ABSTRACT
From Figures 5.14 and 5.15 it should be
obvious that the HF antenna, whilst well matched
at 21 MHz, would be severely mismatched to a
Figure 5.14 Variation of SWR with frequency for an HF notch antenna (note the logarithmic scale used for SWR)
Figure 5.15 Variation of SWR with frequency for a VHF quarter-wave blade antenna (note the linear scale used for SWR)
Figure 5.16 Variation of SWR with frequency for an HF notch antenna fitted with an antenna coupling/tuning unit
conventional 50 Ω feeder/transmitter at most
other HF frequencies. Because of this, and
because the notch antenna is usually voltage fed,
it is necessary to use an HF coupling/tuning unit
83 HF communications
Figure 5.18 Interior view of an HF antenna coupler showing the roller coaster inductor (top) and vacuum variable capacitor (bottom). The high-voltage antenna connector is shown in the extreme right
Figure 5.19 SWR bridge circuit incorporated in the HF antenna coupler. The output from the SWR bridge provides the error signal input to the automatic feedback control system
between the HF radio feeder and the notch
antenna. This unit is mounted in close proximity
to the antenna, usually close to the top of the
vertical stabiliser (see Figure 5.12). Figure 5.16
shows the effect of using a coupling/tuning unit
on the SWR-frequency characteristic of the same
notch antenna that was used in Figure 5.14. Note
how the SWR has been reduced to less than 2:1
for most (if not all) of the HF range.
