HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

Therefore the size of his loop is a square of sides = 5 metres. Total
length of wire = 20 metres. Or somehing similar.

Assume the wire diameter is a conservative thick 2mm.


Assume nothing Reg.

The loop is indeed 5M per side.

But, it is however made out or 2/0 copper cable used for arc welding.

I got a good deal on it. It has many many fine strands, never bothered
to count them or to measure the diameter of the individual strands.

But, suffice it to say that it is much larger than 2 mm.

I'm hoping for a Q of 600 at 60 Khz.

I looked at 3 inch copper pipe, couldn't estimate which had the better
ac resistance, so I went with the welding cable rather than the large
copper pipe.



And getting down to practicalities, this means that the 0.2 uF tuning
capacitor has to be adjusted to an accuracy of about 0.3 percent, or
within a few hundred pF.

That is why I suggested a 2000 pF variable capacitor be included in
the bunch. A 2000 pF variable capacitor consists of an old fashioned
4-gang, 500 pF, receiving-type capacitor with all sections connected
in parallel.

As the loop is to be installed outdoors (with 5 metre sides it HAS to
be) the variable 2000 pF component might be useful to re-tuning it
between summer and winter temperature variations.


I have some 365 pF air variables I planned to use one for fine tuning
the loop.

There will be no ssb reception. Generally I'm interested in 6 to 10 Hz
wide channels at 185.3 Khz and in the 137 KHz ham band.

Fortunately, I have a DDS with a tcxo, so I can spot the frequencies I
want to listen on and then tweek the loop with the variable cap.

Regards,

T

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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.

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

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

HIGH Q CAPS FOR VLF LOOP ANTENNA?
 

On Tue, 25 Oct 2005 23:01:01 -0400, TRABEM wrote:

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.

It is not an effective antenna shield if it is wholly continuous - and
it is not, it has a gap opposite the mounting point which is generally
at ground/reference potential. Part of the point of being "shielded"
is to enforce a symmetry and that ground/reference is electrically
neutral as long as you guarantee it is equidistant both sides around
the loop to that gap.

The shield tends to protect the wire from electrical field
inputs and allows it to only respond to magnetic field variations.

There is no such thing as "only" magnetic fields variations.

I thought the capacitance between the wire and the surrounding shield
material represented a loss in Q,

Q is a simple relation between loss and storage. Lower Q for the same
storage (be it in a capacitor or an inductor) can only result from
resistive loss of Ohmic conduction or radiation. Any loss
attributable to a capacitor is conductive loss - hence the discussion
of ESR. You would have to go back to the stone age of electronics
with paper and wax dielectrics to find loss BETWEEN the plates.
Equivalent Series Resistance for garden variety capacitors, when
compared to radiation resistance, is not trivial. That is, unless,
you swamp that loss by putting your loop in the closet with your
mothballed summer wardrobe or burying it in the garden mud. Design
for failure is easily achieved if you need a rationale to ignore
simple considerations.

Consult:
http://www.w8ji.com/magnetic_receiving_loops.htm

There are countless horror stories about those attempting to use
surplus hardline as shielded loops on LF and VLF, all with
disappointing results.

Such disasters that arise are one of two possible scenarios:
1. They don't have a gap (short circuit city);
2. They don't guarantee symmetry (poor balance, poor tuning, poor
response).

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:))

We get that traffic - yes. They suffer the same learning slope.

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

Because you have to have a conductor pair back to the receiver. The
grounded "shield" serves as one half of the pair, the other spans the
gap connecting to the other half's "shield" (it looks like you are
shorting the inner conductor to ground) to thus pick up the opposite
potential. The voltage across the gap is thus sensed and it only
takes one wire. Look closely at any such standard "shielded" loop.
The sense of what is being shielded is THAT conductor which you
contrive to keep in a controlled environment (a coaxial shield) and
away from the imbalance of nearby capacitive couplings. The
"shielded" inner conductor spans a very small distance whose opposite
poles' capacity is balanced to all neighboring paths to ground. That
is, unless you push one side up against the wall.

Stretch out the gap of the shield loop and you have a conventional
dipole. A conventional dipole exhibits high Z and high V at its tips.
The middle of such a dipole has a low Z and a high I. With respect to
both ends, the middle is neutral and strapping it to a conductor does
nothing to change that topology (and is a common tower mounting
benefit). Being curved into a loop does not change this and allows
you to connect your transmission line to both sides without greatly
exposing a significant length of the transmission line (and thus
forcing an unbalance and upsetting the applecart).

This dipole is obviously very small with respect to its wavelength and
thus some form of end loading is required. Thus the capacitor arrives
on the scene. The circulating currents and potentials become
astronomic for progressively smaller antennas. Those currents flow
through and to the plates of the capacitor. If you don't choose the
right components for that capacitor (and manufacturers of HF loops
like to crow how they achieve this) then your design efficiency goes
TU. Hence to speak of capacitor Q is not appropriate as the correct
term is D (dissipation factor). It is certainly related (an inverse
relation) and despite comments to the contrary, D is resolvable with
standard bridges (although those bridges are of considerable design
sophistication in maintaining balance and their own shielding - not a
trivial matter).

There are simpler ways of achieving the same thing by building a
completely exposed loop with capacitor (still paying attention to the
ESR and keeping the whole shebang out of the mud), and simply building
a shielded coupling loop. Reg has adequately described this before
many times.

73's
Richard Clark, KB7QHC