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Robust transponder antennas

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 18, 2011 6:18 pm Post subject: Robust transponder antennas

I've been away from the home 'puter for a few days

and didn't have access to my big archive of reference

documents when the List was discussing transponder

antennas.



Got home a few hours ago and posted a patent issued

in '61 for what is euphemistically called a "scimitar"

antenna.



http://www.aeroelectric.com/Reference_Docs/Antenna/Scimitar_Patent_3015101.pdf



I recalled seeing these all over the empannage belly

of the B-52. They looked like tie-down rings except that

there was about a half dozen of various sizes. They

were associated with the electronic counter-measures

system. I was a bomb-nav guy so I didn't get any schooling

on them.



Years later I was thinking about making a robust

transponder antenna using this design philosophy

but never got around to it. Others here on the List

might be interested in trying it out. Note that

where the SWR on the exemplar antenna in the patent

goes below 2:1 (4,000 Mhz) the horizontal radiation

pattern goes pretty circular.



An antenna could be crafted from aluminum or perhaps

brass and thick enough to offer the wished-for mechanical

robustness. The edges could also be sculptured as

necessary for aerodynamically appealing affect.



It was interesting to contemplate the development history

of this antenna. While based on development work by

the Turner guys pre 1958 the patent wasn't issued until

1961. The airplanes I was working on were built in the

1954-58 time frame and already had several sizes installed.



Similar antennas on the spacecraft programs were faired

with radomes. The antennas on the B-52 were bare and

hung right out in the breeze.



Anywho, for what it's worth, this might be the basis

for a nice DIY antenna project. It would lend itself

well to an article featuring fabricate-to-drawings

illustrations.

   Bob . . .

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dgaldrich

Joined: 24 Apr 2009
Posts: 267
Location: Naples, Fl and Belfast, ME

Posted: Thu Jan 20, 2011 6:00 pm Post subject: Re: Robust transponder antennas

I believe the antenna design in question is an approach to getting a VERY broad range of frequency coverage (10-1) from a single antenna.  Note also that it was designed to operate from 30 to 30,000 MHz and the ATC transponder frequency is a single one at 1090 Mhz.  



You certainly don't need the broad coverage and are almost certainly better off with a single blade or stub designed for 1090 MHz than this.  



The antennae on the B-52 needed to cover almost DC to light (400 khz to 30 GHz) and this design was very workable.



Ex B-52 Pilot



Dave Aldrich



do not archive

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 21, 2011 5:35 am Post subject: Robust transponder antennas

I believe the antenna design in question is an approach to getting a 

VERY broad range of frequency coverage (10-1) from a single 

antenna.  Note also that it was designed to operate from 30 to 30,000 

MHz and the ATC transponder frequency is a single one at 1090 Mhz.



You certainly don't need the broad coverage and are almost certainly 

better off with a single blade or stub designed for 1090 MHz than this.



The antennae on the B-52 needed to cover almost DC to light (400 khz 

to 30 GHz) and this design was very workable.



      All true. But keep in mind that the roots of

      this thread went to robustness. All of the

      antennas offered commercially for transponder/

      DME service ran from cheap (single exposed

      monopole) to rather expensive (aerodynamically

      faired blade). All active portions of the antenna

      are insulated from the airframe.



      The feature of the scimitar antenna that attracted

      us was the one-piece, all metal design grounded

      to the airframe. Such antennas could be

      crafted from two pieces of metal, a base and a

      blade. Further, they could be rather thick, say

      1/8 to 1/4 inches. Such a construction would be

      MUCH more robust than any commercial offerings.

      If such an antenna were attached to good structure

      in the airplane, it might well double as a tie

      down ring.



      As I noted before, the patent given to this design

      was curious. It was applied for years after some

      of these antennas were already in production.

      The patent was assigned to government in the

      first paragraph. Finally, the patent was

      long on geometry (you can make one "this" way,

      or make one "that" way) and long on radiation

      patterns. But not one word as to the reason

      why one would make one this way, that way, or

      dimension it for optimum performance at any

      frequency.



      Methinks this patent was issued as an afterthought

      during the cold war when unwashed citizens

      began to observe such antennas on military

      aircraft. The patent was more a deterrent

      to commercial development than to protect

      new art.  This leaves the modern day DIY

      developer with the task of re-discovering

      the simple-ideas behind the design. Not

      difficult and even fun if you have the

      test equipment.



   Bob . . .

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