Howard
Long G6LVB
Many
of us live in restricted locations. This article presents a method of
controlling preamps, antenna switching and polarization switching remotely
without the use of control cables.
Each
location has its own restrictions, but mine are problems of primarily of
external appearance. I live in a basement flat in a Grade II listed building
bang in the middle of the museum area of South Kensington, and I’m restricted
not only with antennas but also with feedlines.
On
the plus side I do have limited access to the roof as its primary function is a
fire escape. It’s also a mighty 120’ up, with marvellous views over Central
London. This means, however, long feedlines. I calculate the feedline length to
be about 180’ by the time it gets to the shack, so preamps are essential,
together with some fairly good coax.
I’m
not permitted to run coax down the front or sides of the building but luckily
there’s a light well which can be used. However it’s also someone’s patio,
so I have to make sure I keep on side with the neighbours. Keeping the number of
cables down to a minimum is essential.
Over
the past year I’ve gradually been rebuilding the shack from a lapse of my
licence of nearly ten years. My previous location was an absolute no hoper for
any operating, being a basement with no roof access.
In
order to control a plethora of relays and preamps, I looked into ways of
remotely controlling them from the shack. I’m aware of feeding power up the
coax to control a single preamp, and also of remote switch boxes. I purchased an
Ameritron RCS-4 four way remote coax switch to see if this would help, only to
find out it’s only rated at HF. It also wouldn’t let me control the preamps
at the same time. The Ameritron uses various rectification methods to control
its relays, so there’s four possible solutions (zero volts, positive, negative
and AC).
I
needed something which would control a lot more. So I butchered the RCS-4,
keeping few of the existing components. In the end, only the weather proof
remote enclosure, the console enclosure, the connectors, two inductors and two
capacitors remained.
The
console is connected to a PC which runs some software to allow switching of all
the preamps and relays.
In
the end, by strategically adding some triplexers, I had a complete HF, 6m, 4m,
2m and 70cm station with only one run of coax. I have worked all analogue
satellites except AO-10 with the help of the control box, a triband vertical and
a 10m dipole.
All
the components are easy to find (either in the shack or Maplin) barring the
inductors which could be found at rallies, or alternatively RS.
The
schematic of the console unit is shown in figure 1. L1 is any high current high
inductance type, say 2A and 10mH. None of the component values are very
specific. Indeed, the esoteric values (ie not the standard 1k, 10k etc) used
reflect the fact that I had plenty of other close enough values in the shack.
All transistors can be substituted for similar rating devices.
Figure 1: The Console Unit
The
RF (upto 50MHz) from the radio is fed into the control box at J3. To use higher
frequencies, use a duplexer after J2. Duplexers’ and Triplexers’ low pass
filters let through DC in my experience, although it’s very wise to check it
doesn’t present a DC short, or you’ll blow one of the components. I put in a
3A fuse after blowing the bridge rectifier three times because a manual coax
switch I was using switch on J2 presented a DC short to the unit. The L1 gave
off quite a bit of smoke too.
The
DC (about 35V smoothed, not regulated) is passed onto J2 via L1. L1 and C2 block
any RF back into the circuit. C3 couples RF onto J2. 35V was chosen because some
of the coax relays to control were 26V types.
The
raw RS-232 TXD signal at 300bps is passed into Q1 via R1. It’s inverted by Q2,
so that when the RS-232 is idling at MARK (-12V on J1), Q1 is off, Q2 and Q3 are
switched on and 35V is passed by Q3 onto J2. When the RS-232 goes to SPACE (+12V
on J1), Q1 is on and so Q2 and Q3 switch off, leading to zero volts on J2
(assuming there’s some load on it).
The
mast mounted remote box has two parts: the PSU (figure 2) and the PIC controller
(figure 3). Again all parts are easily obtainable, except for perhaps the
inductor which is similar specification to the inductor of the console unit.
The
remote box PSU splits the console input (J1) into RF (C1 into J2) and
the DC and signalling components (via L1). The signal is split off through a
potential divider (R1 and R2) and then into R2 and D1. D1 limits the signalling
voltage to 4.7V for subsequent input into the PIC controller.
The
DC component goes through a standard set of voltage regulators. The 24V provides
voltage for the 26V coax relays. The 12V provides power for the 12V coax relays
and the preamps. The 5V is only for the PIC controller. The 24V and 12V
regulators must be mounted on good heatsinks.
Figure 2: The Remote Box PSU
The
PIC controller takes the 300bps signalling component from the PSU. The crystal
is a 4.9152MHz unit. This is slightly out of spec for the PIC16F876 (U1). A 4MHz
crystal could also be used, but a change to the RS-232 software timing
parameters would be required. A PIC 16F876 was chosen because it has analogue
inputs available for future use with rotators. A 16F84 could equally well be
used in this application.
Figure 3: PIC Controller
U2,
the UCN5821A is a shift register with high current sink capability. These will
directly drive the relays. Q1 and Q2 provide 12V current source to the preamps.
Q1 and Q2 can be substituted for any medium current PNP switching transistor.
The
PIC takes an 8 bit RS-232 stream and writes it to the UCN5821A ports.
For expansion purposes, two more UCN5821A ports have been cascaded, although they are not in use currently.