"Art Unwin" wrote in message
...
On Jan 25, 3:51 am, wrote:
On Jan 23, 8:45 am, Allodoxaphobia wrote:



On Fri, 23 Jan 2009 13:57:40 +0000, dave wrote:
Ian Jackson wrote:

What about ribbon cable? I've got a fair length of ribbon cable
(something like 25 wires - the sort used in PCs to connect hard
drives
and the like) put away for a rainy-day loop antenna project. While I
could make one large loop using all 25 wires for the really low
frequencies, I'll almost certainly cut it up and make several smaller
loops with fewer wires for the higher frequencies.

Stick with a single wire and relatively few loops. More wire won't
make
the loop bigger, just harder to tune.

I don't think he meant to connect all those wires in parallel.
But, it would be a little tedious to connect each wire at one end to its
neighbor at the other end (of the loop), and _not_ create an ugly bird's
nest at the 'joint'.

Jonesy

Yep, it's a continuous single wire. If you used ribbon cable, you
would have to put a cut, and the jumper to jump over to the next
wire on each turn.. Would be a pain.
You just take a single length of wire and thread it around
through the holes until you have the number of turns you need.
You are moving over a row of holes on each turn.
The main thing to consider is you end up building the loop
and deciding the proper number of turns around the
capacitor you have, not the other way around.
A double 365pf cap "730 pf total if jumped together" will let
you be able to tune the whole AM-BC band with most loops.
My 44 inch per side diamond loop for MW has five turns.
My 16 inch diameter circle loop for MW uses 12 turns.
Both are using basically the same cap values.
I also use a single turn coupling loop that is inside and
slightly smaller than the main loop. But it does not effect
tuning, and it's size and spacing from the main loop is fairly
uncritical. The cap is in parallel with the main loop winding.
If you use a portable with a built in loop stick antenna,
you can just couple the radio to the loop and it will
work. But all my radios require a feed line to the antenna.

I would not be so quick to dismiss the ribbon wire on the basis of
capacitance build up ! If you start from the middle of one end by
joining
the two center wires together and from then on joining the end to end
wires
moving outwards what you have then done is to cancel not only the
capacitance build up but also the inductance build up.
You can then unfasten the first step on the center winding and feed it
from that point
i.e. center fed
Art

.... but a multi-turn loop in which the self-inductance cancelled wouldn't be
much of a receiving antenna! Production of EMF from the magnetic field
caused by current flowing in the adjacent turns and production of EMF from
the magnetic field component of an incident radio wave rely on the same
principle.

Chris