Noise bridge used to measure resonance of antennas

[Huttinger]

Hi,
    I have a technical question regarding the use of a noise bridge to
measure the resonance point of a vertical and would really like to get any
information I can from list members on what is going on with my minimum SWR
measurements using the noise bridge.  I have a vertical antenna that covers
40-10 meters and uses radials.  I have a heathkit noise bridge I used to try
to measure the resonant points of the antenna so I would know how to adjust
the various sliding joints to give resonance on all bands.  There is about
50 feet of RG 58 coax connecting the receiver (and noise bridge) to the
antenna.  The manual says to have the coax as short as possible to avoid
masking the antenna characteristics from the noise bridge.  I measured the
SWR at both ends of the bands (and other points) this vertical covers using
an ordinary SWR meter and noticed that the SWR minimum points were quite a
bit different from what the noise bridge indicates.  When the manual speaks
of coax "masking" the effects of the antenna from the noise bridge what
exactly does that mean?  Does that mean the too long coax makes it difficult
to find minimum SWR points because the noise from the noise bridge doesn't
appear to vary much as you tune the receiver or does it mean the minimum SWR
points get thrown off by the too long coax?  I get what sounds to be very
clear indication of minimum SWR points by nulls in the noise from the noise
bridge.  The only thing is the nulls don't agree with the SWR meter.  Any
insight, hints, ideas etc welcomed.  I am really curious about this.
Thanks,
Mike

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