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