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Checkout and Adjustment
Adjustments
will
have to be made before trying to transmit with this antenna system. Installations
vary considerably and there are no ‘set’ measurements that will work for all the variables in the
installation. The dimensions and coil settings that were used during assembly are somewhat close
and may produce reasonably low VSWR readings over the entire 10, 15, 20 and 30 meter bands and
at least 250 kHz of the 40 meter band. Bandwidth on 80/75 meters should be at least 30 kHz for
VSWR of 2:1 or less at the low end of the band and may be as much as 100 kHz at the high end of
the band,
depending on the efficiency of the ground system used,
greater bandwidth being
associated with lossy ground systems. It should be remembered that on those bands where the
physical height of a vertical antenna is less than 1/4-wavelength, the earth (or the resonant radial
system in above-ground installations) will have a good deal to do with VSWR and antenna tuning,
bandwidth and overall performance.
Low VSWR by itself does not mean that a vertical antenna is operating efficiently, and if low
VSWR is obtained with no more than the usual
quick and dirty
ground connection, it most likely
means the opposite. In general, poor operation or improper tuning of vertical antennas can usually
be attributed to inadequate (or even reactive) ground systems or to other vertical conductors in the
vicinity of the antenna. For these reasons it is suggested that the antenna be placed as much in the
clear as possible and used with the best ground system that conditions permit. For a more complete
discussion of the interrelationships between vertical antenna efficiency, bandwidth, VSWR, etc., a
standard text such as the
A.R.R.L. Antenna Book
is recommended.
For adjustment purposes a simple VSWR indicator may be used. More accurate measurements may
be made at the antenna (i.e., at the junction of the coax 75 ohm matching (R) and the main
transmission line) than at the input end of the line, but the tuning conditions that exist at the
transmitter will usually be of greater interest in that one's principal concern will be to couple power
from the transmitter into the transmission line.
1. Check VSWR on 10 meters. To raise the resonant frequency loosen the small clamp over the
slotted end of tube (I) and slide tube (J) farther into tube (I). To lower the frequency, slide tube
(J) farther out of tube (I) and retighten the small clamp. A length change of 3 inches should
move the VSWR curve approximately 200 kHz.
NOTE: you can measure then adjust 15 and 10 meters at the same time since they don’t
interact like other band adjustments. This may help save some time and effort when
tuning. The 12M coil MUST be compressed as much as possible upon initial tuning. If
not, it is highly probable that 12M will actually be tuned into 10M throwing the ability to
tune 10M at all.
2. Check VSWR on 15 meters. The VSWR curve may be shifted upward or downward by
changing the length of the stranded wire between wire clamp 0.500" 15M with wire (L) and
wire clamp 0.875" 15M with insulator (K).
To raise the frequency, simply shorten the wire by wrapping a longer
tail
back on itself and
sliding the lower clamp upward to maintain tension. To lower frequency, feed more of the
tail
back through the hole in the insulator to increase the length of the wire between wire clamp
