The quest for the ultimate Rcv Antenna continues...
After reading the article in the ARRL Antenna Book on ferrite loop antennas,
I set out to design the better mousetrap. I'm now working on version 10 (or
is it 11?)
Belrose indicates that the sensitivity of the loop is determined by the
geometry of the loopstick. Generally, the formulas indicate that the
leakage currents are minimized when the windings are spread over the surface
of the core. In addition, the diameter of the loop is critical to
performance--the bigger the diameter, the better the antenna.
I priced a 3/4" x 12" ferrite rod at $100. Ouch! Instead, I've been using
1" diameter type 43 beads (I bought 125 for slipping over RG-8 sized coax).
The best performer so far is 19 beads (16") and wound on my lathe with about
65 turns of #28 wire. The secondary is about 12 turns. When I resonate the
primary (electrically isolated from everything else), I am able to get a 50
+j0 match rather easily. The loop is fairly sharp and can be resonated with
about 50pF.
I have several questions. Perhaps others have experience with the fields
surrounding these loopsticks and can answer.
1) When I operate the loop outside, holding the stick horizontal to the
ground works best. I anticipated phasing a couple of these by holding them
vertical. After playing with the first viable design by itself, I discover
that the sensitivity of the loopstick goes to nil when I hold it vertically.
When horizontal, the signal maximum is broadside to the loop. Why do I get
no (or very little) response held vertically? These experiments are done at
4' to 7' height above loamy ground. Can anyone explain the polarization
differences?
2) The permeability of the ferrite is listed in the specs as "minimum 1000
and maximum 3000" All the formulas and information I can gather from
experimentation indicates that with the loop geometries the way they are, a
permeability of 90 to 100 is obtained, which is consistant with the article
in the antenna book and suggested by the formulas presented therein.
3) If I understand the physics of these little devils correctly, I am led to
believe that the reason they work at all is that the permeability of the
core (effectively 100) versus air (assumed to be 1) is such that the
magnetic field lines are able to pass through the core easier than through
air. Belrose suggests that the field lines in a traveling wave are actually
distorted at or near the loopstick causing the field lines to pass through
the stick rather than through the air around it. (Bizarre..... I wonder how
the speed of light factors into all this?) The field lines thus distorted
will then induce a current through the core and couple into the windings
attached to the core. My question is, should I be expecting to phase (e.g.
a pair of them) using principles similar to those used in traditional
verticals (e.g. 130' in air for 1/4 wavelength spacing) or will some other
principle apply?
4) I have attempted to wind various loop geometries on the loopstick. The
trick throughout all this is to completely encase the ferrite in wire
(minimizing leakage) and thereby maximize sensitivity. Under the
circumstances (p ~ 100) inductance is not our friend. It is very easy to
get this type of core to self resonate with as few as 35 turns or so
(closely spaced). I reasoned that (e.g. 5) separate high inductance
windings tied in parallel and resonated with a single capacitor was the way
around it. Nope! In fact, they make a heck of a notch filter. The
resonance becomes a notch rather than a peaking device. Further
investigating indicates that the Multi-multi guys should use this technique
to notch out other transmitters on the same band since the notch is quite
effective (>45 db) and very sharp (<15kHz). E.g. Wind three coils on a
rod. Couple the antenna to the end coil, the receiver to the other end, and
resonate the middle core. I was shocked at the depth and sharpness of the
filter--and incredibly simple! My question is, why is this so? Why can't I
wind three, five, or 15 coils in parallel and resonate them together?
Instead, they act as attenuators. What's up with that?
If anybody has been experimenting with large ferrites on 160, I want to hear
from you. Through Topband is great unless the others would prefer this to
be a private discussion, in which case, email direct.
Thanks for the (substantial )bandwidth
Ford-N0FP
ford@cmgate.com
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