Practical_Antenna_Handbook_0071639586

422 p a r t V I : a n t e n n a s f o r O t h e r F r e q u e n c i e s
I
80–meter dipole
feedline
SO-239
Coaxial jack
OWL
Transmitter
or
Amplifier
or
ATU output
Figure 18.8 Pseudo-T from an 80-m dipole and feedline.
to begin with. If you are using twin-lead or open-wire line, simply twist the indoor ends
of the two sides of the feedline together and stuff them back into the center (or “hot”)
receptacle of the SO-239. If you are using coaxial cable, you may find it best to build up
a small adapter consisting of an SO-239 socket and either a wire pigtail (to push into the
SO-239 socket) or a PL-259 plug attached via a short wire to the added SO-239. Some
important considerations, however:
• A really good RF ground system is an absolute necessity if this antenna is to be
effective. Especially if the radio equipment room is above the first floor of the
building, this can be a challenge. Without a ground system, not only will the
antenna fail to perform but the radio equipment will be “hot” with transmitted
RF and capable of causing very painful burns to your skin. At the very least, a
single l/4 counterpoise wire must be attached to the chassis at the point where
the new feedline is connected. Use insulated wire (except where it connects to
the chassis, of course!) and try to get it outdoors and close to earth in as short a
run as possible.
• One negative of this approach is that you have to disconnect and reconnect the
80-m feedline whenever switching from 80 to 160, or vice versa.
• This technique works only if neither side of the feedline is grounded between
the transmitter SO-239 and the antenna. If, for instance, all your coaxial feedlines
go through a remote switch or lightning arrestor connected to ground, your
signal will be shorted to ground before it gets to the antenna.
• As described in the preceding T-antenna paragraphs, radiation from the two
sides of the dipole mostly cancels at high elevation angles. However, because
the feedline in this example travels both horizontally and vertically on its way
to the dipole center, it generates both vertically and horizontally polarized ra-