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