The Autek is modifying the impedance seen by the Omni differently than
the Bird. Check the antenna SWR with the Bird alone and then with the
Autek between the Bird and the antenna at the critical frequency (using
the Bird to make both measurements). The Bird modifies it differently,
neither passes the load to the radio precisely the same as it was
because the coupler has some transmission line length and coupling
elements. Likely in the Autek there's a spurious resonance in the
coupled elements to make it differente between the two frequencies.
Different slugs in Birds will give different results.
A standard test set for tuning VHF radios with Birds adds enough
transmission line to the line segment of the Bird so the whole assembly
is 1/2 wave long. That way the Bird changes the antenna impedance the
least. This might work with the Autek or might not depending on whether
its simply a transmission line effect or a spurious resonance effect in
the coils and capacitors of the Autek. I presume the Autek is using a
circuit similar to that from the Bruene or the DeMaw degredations of the
Bruene wattmeter that has a toroid coupled around the line with
capacitive dividers for forward and reflected power. There's a resonance
in that circuit I believe.
73, Jerry, K0CQ
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