when the metal plates move and more surface is used/covered by the cap,
is the capacitance higherb (farads) ??

I see that with loop ant, the more the plates cover each other, the
lower the tuned freq is ...

So with a high cap, you could make an ant with less turns of wire ...

Sometimes I find a large cap (1 F) at flea markets, for 1 euro/$ or so.

They cost 100 or so new. Would they make sense ??

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For transmitting purposes there is a high voltage across that capacitor.
I have a friend who uses 10,000 volts rated capacitors in a loop for 50
watts.

The 1 farad capacitors are generally very low voltage.

switcher wrote:

when the metal plates move and more surface is used/covered by the cap,
is the capacitance higherb (farads) ??

I see that with loop ant, the more the plates cover each other, the
lower the tuned freq is ...

So with a high cap, you could make an ant with less turns of wire ...

Sometimes I find a large cap (1 F) at flea markets, for 1 euro/$ or so.

They cost 100 or so new. Would they make sense ??

Not only what Dave wrote (that the voltage is likely to be too low),
but LOTS of other things too...
-- the RF losses of such a capacitor are often terrible
-- they are almost always polarized, not intended for use with an AC
signal but only as a filter for DC (especially one you'd pick up at a
flea market!)
-- I have some 1F 5.5V caps which are not so expensive new--but they
have very high series resistance. They are intended only as system
memory backup or similar for systems in which the backup current is low
microamps.
-- with such high capacitance, the loop would be so tiny that it would
be inefficient.
-- the self-resonant frequency of the capacitor is likly lower than the
frequency you want to tune the loop to. That means at the operating
frequency, the "capacitor" would look like an inductor and not tune the
loop anyway.
-- if you can manage to keep the losses low in the loop, even with a
reasonable tuning capacitance that you might actually be able to use,
the Q may be so high that the bandwidth is unuseably low.
-- the stability over temperature and time is terrible; the loop would
not stay tuned on one frequency.

These are broad generalizations. What you really need to do is look at
a SPECIFIC design and decide what capacitance makes sense from a system
performance standpoint. Will it be sensitive enough (as a receiving
loop)? Will it be efficient enough (as a transmitting loop)? Will the
bandwidth be large enough? Then the design will tell you what
capacitance you need, and you can ask the additional question: can I
tune it--how will I vary the tuning?

On very low frequencies, I can imagine using perhaps polypropylene caps
that are designed to work in switching power supplies and have low
inductance and low effective series resistance, but probably only for
use on one fixed frequency since changing the capacitance would be such
a hassle.

Assuming you're interested in low frequency receiving loops, have you
had a look at Reg's loop program? It can help you make decisions about
how to make your loop.

Cheers,
Tom

There are other magloop programs beside mine.

Most of the others calculate the size of the tuning capacitor in pF to
be that which resonates the calculated loop inductance in
micro-henrys.

This is incorrect.

It may be OK for very small loops. But, taking the extreme example.
when the loop circumference approaches 1/2-wavelength at the working
frequency, obviously no more capacitance is needed regardless of the
value of the loop inductance.

But if there is no capacitor then the loop cannot be tuned precisely
to resonance! A variable capacitor, with its minimum stray
capacitance, is always needed. The inductance being somwhat less than
the theoretical value.

It is popularly assumed that the diameter of the small coupling loop
must be 1/5th of the diameter of the main loop. Actually this is a
rule of thumb and, for an accurate impedance match, the diameter
changes somewhat with frequency and the conductor diameter of the main
loop.

But 1/5th diameter applies only when the impedance to be matched is a
50-ohm coax. Other feedpoint impedances need a different diameter. The
diameter of a 75-ohm coupling loop should be about 1/4 of the diameter
of the main loop.

It is common practice to screen the coupling loop by making it from
coax cable. This is a waste of time and materials. No useful purpose
is served either on receive or transmit.

The coupling loop can be a self-supporting circle of wire of no
greater thickness than the inner conductor of the coax cable which
serves it. It can be a square or other loop of the same area but a
circle is neater.

It is best to make no direct connexion between the main and coupling
loops. Isolate the main loop from the feedline and everthing else.
Although this may be difficult to do when there are control wires for
a motor-driven variable tuning capacitor. Chokes can be used in
control wires or bundles of wires.

Just a few hints and tips which come to mind.
----
Reg.