When ATU Bypass Isn't Necessarily So.

A long time ago, when I was even more ignorant of ham radio set-ups than today, I tried to create a well-matched delta loop by using an ATU's SWR meter, with the matchbox set on 'bypass'.  I remember trudging backwards and forwards so much in that exercise that I'd worn a very muddy path in the garden!

Beware stray currents - and false SWR readings - on ATU 'bypass'.

I never did achieve a low SWR with the ATU meter, and wondered why that was.  The answer, it seemed, was that, even when switched to 'bypass', there is some kind of interaction between the RF and the ATU circuitry, leading to strange and usually meaningless SWR meter readings. 

This phenomenon made itself known again the other day, when I switched over to an extremely reliable 2-ele quad for 6m that has a 1:1.05 SWR at worst.  It went via a coax switch to the ATU on 'bypass', and then to the rig.  I keyed-up to check the antenna, and the rig SWR meter was reading 1:1.5!  I scratched my head a bit and thought about what had changed recently.

It turned out that the changed item was the position of the inductor switch on the ATU.  I'd been using an inverted-L on 60m, this antenna design necessitating the use of a matchbox.  I turned the switch back to its earlier position, and the problem 'high' SWR on the 6m quad vanished.  I connected to the quad directly via a separate SWR meter, and that also reported the expected, 1:1 SWR.   It was the same with the delta loop; only by using a standalone SWR meter and direct connection to the rig, rather than via an ATU on bypass, was I able to cut the right wire length with ease.

Some ATUs might be OK in this regard.  But seeing as many of us use relatively cheap units by the well-known producers, it's a salutary point to note next time you want to check what your antenna is doing!

Update: Someone pointed out to me that the ATU I was using is only rated up to 30MHz which, they opined, was also true even when set to bypass.  They're doubtless correct in this, but I think most people, perhaps naively, expect 'bypass' to mean 'completely bypass, whatever the frequency', which isn't the case.