Andre' Kesteloot akestelo@bellatlantic.net
Mon, 30 Nov 1998 16:09:57 -0500

vernall wrote:

> A second round of comments:
> Bob ZL2CA
> > > I can pass on some experience from New Zealand.  Computer ribbon is an
> > > attractive way to make a "coil" by connecting wire1 end2 to wire2 end1,
> > > and so on ... and there could even be tapped turns.  However, actual
> > > testing has shown that computer ribbon has quite high capacitance per
> > > unit length, and if nothing else this limits tuning range, but it also
> > > lowers Q.
> Peter G3LDO
> > I have made an LF receiving loop with a square configuration, one
> > metre per side. I used 14 wires of a section of ribbon cable and used
> > one separate turn for coupling to the receiver.
> > The main loop is tuned with a 1000pf air-space capacitor.
> Bob ZL2CA
> I would agree that 14 wire ribbon is a better starting point than 50
> wire ribbon.  Also in ZL experiments have generally shown that anything
> less than a metre per side (one square metre) loop is insufficient
> aperture (as in capture area) for external noise to outweigh internal
> noise (in the input device).  A BB212 varicap has been found to work
> well, with two sections in parallel, and can of course be tuned from the
> shack.  This gives tuned coverage of amateur and the lower part of the
> beacon band.
> > > The basic test of a frame receiving loop for LF DXing is that it can
> > > distinguish QRN as the main component of background noise.  If it can't
> > > then you are probably missing some weak signals ....
> >
> > Very true. I used a MAR amplifier as described by John, G4GVC, in
> > 'The LF Experimenter's Source Book' to improve the sensitivity.
> Noise figure is likely dominated by losses in the loop itself,
> irrespective of aperture.  Increasing loop aperture is the main way of
> "scooping in" more signal, for a given noise floor set by the receiving
> system.  Unfortunately, a bigger sized loop becomes awkward to handle
> (my biggest one tested would not fit through the workshop door!).
> > > So the computer ribbon is expedient, but unlikely to be a high
> > > performer.  I was hoping to stuff the ribbon inside plastic conduit
> > > pipe, and have a weatherproof frame loop for mounting outside, but it
> > > has not worked out that way.
> >
> > I used the plastic conduit construction and it kept the wires dry
> > even in this very wet year we have had in the UK. The tuning
> > capacitor is mounted outside the conduit and wrapped in polythene.
> That was what I was aiming for too.  Using standard conduit pipe and
> "elbows" or "rounds" for loop corners, and a "junction box" to house a
> JFET source follower and varicap tuning.  However, testing of a
> prototype showed that, while it did work, the noise performance was
> significantly worse than my active whip.  As I reported before, the
> "best" frame loops made in ZL have hand wound wires, with air spacing
> between turns.  I have to assume that the computer ribbon has lossy
> dielectric as well as rather too much capacitance between turns to yield
> a high Q result.
> One ZL (Nat ZL3VN) has a favourite loop made of 300 ohm ribbon, as a 2
> turn loop, tied in a tree, and fixed tuned with around 10,000 pF (a
> parallel bundle of selected polystyrene capacitors to tune to the
> desired spot).  The area is several square metres, and he reports that
> it works well in wet or fine conditions.  I think he uses a JFET source
> follower to feed to a transmission line.
> The approach using plastic conduit pipe housing has potential, but it
> needs wire that is better than "computer ribbon" for performance to
> rival an airspaced winding.
> Regards,
> Bob ZL2CA