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

"Walter Maxwell" wrote in message
. ..



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 confidence. It is coincidental 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 hank 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

configurations. 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 elements 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





Walter

I found pre-cut to 5 foot length two copper pipes at Home Depot. The large
diameter pipe is 2.00 ID and the smaller is 0.875 OD. I made some ID and OD
tapers about 3 inches long so they matched some N connectors. I dont
actually know why I measure a reflection of 1/10 th of the incident voltage.
I must have been too sloppy with the construction somewhere. Still, I wasnt
at all sure I could build something that would work so I wasnt sure how
careful I'd have to be. I can probably learn where most of the
discontinuities are. But, I can do everything I need to do with this "quick
and dirty" slotted line.

As for the Helix for NOAA satellite reception, I agree that the helix is the
best way to go for polar orbiting satellite pictures. I had heard alot of
tales of difficulties about building QHAs and QFHs. I have a home made QHA
that works pretty well. I dont understand how they work. But I like the way
the work.

Right now I'm having fun trying to build a collapaable/foldable antenna for
VHF polar orbiting weather satellite reception. The building of an antenna
is the major reason for me being interested in WXSAT. I sure couldnt improve
on the omni azimuth reception of a helix. But, what do you think about there
being some worth in a crossed dipole that is oriented to look along the
North-South? It is fairly easy/cheap to build and quite tolerant to errors
in dipole element length.

I am sure that you are vastly more knowledgeable about the radiation pattern
and coverage of the Cross than I am. It seems to provide good signals toward
North and South with good RHCP within about 90 degrees both North and South,
without "aiming". I oriented the antenna so its nulls are East and West at
low angles. I reasoned that satellites traversing those low angle toward the
East or West would be viewable for only minimal times when only polar
orbiting satellites are considered. I tried orienting the antenna so the
null occurred at zenith, so the East-West sensitivity would improve, but I
cant fill the overhead nulls.

I'm getting carried away with a discussion that probably interests no one.

Jerry