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NOTES ON ANTENNA MATCHING
(1) It is very important that thé maximum time
limits during tune-up conditions for thé
transmit ter are not exceeded when thé trans-
mitter is being used at full power.
(2) Any matching
perfbrmed by thé FC-700
in thé shack will hâve no effect on thé
lOSSeS due to SWR on thé coaxial Une
between thé FC-700 coupler and thé antenna.
The operator should consult one of thé
popular antenna handbooks
to détermine
whether
or not
matching
between
thé
coaxial line and thé antenna must be per-
formed at thé antenna. For example, a 100-
foot length of RG8A/U coax typically has a
loss (with 1:1 SWR between it and antenna)
of less than 1 dB at 21 MHz. If this line is
operated with a 3:1 SWR due to a low or
high antenna impédance, thé loss due to SWR
will increase roughly 0.5 dB,#n imperceptible
dégradation as compared to thé 1:1 condition.
In this case, attempts to reduce thé 3:1 SWR
at thé antenna end would serve no useful
purpose as far as reducing losses in thé coax,
through matching with thé FC-700 would
improve thé impédance presented to thé
transmitter output circuitry- However, if a
500-foot length of thé above coax were used
instead of oniy ioo feet, somewliat more
than 1 dB of loss would occur in thé ooflv due
to thé 3:1 SWR, possibly justifying further
matching attempts at tlic untciinu.
(3) When using a transceiver such as thé FT-77
which has protection for thé output tran-
sistors against high SWR, it can be seen that
thé matching action of thé FC-700 will ensure
that a 50-ohm load is presented to thé output
circuitry, thus ensuring full transmitter power.
(4) It may be useful for thé operator to record in
a notebook thé proper TUNE and LOAD
positions for a particular antenna for quick
référence. Alternatively, appropriate labels
may be fabricated and applied to thé FC-700
front panel showing thé proper positions of
thé TUNE and LOAD controls for a particular
frequency.
JOf
FC-70Q
CIRCUIT DIAGRAM
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