Rajar was attacked last week by The Wireless Group chief executive Kelvin MacKenzie for launching a second period of testing of two electronic measurement systems.
MacKenzie accused Rajar of kicking the issue "into the long grass" and of unnecessary delays that were costing him and his TalkSPORT station money.
However, MacArthur is taking the view that Rajar is right to insist on further tests of the Arbitron and Radiocontrol systems.
"Rajar has budgeted a further £0.5m to test the second generation as soon as they are available. Though I am not a member of the Rajar board, I have no doubt that they are acting sensibly given the test results," MacArthur wrote.
He outlined that the results of Rajar's first period of testing of the meters, which could potentially replace the current diary-based system, showed differences of up to 600% in individual station measurements between meters and an overall difference of over a third in terms of hours listened.
"I support the Rajar board decision. This decision wasn't, as some suggest, a 'stitch-up' by the big boys of commercial radio. I understand that this was a unanimous decision by the whole Rajar board -- including BBC Radio, the IPA and ISBA," he concluded.
MacArthur called for advertisers to "hurry along" Rajar with the next stage of development but pointed out that it had already earmarked over £1m to testing and development.
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