Let me describe quickly what the helical winding or lumped loading does to
the antenna, and what to watch out for.
All any method of loading does is slow the wave in the antenna antenna down.
It reduces the velocity of propagation. If the velocity of propagation is
slowed to 1/2 freespace, the antenna pattern looks more like the pattern of
an antenna twice as long. It creates nulls broadside to the antenna.
There is a limit to how slow the wave can be made before the pattern no
longer improves, and it is generally at about 1/2 free-space speed. Slower
than that, or faster, and the pattern deteriorates. With too much slowing,
the pattern deteriorates AND sensitivity of the antenna decreases. In either
case the resulting pattern will not be optimized. If an error is made it is
generally best to keep the antenna's current propagation on the faster side
of 1/2 freespace.
Like a regular Beverage, a slow wave structure wants to receive away from the
feedpoint in the direction of lagging phase or termination.
The helical winding of the slinky mainly adds distributed inductance, we all
know series inductance retards current flow. The same slow wave effect will
occur with a thick dielectric surrounding the wire (it increases shunt
capacitance), a string of beads, or conventional inductors in series with the
antenna and spaced every 1/4 wl or less. Any of these methods are just as
effective, so the method used is a mostly matter of personal choice. There is
no "magic" caused by cramming 500 feet of wire into a 200 foot area, it's all
current distribution and phase. You will still have a 200 foot long antenna,
just a bit better one if the phase and current is correct. If the phase and
current is distribution wrong, it will be a poorer antenna.
I used seven lumped inductances placed about every 1/16 wl on a 250 ft wire.
As I recall the best value was about 400-600 ohms XL, and termination was
about 1000 ohms with a number 12 or 14 wire eight feet high.
I adjusted the inductances to the correct value for 160 meters. I did this by
exciting the antenna with RF and measuring phase shift from end to end.
Instead of terminating the antenna, I returned the far end through a step
down transformer via equal lengths of coax to a phase meter.
Knowing that it is worse to slow the wave too much as not enough, it seems
unlikely a slinky stretched ransom amounts would provide optimum performance.
It would be pure blind luck if it was, and likely not reproduced by the same
luck in other installations unless the original "inventor" made measurements
of Vp and winding pitch and diameter, and the installation was duplicated
exactly.
I'm open to any technical correction on this. My experiments were done in the
70's, and my recollection is vague. But page 72 of Antennas and Transmission
Lines by Dr. John Kuecken seems to confirm what I recall.
Slow wave antennas will not work as well as a simple phased array occupying
the same physical area, but they are capable of being better than a
conventional terminated wire the same length.
73 Tom
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & N4VJ / K4AAA
|