g'day ford
is there an faq on this topic?
(Try "List Search" at the left side of http://www.contesting.com/ ...de W4ZV)
i looked into these things several years ago and will pass on what i recall
windings :
(1) do not wind wire direct onto the ferrite core - the dielectric losses
will be greatly increased - just wrap the winding where the core will go
with paper or light cardboard
(2) best Q is obtained with a winding diameter to winding length ratio of
about 4/5:1 - so when combined with optimium rod length the winding will
cover less than 25% of the rod
(3) for a given winding wire cross sectional area, litz wire will give
higher Q, but it is often simpler to just double the area
ferrite core :
(1) forget the 43 material with the ur over 1000 - in your dreams buddy -
this material is great for broadband tightly coupled transformers especially
the "balun" 2 holed beads - but even at MF the Q will be about 10 or even
less - when i tried the stuff it was so poor that i didn't even bother to
measure it - it is just not suitable for either resonant or non-resonant
circuits - that is why it is used around coax cables etc to absorb the
shield currents - i bought one of those big rods from amidon years ago and
it just clutters up my junk box now
(2) go for the 61 material, the ur is about 100 at the 1-2 MHz mark and is
what is normally used in am broadcast receiver loopsticks - for good
reason - i made a winding resonant at about 1MHz with 100pf using the
biggest 61 toroid then available from amidon and measured unloaded Q in
excess of 1500. selectivity was so high that even with a matched load there
was noticeable reduction in audio highs on am when peaked on the carrier -
this with a single LC circuit
(3) best sensitivity is obtained with a cylindrical structure (rod) with
length to diameter ratio about 15/20 to 1.
(4) if you are making a really large core - even the largest available
single ferrite rod will not give you performance better than a 1 metre box
loop - the core of the manufactured rod can be hollow with minimal impact on
performance - i would suggest 50mm diameter upvc pipe 1000mm long covered
with the largest diameter/longest length 61 material ferrite rod you can
find (your xyl will just love one of these things inside your shack - you
will get 1 metre square air-cored box loop performance when the
cross-sectional area of the manufactured cylinder of rods (including the air
core) times the ur of the ferrite is equal to one square metre (assuming
equal number of turns and equal Q) - another approach would be to get a
number of the largest 61 ferrite toroid available (especially the double
thickness ones) and join them face to face to make a hollow cylinder - use a
wooden dowel as the former
secondary winding for connection to receiver
(1) experiment with a small number of windings maybe even 1 to minimise the
loading of the loop Q by the input impedance of the receiver. for a really
big ferrite rod you may even want to try a 9:1 ferrite broadband impedance
transformer between the receiver and the secondary winding. increase
coupling till there is no further increase in Q, back off a bit and leave it
at that
phasing 2 or more units
(1) a single big resonant loop will give deep nulls in its figure 8 pattern
(2) you can phase the resonant loop against a longwire to get a cardiod or
other pattern but with super high Q the rate of change of phase shift can be
very high and it gets real fiddly
(3) i have never tried phasing 2 non-resonant ferrite loops but if the
individual sensitivity can be made high enough then phasing should be a
breeze - but then remember that you are now phasing two elements both of
which have figure 8 patterns not omnidirectional patterns as in the two
vertical elements of a single loop - i would suggest that you keep the 2
loops at least 3 rod lengths apart
(4) the recent popularity of the k9ay et al loops has made me wonder whether
a ferrite equivalent is feasible - perhaps with a large non-resonant ferrite
loop the insertion of an appropriately valued resistor across the primary
winding or in series with the winding at the centre of the winding and a
centrally placed balanced secondary winding would generate a broadband
cardioid pattern - now that would be kewl
73s gd dx de
sam dellit vk4zss
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