I think there are several issues that are contributing to
the wider measured bandwidth.
First, the resistance of the loop is going to be that of
the tubing when fully stretched, because the length
of the aluminum doesn't change when it is collapsed.
There may be some shortening due to contact between
the aluminum as it folds together, but due to oxidation
or any protective coating, I think you should assume that
the effective resistance of your shorter loop is the same
as if the tube is stretched to its full length.
There is also the effect of the spiral steel wire "spring"
that keeps the aluminum tube from collapsing. In a
perfect world that would not have an effect if it is totally
inside the tube, but I don't know how well that applies
in this situation. Any current in that steel will increase
losses.
Another potential problem is the matching system:
The original "Army Loop" did NOT use a split-stator
capacitor. The two tuning capacitors were ganged, but
they did not have a common terminal.
I would have to go find a link, but imagine having
3 capacitors in series between the ends of the loop:
the outer two (which would be C1 and C2) are ganged
to tune the loop, but note that they do not have a
common terminal. The inner one (with the feedline
connected across it) is larger and used for impedance
matching. With a value of 3 pF for C3, that clearly is
not the case here. For a single band operation, the
middle capacitor can be a fixed value whlie still allowing
the antenna to be tuned across the band.
Your configuration was an adaptation of the "Army
Loop" matching method that allowed using a standard
split-stator capacitor (with a common frame). While
it is often used, it does not provide the balance that
the original one did, and so it is quite possible that
radiation from your coax is contributing to your radiation
resistance, and hence your wide bandwidth.
While split-stator capacitors are often used to tune
loops, most often they are wired so the loop connects
to each stator, and there is no connection to the frame.
That is because the resistance can be relatively high
in the rotating joints between the shaft and the frame.
Basically, the two sections of the the capacitor are used
in series, which reduces the available capacitance, but
means that the RF path is through one capacitor, along
the shaft, and out the other capacitor, without passing
through the rotating joints. (In that application there
needs to be a separate method of matching the impedance.)
Also, that particular style of capacitor is not well suited
for tuning small transmitting loops because the capacitor
plates are only press-fit to the shaft, creating a high-
resistance joint. That's not a problem when the capacitor
is used in a high-impedance parallel-tuned circuit, but
the resistance can be an issue when there is significant
RF current though the capacitor.
One other comment - the original "Army Loop" used
aluminum sections to make an octagonal loop. After
testing, they had to gold-plate the joints to achieve
good efficiency. That gives you an idea of how
critical contact resistance is in a small loop antenna.
I think your approach is worth further experimenting.
My suggestion is to try modifying the matching method
first to improve balance, and possibly adding a feedline
choke to reduce radiation from the coax. Then you
will be measuring the characteristics of the loop itself.
You might make a loop of plastic pipe to go inside the
aluminum as a support. Some of the polyethylene pipe
can be bent into a loop of suitable diameter.
Good luck!
Author
Topic: Torus loop antenna (Read 591 times)