Electronic – Help interpreting a NASA design

antennaelectromagnetismRF

I'm very interested in replicating a design published by NASA here : https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027843.pdf

Specifically the arrangement on Page 11. I have a similar design using a hybrid coupler, and this is very nearly what I want to build in terms of antenna system, but I don't understand how the coax is attached to the antenna elements as shown in this diagram. Phasing Network

I already got an answer to this exact question on ham.SE:

It doesn't make much sense to me either, or rather, the details are
not specified adequately. What is specified in the document you
linked is:

The near quarterwave monopole, fed as one half of an opposed pair of antennas has an input impedance of the order of \$80 \pm45^{\circ}\$. It
is therefore possible to match the impedance to either 50 or 100 ohms
depending on system requirements. Usually a plot of impedance vs
length of the antennas in the proper structure is made and a
convenient value of length selected to facilitate matching. The
matching components (all reactive) are installed in a small chamber in
the base of each antenna. This permits the rest of the feed system to
operate in a low loss balanced condition.

(I assume that \$80 \pm45^{\circ}\$ is a typographic error and \$80 \pm45
> \Omega\$ was meant.)

The only “opposed pairs” in this system are the pairs of antennas 180°
out of phase, such as the pair of #1 and #3.

Since #3 is (called) a monopole, at the end of a 50 Ω feed line, it
must be fed in the manner of a ground-plane antenna, with the spacecraft's outer shell connected as the ground plane.

Then since there is a 50 Ω line to the left and a 50 Ω line to the
right of #1, we must conclude that the thing called a matching network
in fact incorporates a power divider to split (or combine) the
signal between #1 and the line to #3.

I don't find this very plausible, but it's the best interpretation I
find so far of what's written there.

I would assume that it would simply be the center of the coax, but this doesn't really make sense with the way I understand connecting coax to an element.
If someone could explain the connection to the antenna that would be appreciated.

Best Answer

They are analyzing Antenna Diplexers from different Satellites which are not the same design.

In general, when reading mature commercial datasheets and NASA documents it is better to ASSUME that you do not understand , BEFORE you ASSUME the document is wrong.
( my opinion from 40 yrs experience)

However, this logic does not apply to random web schematics that are poorly documented and then you can assume high risk to error from poor specs and missing assumptions.

  • The diagram you show in this question is for Explorer 34 while the added text from previous question is for Explorer 32, which are completely different designs.

  • the Explorer 32 text was correctly stated.

  • "an input impedance of the order of \$80 \pm45^{\circ}\$ ...to match the impedance to either 50 or 100 ohms depending on system requirements."

    ( this is because ¼λ inverts impedance and is very sensitive to tune Z by length. )

    • Explorer 32 uses a ¼λ splitter at fo=c/λ, produces a 90° diff. phase or ± 45° single ended to each antenna. It used a 90° splitter with 2 diff. stripline antenna to make a quadrature 4 port antenna.
    • Explorer 34 uses a quadrature 70Ω coax hybrid splitter to four (4) quadrature 100Ω antenna. Using a combination of direction, diversity and phase splitters so that each antenna is phase coherent at the resonant wavelength
      • (you should already know from transmission line Theory, that ½λ produces 180° with same Z at each end at fo.)

For those who do not know already; combiner, splitter and coupler are synonymous in RF components since they may be used in either direction hence the unique names implying use. Coupler is more universal, bi-directional in meaning. e.g. dir. coupler, DC-3 is also a splitter or a combiner.

Which number do you think is best and why?

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Personal note

In 1976, I had to design a VHF telemetry antenna for one of our company's smallest rockets, the Black Brandt VI which were all for plasma research and carried experiments for upper atmosphere from 50 to 1000km altitude. This antenna was to be coiled during launch then eject after the reaching top supersonic speed while spinning around 5~10 Hz. The nose cone was then ejected exposing my flat rolled braided wire on a nylon spool which by centripetal force spun out like a perfect dipole in the vacuum of space then descend by parachute once air was reached. There was an official spin test in the lab for my nose cone antenna where they spin it then eject the clamshells and nose cone. It worked. In flight from Churchill, the ground station used quad Helix auto-tracking antenna. The signals were perfect until it approached the horizon where the spin would be orthogonal and thus have nulls every 1/10th of a second in data when the ends of the antenna are pointed straight towards us. Fortunately, all the data needed was before this time. I had several 2 week very kewl trips to Churchill , also very cool Aurora and "whiteouts" where you can barely see the tail lights in front of you and tracking Polar bears that leap over 14ft Razor wire protected garbage dumps like it was a simple babycrib. (I also designed Dopper Azimuth tracking systems for BB-IV, BB-V etc.)

p.s. the block diagrams need to be combined with the antenna length, azimuth and elevation angle along with; ground plane; ferrite balun and LC reactive tuning to get > 15 Return Loss and get (good) transmission loss for different receiving antenna ( polar and linear ) as shown in the polar plots.

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