chapter  5
6 HF antennas and coupling units
Pages 3

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.