User Manual

This is the electrical equivalent of a series of five coupled helical resonators.
Similar lower power filters are built using conventional air wound coils and
ceramic trimmer capacitors, and these will be described next:
2. 20B704G1 Low Power Bandpass Filter for High Band:
Please refer to Figure 5. The configuration of this filter is similar to the
previously described helical resonator type in that it uses five LC resonant
circuits, but it differs in that two of these resonant circuits behave as high Q
traps for frequencies outside the band edge (_4.5 and +9 MHz), so that the
overall response has a reasonably flat top and steep sides. Factory adjustment
is made to achieve the same in-band response (carriers must be f
V
< 0.6 dB and
f
A
< 0.7 dB departure from flatness) as described for the helical resonator
filter. (We would have preferred to use this 20B704G1 filter for the 50 watt
high band system as well, except that the ceramic trimmers overheated due to the
higher RF currents at the 50 watt level, so the decision was made to go with the
higher power helical resonator filter for 50 watt high band transmitters).
Like the helical resonator filter, there are nine screw adjustments and two I/O
matching (with soldering iron) adjustments that need to be made simultaneously,
and all of them are interactive. Accurate adjustment is impossible without the
aid of a network analyzer, and because of the expense of this gear it is not as
likely to be available in the field; for this reason we say the filter is not
user-adjustable.
Sure, it is possible to use a sweep generator and detector for setting the
response of either filter, but unless an accurate 50 ohm return loss bridge is