This remote controlled filter and LNA is used with a simple vertical antenna by a (receive-only) multiplier spotting station. One of six band-pass filters is selected by applying power in an encoded combination to 3 cores of the control cable.
A high intercept-point low-noise amplifier (LNA) follows the filters. The LNA is protected against overload by PIN diodes.
The LNA can be also be bypassed to reduce signal levels into the spotting receiver if necessary.
A switch on the control box bypasses the LNA. It works by reducing the supply from 24V to 14V. A voltage sensor in the remote filter box power supply detects this change and switches off and bypasses the LNA.
Each band-pass filter has a L-network at its input providing a match to the vertical.
Click on a thumbnail below to see a larger picture
The 20ft vertical with remote filter box at its base. The vertical is 150m from the transmitting station, shown here operating the 15m SSB station for GB5HQ in 2003. The vertical has four 20ft radials at ground level.
The operator control box.
Inside the operator control box.
Inside the remote filter box.
Close-up of the LNA. The design is a high intercept-point, common-base, lossless-feedback amplifier, followed by common-emitter stage. Both transistors are biassed at 20mA.
Three 50m-long control and RG58 feeder cables. The vertical can be located 50, 100 or 150m away from the transmit antennas.
The frequency response of the six contest-band filters are shown below. The red traces are with the LNA on, the blue traces are with the LNA bypassed. These measurements were made with a N2PK VNA using Exeter software.
160m filter with & without LNA.
80m filter with & without LNA.
40m filter with & without LNA.
20m filter with & without LNA.
15m filter with & without LNA.
10m filter with & without LNA.
Coax cable measurements
The loss of four 50m RG58 cables used with the remote filter bank are recorded below.
The black trace is the through calibration reference.
Cables 1 & 2 (brown & red) are hollow air-spaced dielectric RG58.
Cables 3 & 4 (orange & yellow) are foam dielectric RG58U
While set up for measuring cable loss, the 20 & 15m yagi feeders were also measured.