Whether or not the unused slopers act as a counterpoise or not depends
on the switching arrangement.
If they are 'floating free', then you are right; however, if the unused
slopers are electrically grounded to the tower at their 'feedpoints',
then they DO act as 'counterpoises' [in reality, they are part of the
'tower side' of the sloper system].
I used a 'sloper' system like this before they were called 'slopers' ..
used the guywires for my tower, insulated at top and bottom, as a 4-wire
array, with remote relay selection of any one or two of them as
radiators.
Using one gave me a 'sloper' system, with the other three 'grounded' to
the tower, and using two of them gave me an 'inverted-V' configuration.
Should mention that the guywires were each 66-feet long, and my feedline
was 450-ohm open-wire line, fed through a Johnson Matchbox to my Viking
I transmitter ... [shade of the 'GOOD OLD DAYS']
Don
N8DE
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