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Balanced antenna systems should be fed through an antenna coupler sothat neither leg of the line
nor the antenna will be at ground potential. Normally the output of the transmitter ie unbalanced
since one side of the output circuit is grounded.
When loading to elther a balanced or an unbalanced system, the maximum loading point of 260
milliamperes in the final stage should not be exceeded.
In many instances, the transmitter will
Joad to a higher tevel, but the harmonic suppression of the pi network is better if the output
coupling capacitor is not reduced to absolute minimum value. Then too, the extra 10 or 15 watts
which would be gained by maximum loading would not make any appreciable difference at the
receiving end of the tranamission.
END-FED HERTZ AND MARCONI ANTENNAS
This type of antenna consists merely of a single wire from one-fourth wave length long to any
even multiple thereof. One end of the wire is coupled to the transmitter and the other end aup-
ported in space. [If this antenna is operated against ground, it is known as a Marconi antenna,
while if the length is one-half wave length or more, it is known as a Hertz antenna. The great-
est disadvantage of feeding a single wire is the necessity of bringing part of the radiating element
into the radio room where its proximity to nearby objects increases losses. The greatest ad~
vantage of such an antenna is the simplicity with which it may be constructed and its compact
size where space is at a premium. Providing the reactance of such antennas is not excessive,
they may be loaded satisfactorily with the pi network output circuit of the DX-100.
The length of such antennas should be calculated from the information given in the Radio Hand-
book or the ARRL Antenna Manual. Random lengths may also be used effectively except for some
critical antenna lengths where the antenna impedance becomes too high or too low to be matched
zatisfactorily at the output of the transmitter. When such a situation is encountered it can usu-
ally be recognized by the action of the loading controls on the transmitter. Should it be found
that increasing the loading (reducing the output coupling capacitor value) does not raise the final
amplifier plate current reading on the meter and yet the final amplifier has not become fully
loaded, it is very likely that the antenna being used has a higher than normal reactance at the
frequency of operation, resulting in the transmitter looking into a higher overall impedance.
If it is found that a change in coupling capacitor value has little effect on the final amplifier load-
'ing and does not affect the final amplifier tuning in the normal fashion, the antenna ig use is
probably presenting lower than normal impedance to the transmitter.
When the antenna impedance is too high or too low for satisfactory loading, the situation can
usually be remedied by changing the length of the feedline to the antenna, the length of the an-
tenna itself or the point at which the feedline is connected to the antenna. Small changes, not to
exceed one-quarter wavé length should be effective in correcting the impedance
mismatch.
Another cure for such difficulty would be to introduce inductance or capacitance in series with
the antenna and feeder to cancelout the undesirable reactance inthe antenna. Of course, a more
elaborate method would be the use of an antenna tuner if some element in the situation made it
impractical to employ the methods outlined above.
BALANCED ANTENNA FEEDERS AND SYSTEMS
In the average station it will be found expedient to have the radiating portion of the antenna some
distance from the transmitter. This statement assumes that the amateur will have his antenna
'up high and clear of nearby objects, whereas the actual transmitter may be in the basement or
any other room in the home. In such a case, some form of transmission line must be used to
efficiently connect the transmitter to the antenna. In many instances, the transmission line is
of the "balanced" type in which neither leg of the line is grounded. Such transmission lines may
be either tuned or untuned, but in either case, a balanced line miust be fed through an antenna
coupier or "balun" coils. 'The coupler also provides the means for matching the impedance of
the antenna, which will vary depending upon the type of antenna employed. A typical antenna
coupler circuit is shown in Figure 24 on Page 61.
Page 60
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