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Discussion at this answer to Why does DSN sometimes uses two dishes at the same time to receive Voyager-1? include the possibility that in some cases it would be preferable to use two 34 meter DSN dishes instead of one 70 meter dish, and if a 70 meter dish somehow reached end of life it would be replaced by two 34 meter dishes that would also be lower cost to operate.

The primary goal of this question is to address the usage aspects rather than the operation costs or the likelihood that a 70 meter dish would "wear out" any time soon. But one is welcome to touch on those too.

Question: Given three different configurations:

  1. One 70 meter DSN dish
  2. Two co-located 34 meter DSN dishes
  3. Two 34 meter DSN dishes at different sites

what are the tradeoffs between them, and under which circumstances or usage scenarios are each the best or worst option?

Related questions and answers likely helpful to support answers here:

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    $\begingroup$ The use of the Soviet analogs of these radio telescopes (P-2500 = 70m, P-400 - 32m) was purely economic. The cost of operating a small antenna is less than that of a large antenna. In most interplanetary flights, a small antenna was used. And at a distance of the orbit of Mars, Venus, a large antenna was connected to work. For those modes where it was necessary to transmit information at high speed, for example, take photos. $\endgroup$
    – A. Rumlin
    Aug 6, 2021 at 17:02
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    $\begingroup$ According to Wikipedia: NASA Deep Space Network- By 2025, the 70-meter antennas at all three locations will be decommissioned and replaced with 34-meter BWG antennas that will be arrayed. $\endgroup$
    – Fred
    May 8 at 10:11
  • $\begingroup$ @Fred cool! wow! nice! thanks! $\endgroup$
    – uhoh
    May 8 at 15:27

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The major downside to using a pair of 34-meter antennas is that they have only half the collecting area of a single 70-meter antenna, and produce only half the signal strength. Beyond that, on the plus side for a two-dish array:

  • You can operate two (or more) antennas as a phased array, permitting more precise pointing than a single antenna with a comparable physical steering system can manage.
  • In a similar act of physics, you can use the antennas as a radio interferometer, giving an effective beam width comparable to that of a single antenna with a diameter equal to the distance between the antennas.
  • Bigger antennas are more expensive to build and operate, with the costs increasing disproportionately to their size. Two 34-meter antennas cost less than half of what a single 70-meter antenna does.
  • Antenna arrays are more reliable. The Deep Space Network has eleven 34-meter antennas; you can theoretically build an array from any pair that can see the target spacecraft, giving more options if an antenna fails. In comparison, if one of the three 70-meter antennas fails, your only option is to wait until the target comes into view of one of the other two.

And on the minus side:

  • Antenna arrays have a more complex beam pattern than single antennas. This results in power being wasted in unwanted directions when transmitting, and increased interference from side lobes when receiving.
  • As mentioned in the introduction, you need four 34-meter antennas, not two, to get the same power as a single 70-meter antenna.
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  • $\begingroup$ If each dish has its own front end (the first stage amplifier connected to the little dipole inside the feed horn) then won't an other disadvantage be something like $\sqrt{2}$ more noise? Isn't signal to noise ratio really the important metric here? $\endgroup$
    – uhoh
    May 4 at 23:51
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    $\begingroup$ @uhoh, noise on antenna arrays is complicated. Noise from correlated sources goes up; noise from uncorrelated sources goes down. $\endgroup$
    – Mark
    May 4 at 23:53
  • $\begingroup$ Ya it does need to be treated in a rigorous way and I'm definitely being evil by asking the question rather than trying to do the work :-) For DSN antennas I think they work hard to minimize all other sources and the thermal $k_B T$ noise of the front end dominates. To my understanding not all dishes have front ends cooled with liquid helium but certainly some do Why doesn't thermal radio emission from a DSN "hot dish" completely swamp the benefits of a cold LNA? $\endgroup$
    – uhoh
    May 4 at 23:57
  • $\begingroup$ "if one of the three 70-meter antennas fails, your only option is to wait until the target comes into view of one of the other two." Or maybe are completely doomed if the light distance to the target is such that you need a certain combination of antennas for TX and RX. $\endgroup$
    – asdfex
    May 5 at 7:33

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