop
size changes. Presumably your loaded Q won't be so high that it's a
problem: 50ppm/C over 20C is 0.1%, which wouldn't be noticable, most
likely, with a loaded Q of up to 500 or so. At 60kHz, Q=600 is only
about a 100Hz bandwidth, so I suppose you won't want a higher Q than
that anyway (assuming you could get it). I'm curious: what loaded Q
do YOU expect to get? How big is your loop going to be? What
impedance do you expect with the loop resonated?

Cheers,
Tom

Thanks Tom,

I hope for a Q of 600.

At 60kHz, Q=600 is only
about a 100Hz bandwidth, so I suppose you won't want a higher Q than
that anyway (assuming you could get it). I'm curious: what loaded Q
do YOU expect to get?

How big is your loop going to be?

Around 5.2 Meters per side.

What
impedance do you expect with the loop resonated?

It should be under 1 ohm. I don't exactly know the ac resistance or
how the Q of the C and the Q of the inductor combine.

My loop material is 2/0 copper welding cable, many fine starnds. I
considered 3 inch copper pipe, but couldn't get an estimate of the ac
resistance for either, so I chose the copper cable.


Regards,

T

I'm puzzled. My copy of rjeloop3 suggests the Q will be about 200 at
60kHz with a 9mm wire diameter, and you'll see about 2kohms when it's
resonated. Are you not taking the output across the ends of the loop
(across the capacitor)? And with a skin depth of about 0.01" at 60kHz
in copper, certainly 3" diameter soft copper pipe would have the lower
resistance. You might have some trouble finding soft copper pipe,
though. But even hard copper pipe should have a low RF resistance.
"Reference Data for Radio Engineers" (or "Reference Data for Engineers"
in newer incarnations) has lots of good info for figuring out things
like RF resistance of copper wire. I assume your welding cable doesn't
have strands that are insulated from each other like Litz wire.

Consider that Q is energy stored divided by energy dissipated per
radian (1/2pi of a cycle). Then the net Q will be 1/(1/Q(inductor) +
1/Q(capacitor)). So if the cap and inductor have the same Q, the net Q
will be half that. And if you put a resistive load across the
coil+cap, that will dissipate power and lower the Q further.

Cheers,
Tom

Stranded, layer-wound wire, even when strands are individually
insulated, behaves similar to solid wire of slightly smaller diameter.

The strands in true Litz are WOVEN such that every strand spends the
same length in inside and outside and intermediate layers of the
cable.

Current is then more uniformly distributed throughout the conductor's
cross-section.

The diameter of an individal strand should not be greater than about
about twice skin depth. Otherwise effectiveness decreases. Thus, at
high frequences where skin depth is very small, very fine wire must be
used.

There are practical and economic limits to the fineness of drawn
copper wire. There is little to be gained by using ordinary Litz
above 3 or 4 MHz. At high frequencies with small coils of few turns,
such as receiving coils, tank and loading coils, it is far more
economic to increase Q just by increasing the diameter of solid copper
wire. Litz is at its best from VLF to IF and MF.
----
Reg.

On 24 Oct 2005 16:57:07 -0700, "K7ITM" wrote:

I'm puzzled. My copy of rjeloop3 suggests the Q will be about 200 at
60kHz with a 9mm wire diameter, and you'll see about 2kohms when it's
resonated. Are you not taking the output across the ends of the loop
(across the capacitor)?


No, you are describing a parallel tuned loop, aren't you Tom,

That is NOT what I'm trying to build.

I am planning a series tuned loop, which is C in series with L and the
output is taken across the unused loop terminal and the unused cap
terminal.

I think it should be called a series tuned loop, shouldn't it?

I know I suggested a whole bunch of times that your 2K loop impedance
sounded like a parallel tuned loop value and you keep insisting that
my series tuned loop will have an impedance of 2K ohms.

You also told me that "Tom has already carried the water describing
what your antenna Z looks like". I suggested that perhaps Tom and you
thought I was referring to a parallel tuned loop and said several
times that it was a series tuned loop.

Then you ranted on and on or maybe I ranted........

Did Tom and you not hear me when I said it was a series tuned loop or
did I not make it plain enough.

Isn't the impedance of a series tuned circuit LOW at resonance??? It
was when I went to school.

If I've err'd, please let me know how.

Thank you.

T




PS:

And, yes......I expect the Q to be cut in half if I attach a receiver
and a loop with identical impedances to each other. I call it loaded Q
and it's a necessary evil if one doesn't want to resort to electronic
(active component) impedance matching.

Put another way, if my receiver had a 50 ohm input impedance and my
loop had a 50 ohm output impedance (with a Q of 100, unloaded), I'd
expect to have a (net) Q of 50 after the receiver was connected to the
antenna.

Reg's software tells me I have a Q of around 221. I assume that's net
Q for the loop itself (unloaded). If my receiver is made to have the
same Q as the loop, then I expect the loaded Q to be around 110 after
they are connected together.

I know you mentioned an active buffer amp to transform impedances. No
doubt this would help to keep the loaded Q up, but I'd like to avoid
any active antenna preamp/rf stage if possible......as previously
discussed.

And with a skin depth of about 0.01" at 60kHz
in copper, certainly 3" diameter soft copper pipe would have the lower
resistance. You might have some trouble finding soft copper pipe,
though. But even hard copper pipe should have a low RF resistance.
"Reference Data for Radio Engineers" (or "Reference Data for Engineers"
in newer incarnations) has lots of good info for figuring out things
like RF resistance of copper wire. I assume your welding cable doesn't
have strands that are insulated from each other like Litz wire.


I thought about litz, and it probably would have been cheaper than the
copper welding cable I bought. But it's fragile in the outdoors and
breaks easy when the wind blows it especially in long spans like I am
going to have. Rather than encase it in some sort of protected sheath,
I decided to use the welding cable.


Consider that Q is energy stored divided by energy dissipated per
radian (1/2pi of a cycle). Then the net Q will be 1/(1/Q(inductor) +
1/Q(capacitor)). So if the cap and inductor have the same Q, the net Q
will be half that. And if you put a resistive load across the
coil+cap, that will dissipate power and lower the Q further.


I think I understand that now and understood it before you explained
it. But, thank you.

Another set of questions: Given the high atmospheric noise level at
LF/VLF, is there really a need for such a large loop as you propose,
for receiving? How quiet is your receiver front end? In other words,
will such a large loop significantly improve your SNR on weak signals?
Do you have a reason other than signal level for using such a large
loop? What about the response to nearby strong electric-field noise
generators of a large loop versus a smaller one?

Cheers,
Tom

You are absolutely right about the size of the loop.

A larger loop might not enhance the ability to copy a weaker signal.
And, I spent a small fortune in buying big wire just to make it have a
reasonably high Q. My question about the caps was merely to make sure
that I was buying the right type of caps, so that the investment in
the larger sized wire didn't get negated by having the wrong type of
cap.

At some time I might like to evaluate a smaller loop against the big
one in terms of the actual weak signal reception capability.

The receiver is hot on HF and should be just as good on LF and VLF.

Ultimately I'd like a shielded loop, but the effect of the stray
capacitance seems to really kill the Q. The shielded loop camp makes a
convincing argument in that the magnetic field is significantly
quieter than the electrical field is. But, how to do a shielded loop
without knocking the Q all to Hell is a significant issue. Needless to
say the potential for interference by strong LF broadcasters is much
reduced by shielding the loop as well.

One user I spoke to recently commented on the quality of reception
with his shielded loop.....signals that were buried in noise by quite
a few db seem to pop up into Q5 readability when the shielded loop
antenna is switched in. So, I know they work. Just not sure how to
implement them without incurring a lot of loss in Q from the stray
capacitance introduced by the shielding.

T


On 25 Oct 2005 09:22:10 -0700, "K7ITM" wrote:

Another set of questions: Given the high atmospheric noise level at
LF/VLF, is there really a need for such a large loop as you propose,
for receiving? How quiet is your receiver front end? In other words,
will such a large loop significantly improve your SNR on weak signals?
Do you have a reason other than signal level for using such a large
loop? What about the response to nearby strong electric-field noise
generators of a large loop versus a smaller one?

Cheers,
Tom

On Tue, 25 Oct 2005 19:43:42 -0400, TRABEM wrote:

The shielded loop camp makes a
convincing argument in that the magnetic field is significantly
quieter than the electrical field is. But, how to do a shielded loop
without knocking the Q all to Hell is a significant issue. Needless to
say the potential for interference by strong LF broadcasters is much
reduced by shielding the loop as well.

You are tap dancing in the mine field of nonsense. Once you strip
this stuff out of your thinking, you might find your way to a more
sensible antenna design (maybe even a good shielded one - and shielded
for better reasons than those above).

73's
Richard Clark, KB7QHC

You need first to realize that the "shield" IS the antenna. The whole
point of the "shielded loop" is that you can make it very symmetrical,
which is just what's needed to reject strong local electrical fields.
The symmetry does nothing to reject electromagnetic signals. BUT you
can make an "unshielded" loop which is as symmetrical as a "shielded",
if you are careful, and get the same advantages. If you really want to
build one like a classical "shielded loop" and maintain high Q, just
build the "shield" out of copper pipe and put the capacitor across the
gap. The wire inside the pipe is just the center conductor of a short
piece of coax connected to the feedpoint.

If you don't understand this, please see King, Mimno and Wing's
"Transmission Lines, Antennas and Waveguides." It's explained quite
nicely in the "antennas": chapter. It's also explained reasonably well
in Johnson and Jasik's "Antenna Engineering Handbook."

Cheers,
Tom

On 25 Oct 2005 17:31:02 -0700, "K7ITM" wrote:

You need first to realize that the "shield" IS the antenna. The whole
point of the "shielded loop" is that you can make it very symmetrical,
which is just what's needed to reject strong local electrical fields.
The symmetry does nothing to reject electromagnetic signals. BUT you
can make an "unshielded" loop which is as symmetrical as a "shielded",
if you are careful, and get the same advantages. If you really want to
build one like a classical "shielded loop" and maintain high Q, just
build the "shield" out of copper pipe and put the capacitor across the
gap. The wire inside the pipe is just the center conductor of a short
piece of coax connected to the feedpoint.

If you don't understand this, please see King, Mimno and Wing's
"Transmission Lines, Antennas and Waveguides." It's explained quite
nicely in the "antennas": chapter. It's also explained reasonably well
in Johnson and Jasik's "Antenna Engineering Handbook."


Hi Tom,

No, I don't understand this. I thought a shielded loop meant the loop
antenna wire was shielded by the copper (non-ferrous) surrounding the
wire. The shield tends to protect the wire from electrical field
inputs and allows it to only respond to magnetic field variations.

I thought the capacitance between the wire and the surrounding shield
material represented a loss in Q, therefore a loss in output voltage.
So, a loop that might have a Q of 100 in free space would have a much
lower Q if the loop wire was enclosed in a non-ferrous pipe.

There are countless horror stories about those attempting to use
surplus hardline as shielded loops on LF and VLF, all with
disappointing results. The predominate attitude was that the
capacitive coupling between the wire and the shielding material was
the cause. I don't say the predominate attitude is correct.but, if it
is a false assumption, then I am not the only one who needs
revision)

If the copper pipe IS the antenna, then why have the wire inside it at
all??

I must say I'm more confused now than I was before reading your
message.

I'm sorry, I have to leave now. The director of the asylum is
calling.......

T