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

Hi Jerry,

I hink it's great that you can buy materials a Home Depot from which you can
fabricate a slotted line having 50-ohm impedance. With smooth inner and outer
conductors the inherent swr should be low enough for making acceptable
measurements. In addition, if the teflon spacers are thin they should give only
an insignificant discontinuity at 137 MHz.

You might still want to consider the quadrifilar helix. They are being used
nearly exclusively for all frequencies on the NOAA polar-orbiting weather
satellites. While at RCA in 1976 I performed an R&D project on the quad helix by
investigating the radiation patterns and impedances on over 1000 different
configuations. From the data obtained from my measurements, one configuration
fit the requirements for the TIROS-N spacecrafts, which became NOAA spacecraft
once in polar orbit. RCA built the entire series of TIROS spacecraft until RCA
was taken over by GE in 1986.

The report of my R&D investigation appears in Reflections II, and is also
available for downloading from my web page at http://home.iag.net/~w2du. There
is also a chapter in Reflections 1 and 2 that describes two configurations of
the quad helix, one of which is that flying on the NOAA satellites;. The chapter
number is 22, and is also available for downloading from my web page. That
chapter gives the data for constructing the configuration flying on the NOAA
satellites, which uses an infinite balun and self phasing of the helical
elements. If you're not interested in constructing that particular configuration
you can still feed the two helical elementss separately, using commercially made
baluns and a 90° phase shifter.

Jerry, the beautiful aspect of using the quad helix is that you don't have to
point it toward the satellite. Just place it so the beam is vertical and at
least 1/4wl above ground and it'll give you signal from horizon to horizon
without aiming it.

Good luck with whatever configuration you use.

Walt