TopBand: Autek RF-1 Antenna Analyzer

Mon, 6 Oct 1997 22:39:19 +0000

> From:          Ward Silver <hwardsil@WOLFENET.com>

> I know the MFJ-259 is pretty well unusable below 7MHz at my QTH due to
> strong local BC unless I put in an HPF (Kiwa BC-1).
> 
> The only problem with filters is that they usually need something close to
> their design termination impedance to have their frequency-responses stay
> as-designed.  A highly reactive load or one with an impedance far away
> from the filter's expectations will upset it greatly.  A high-Q stub is
> much less likely to have the same effect.
> 
> 73, Ward N0AX

A bandpass or band reject filter would be the way to NOT go.

The best device is a high Q parallel LC circuit right at the 
analyzer's terminals. By adjusting the parallel circuit for maximum 
impedance on the measurement frequency, the circuit would remain 
transparent for ANY load condition.

The only requirement is the circuits unloaded Q be high enough that 
the equiv parallel resistance (resistance because it is resonant) of 
the L/C combo exceeds 1000 ohms or so.  That's pretty easy to do.

73, Tom W8JI
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