Coll 35/31 'Bahrain: application for W/T amateur transmitting licence for employee of Bahrain Petroleum Company' [‎64v] (128/250)

4G
A GUIDE TO AMATEUR RADIO
V
A popular method is to tap the aerial or feeder
directly into the anode circuit, but this is very poor
practice. Apart from the danger of having H.T.
voltage in the aerial, it is the surest way to trouble,
as it allows everything in the transmitter to radiate,
thereby causing interference in local receivers.
It also upsets the earthing arrangements, often
allowing R.F. energy to flow back into the mains.
A point to note is that the presence of the aerial
circuit may disturb the neutralising adjustment,
and it is advisable to recheck it after the aerial is
set up to its coupling. This effect does not occur
with link coupling.
Power Supplies
The design of power supplies for the anodes is
considered in another chapter. For a small two-
stage transmitter a single H.T. supply can be used,
but with more stages it is usual to have at least
two, one for the driver unit and the other for the
P.A. stage. If a self-excited non-crystal oscillator
is the frequency source, its power supplies should
be well separated from a H.F. point of view, and
should be very pure, heavy low-frequency de
coupling or smoothing being used. Remember
that keying or modulation of the final stages is
liable to react seriously on the voltage of the H.T.
supplies unless the power pack is separate, or has
heavy ballast.
Grid Bias
There are many ways of obtaining the bias
for the various stages. The first is of course from
batteries, but as several hundred volts may be
required this is not very convenient or economical.
Another method is to use grid-leaks. This method
is quite satisfactory for the early stages where
the valves are not liable to come to much harm if
the bias is removed, but in the final stages it is
important to note that if the drive stops so does
the bias, and if the valve is operating with high
voltage applied it will be damaged. A third method
is automatic bias, obtained from the plate current
passing through a cathode resistor, which is shunted
for R.F., as for'example, R 5 , C 13 in Fig. 13.
Automatic bias is not easy to arrange unless a
separately heated cathode is used, but this is often
possible in the drive unit. In the P.A. stage, where
bias to twice cut-off is needed, it results in much
of the H.T. being used for bias, and makes adjust
ment difficult, since when the plate current drops
to zero there is no bias. It is, however, always safe.
The resistor should be of sufficient wattage to take
the power dissipated in it.
Fourthly, a power pack may be used for bias.
This should have a heavy load resistor across it,
liberally tapped, and each tap or lead should have
a IpF condenser between it and earth. The steady
load should be at least three times the total grid
current.
Probably the best arrangement is to use sufficient
battery or power bias to give cutoff, together with
a 10,000 to 20,000 ohm grid leak in each grid return.
This will give the extra bias required under
full drive conditions. In switching on the trans
mitter it is as well to see that the bias voltage comes
up before the H.T. All the various methods of bias
supply are well illustrated in the diagrams chosen,
as well as a good selection of cathode connections,
which are, in general, usable to choice.
A transmitter using the circuit of Fig. 13. This is a rear
view and shows a construction using an Eddystone rack
frame. The short side aerial Is for the 'phone monitor.