"Walter Maxwell" wrote in message
...
On Sat, 17 Jul 2004 08:30:08 +0100, "Ian White, G3SEK"
wrote:
Jerry Martes wrote:
Is there anyone interested in seeing pictures of a home made VHF
slotted
line?? I bought some copper pipe at Home Depot and built a 50 ohm line
that seems to work very well.
Definitely! (I'm always a sucker for VHF test equipment.)
H-P used to make one of these, in the form of a very deep trough-line
with massively braced side castings. There was also a 'lite' copy in the
old ARRL VHF Handbook.
If you can minimize the residual SWR (the variation in probe readings
along the line when terminated in a precision 50R load) and other
sources of variability, then you'll be able to make highly accurate
vector impedance measurements at VHF.
Hello, Ian,
here's a little trivia concerning slotted lines you might find
interesting. I
developed all the antenna systems that flew on the World's first weather
satellite, TIROS 1, in late 1957, which flew April 1, 1960. The system
operated
on four different frequencies, two around 108 MHz and two around 235 MHz.
Circular polarization was required, and was achieved using four crossed
unipoles
working against the body of the satellite. Each unipole used a 1/4wl
sleeve for
the high-frequency portion with a coaxial extension for the lower
frequency,
decoupled by the sleeve for the higher frequency. The sleeve portion was
was
developed to yield a 50 + j0 terminal impedance that included the mutual
impedance of the extended portion. The extension portion was developed to
provide as low an SWR as possible at the lower frequency. The coupling
system
for feeding the four radiators with four transmitters simultaneously was
designed in stripline.
Now, getting to the point of the story, the development of the radiating
elements was performed before there were any network analyzers--I
performed ALL
the work using only a slotted line for hundreds and hundreds of
measurements.
The line was an HP-805, that couldn't cover the frequencies involved, so I
used
4:1 scale modeling at 432 and 940 MHz. The radiator development was done
using a
single element over a ground plane, with the spacecraft modeled with a 4:1
half
shell mounted on a 4' x 4' aluminum plate for the ground plane.
If you have a copy of Reflections II you can see a picture of my lab setup
at
the RCA Laboratories in Princeton, NJ, showing the ground plane and the
HP-805
slotted line. Incidentally, in 1957 there were no hand calculators
either--my
calculations from the line measurements were all made with a slide rule.
Just
picture the amount of work involved using such antiquated equipment
compared
with what we could do now using today's more sophisticated
instrumentation.
Walt Maxwell, W2DU
Walter
I too used a HP slotted line alot when I was developing antennas in the
1950s. I never actually knew exactly what I was doing, and have forgotten
most of what I once did with confidance. It is coincidantal that my
primary reason for wanting to know the antenna impedance at 137 MHz is
because I learned, a few months ago, that weather pictures could be produced
from VHF satellite signals.
I reasoned that the pattern form two crossed dipoles spaced 1/4 wave apart
and fed in phase could produce a bidirectional pattern with circular
polarization. I thought that set of simple dipoles might be easy for me to
build, if the antenna concept was compatible with the radiation from polar
orbiting satellites, since they travel in a North-South path.
I am aware that the helix antennas are superior to the crossed array I'd
like to build. Still, I am having fun learning about how these antennas
work.
If I cant easily refine this slotted line, I may try building an impedance
meter with the 4 coax lines, 1/8th wave long each. Perhaps the accuracy of
the coax impedance meter will provide the accuracy missing from the slotted
line method. Maybe the two 'devices' can compliment each other.
I was sure pleased with the way this slotted line worked last night when I
fired it up. I am open to any criticism. This is only fun for me. But,
since it seems to work so well, I thought there might be some interest in
home made "Home Depot" quality (low cost) slotted lines for VHF.
Jerry
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