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

On Mar 31, 11:32 am, Roy Lewallen wrote:
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

Since there is just one frequency that's causing trouble (apparently),
and since I gather that it's removed in frequency a pretty good
percentage from the frequencies of interest, it may be possible to
block it with a fairly sharp notch filter. As Roy notes, the 100.5MHz
FM signal may not be the only one causing trouble, but assuming it is,
try this: shunt across the input to the analyzer, place a series LC.
The L: 9 turns #12AWG bare copper, 0.75" ID, 1.5" long (about 750nH,
SRF about 150MHz). The C: a high quality 1-5pF trimmer capacitor.
I'd recommend a good piston trimmer, but they tend to be expensive
unless you happen to have some or can get some surplus. Depending on
the exact coil characteristics, the cap will adjust to a 100MHz
resonance at about 2pF. By keeping a high L:C ratio, the notch will
be sharp--not as deep as it could be with lower L:C, but it should be
deep enough if you use good construction practice. The suggested
values should give you at least 25dB attenuation at the notch
frequency, and should look practically like the capacitor value (about
2pF) at HF.

Cheers,
Tom