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@Fred's comment links to the Curious Droid video How Australia Helped Save Apollo 13 and a little after 02:15 and 03:00 it shows images of dish antennas presumably at Australia's Honeysuckle Creek station. It's possible these are two photos of the same antenna on different dates, but I'm not sure.

Is it possible to explain what these three components are, how they work and why they have their distinctive shapes?

  1. small, covered auxiliary dish antenna(?) mounted on one side of the main dish
  2. small, open auxiliary dish antenna(?) mounted on the other side of the main dish
  3. large hourglass-shaped central biconic structure which presumably supports either a convex secondary reflector or the feed horn system itself at the primary focus.

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screen shot Curious Droid's How Australia Helped Save Apollo 13

screen shot Curious Droid's How Australia Helped Save Apollo 13

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The photo you have is of DSS-46, which is actually at Tidbinbilla. It used to be DSS-44 at Honeysuckle Creek, but when that site was closed in 1981 the antenna was moved. When it was re-assembled in 83, a new self-supporting secondary reflector replaced the old quadripod (source).

Here's a better photo of that bit:

DSS-46

(Crop from 2005 image here by Eric Fehr)

And here's comparison with an older feed on the antenna:

DSS-44

(The feedcone is changed during Apollo 8, photo by Alan Foster)

You can sort of see that if you just sliced off all the quadripod bits and kept the secondary reflector and mounted it to the end of the feedcone by a minimal bit of some suitably radio-transparent composite, you'd end up with the biconic structure you're asking about. Nothing mysterious there, at least.

There's a commercial example here for a much smaller antenna:

Microstar X-Band antenna

the manufacturer states,

The subreflector for the 8 ft. Cassegrain feed is supported by a thin wall fiberglass cone which attaches to the short aluminum conical support structure that supports the feed system. The resulting 8 ft. monopod feed support is very stiff, with a very high resonant frequency.

I haven't searched further to find out whether "monopod" or "self-supporting" is the preferred term. I'll leave that as an exercise for the reader.

According to the Canberra Deep Space Communications Complex page, the antenna was "modified to expand its capabilities for early launch acquisition", though it doesn't go into detail. This entailed getting rid of the old S-band acquisition antenna that sat on the end of the quadripod, and replacing it with new gear. The CDSSC page simply states that it the antenna now has an S-Band Acquisition Aid (2200-2300MHz) and an X-Band Acquisition Aid (8400-8500MHz), which will be the two little antennae stuck on either side. I'm not sure which is which, yet.

There's a nice high-resolution image of the dish here (credit: Phil Maier) from which I've grabbed these little details:

Acquisition Antenna 1

which looks like a fairly boring and conventional parabolic-plus-quadripod jobbie, and

Acquisition Antenna 2

which apparently has a radome, for some reason.

A quick rummage through an image search for S- and X-band antennae didn't turn up any examples of S-band devices with radomes . A few X-band examples do, but they seem to be radars rather than merely acquisition antennae (such as this one: XPOL). Presumably it isn't necessary here, either (as the other acquisition antenna lacks one) so maybe it just came stuck to an off-the-shelf commercial unit that suited the operators.

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  • $\begingroup$ This is great, thanks for digging in! For the secondary reflector and feeds, is this roughly what the beam envelope looks like roughly? i.stack.imgur.com/vm9Qc.png Have I got the location of the secondary focus about right? For the "acquisition aids" the appear to have roughly 170 times less area than the main dish, which would be about a 22 dB reduction in signal strength. It's hard to imagine how they could aid in the acquisition of weak signals, but since they only need to detect carrier presence and don't need a good S/N ratio maybe they're enough. $\endgroup$
    – uhoh
    Jan 18, 2020 at 1:31
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    $\begingroup$ The diagram seems OK to me, but I'm no expert ;-) It'll almost certainly be a standard cassegrain antenna design, so you can always check against diagrams of those. The 3ft acquisition antennae used in the Apollo USB system had a 22dB gain. They were only used to track nearby stuff (eg. LEO-ish) so they didn't need to be super high-gain. $\endgroup$
    – Starfish Prime
    Jan 18, 2020 at 21:27
  • $\begingroup$ I'm a bit confused by what you mean exactly. I don't think this large 26 meter DSS dish is used to track LEO-ish objects, so these "acquisition aid antennas" would have to aid acquisition of sources in deep space. Also, if the dish is 26 meters, these small dishes are 6 feet rather than three (estimating a diameter ratio of something like 13 or 14 to 1) Anyway, that's probably the basis of a "next question". Thanks! $\endgroup$
    – uhoh
    Jan 19, 2020 at 1:09
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    $\begingroup$ @uhoh the example was of another acquisition antenna that is unlikely to be substantially poorer. This specific antenna was used on LEO-ish objects... see the quote below the fold, "modified to expand its capabilities for early launch acquisition". The other big dishes at Tidbinbilla do not have any acquisition antennae on them at all, I note, and since DSS-46 was retired none have been modified to add them. $\endgroup$
    – Starfish Prime
    Jan 19, 2020 at 10:36

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