Tam wrote:

I tried a 2.5 MHz HPF with an MFJ analyzer to measure a 75 m antenna.
Unfortunately there is too much phase shift. Really not usable below
about 5 MHz. The MFJ works fine during daytime hours before all the
signals come up.

The "half wave" filter I described makes no impedance disturbance only
at the frequency at which all the components have the same reactance.
The cutoff frequency, as I recall, is close to that frequency. So you'd
have to make a filter with a cutoff close to 3.8 MHz, not 2.5 MHz, in
order to measure a 75 m antenna if you're using this kind of filter.
There are, of course, many other types of filters, but just about any
will affect the impedance measurement except perhaps over a fairly
narrow range of frequencies.

Roy Lewallen, W7EL