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the band. If you use both phone and CW, it
might be well to make the design frequency
near center of the band. Likewise, the design
freq uency may be made in the CW segment
if you desire. Getting back to the swr
measurements, you will note that at the antenna,
or the base of the matching section, if
one is used, the reflected power is nil across
the greater portion of the design frequency,
making the true swr unit. Next remove the
swr meter from the driven element and reconnect
the transmission line. There is no need to
cut the transmission line to any particular
length-just use random length to suit your
purpose. Insert the swr meter at the transmitter
end next and again make the full swr meter
measurements across the band every 50
kHz as previously done with the swr meter at
the antenna. Again, plot the curve on graph
paper and note the similarity of the curves.
Due to a multitude of factors, the transmitter
end of the transmission line swr readings will
tend to be somewhat lower at the band end
extremes.
A dummy load may be substituted for the
antenna. Heath's "Cantenna" is a good and
inexpensive one. The Waters dummy load
power meter is excellent, but more expensive.
An rf (thermo-couple) ammeter in the transmission
line at the transmitter is worth its
weight in gold and much more preferable to
an in-line swr meter at the same point. Good
Western Electric and G.E. rf ammeters may
be purchased on the sur plus market for less
than five dollars. After tuning and loading
the transmitter to either the dummy load or
the antenna there will be no change in transmission
line current when switched from one
to the other and no retuning of the transmitter
should be necessary. This indicates as
nearly as possible with available equipment
whether your antenna is nonreactive or near
pure resistive. If a Bird Model 43 in-line
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wattmeter or equivalent is available. some interesting
overall efficiency measurements may
be made. Insert the wattmeter at the antenna
feed point and adjust the transmitter to a
given plate power input. On all further
measurements keep the transmitter adjusted
to the same power input. At frequencies near
the design frequency the antenna is nonreactive
and there is no reflected power and the
overall efficiency is indicated. For instance,
we adjust the transmitter to 500 watts de input,
the wattmeter indicates no reflected
power and forward power reads 300 watts
(with grounded grid amplifiers and 100 watts
of power output from the exciter, the final
amplifier should be adjusted to 360 watts
input as the output from the 100 watts from
the exciter should appear in the output to the
transmission line). The overall efficiency
from plate power input to actual power output
to the antenna would be 60%. This includes
normal transmission line losses, impedance
transfer from final amplifier, etc.
This percentage may seem high, but is quite
attainable with good linear amplifier design
and proper matching of transmission line to
a resonant antenna. Now tune the transmit·
ter to the same power, but to freq uencies at
which the antenna is nonresonant. Note the
difference in forward power and reflected
power. Subtract the reflected power from the
forward power and figure the efficiency
percentage. Make these same percentage
measurements every 50 kHz over the band as
was done with the swr curve. That does it!
There lies the reason why it is still preferred
to have an antenna with less frequency excursion
and higher efficiency than one of a compromise
nature.
The preceeding procedures are not intended
to be the "ultimate" but will afford the
"working ham" a less expensive and time-consuming
method of getting the most from his
multi-element quad antenna. Although the
reference is to a four element quad on a
thirty foot boom, the same approach may be
used with a two or six element quad. The
forward gain and FIB ration will be as good if
not better than the average when tuned in
this method.
Work has been going on for more than
three years to broad-band a multi-element
quad and yet retain a minimum reactive load
over the entire band, Success seems just around
the corner, but the last ten years working
with the multi-element quad has taken
its toll.
W4AZK
34 73 MAGAZINE