>
> Would it be feasible, where you believe that it's only one AM station, to
> build up a little series trap? Could the notch be deep enough, or does it
> take more elaborate circuitry like MFJ's add-on?
>
>
> 73, Pete N4ZR
>
IMHO, one must be very careful when any sort of network is introduced
between the antenna being measured and the MFH unit, to prevent seriously
altering the reading. A two-terminal or one-port device such as a simple
series trap needs to be sharp enough to be essentially out of the circuit at
the antenna's operating frequency. 10X the expected antenna impedance (500
ohms or more for a 50 ohm antenna) would be good.
For a two-port device, such as filter between antenna and analyzer, it is a
bit more complicated. A passive two-port can do only four things with energy
from a source----pass it through, reflect it back to the source, dissipate
it internally or radiate it. Presuming essentially lossless components and
perfect shielding, such a device may either pass energy through or reflect
it back to the source. There would ideally be no insertion loss.
One is attempting to measure the energy reflected from the load (antenna),
so any reflection due to the filter distorts the reading. Therefore, for the
filter to reject BC band energy without introducing error at the test
frequency requires a ner-perfect match within its passband for a 50 ohm
termination on the other end. Note that filters with ripple in the passband
only meet this criterion at frequencies of zero reflection.
The implication is that, in order to be rid of the gross error due to BC
station pickup, there will likely be some loss of accuracy at the
measurement frequency.
Garry, NI6T
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