Powerful Medium Wave AM Broadcasting Transmitters
The pictures below show various installations at
Brookmans Park in the UK
BBC RADIO 5
MW AM TRANSMITTER (150 kW)
The above photograph shows the three 50 kW Doherty
AM transmitters used by BBC Radio 5 at the Brookmans Park site.
The output of these are combined into a Triplexer to achieve
150 kW of RF power. These are tube transmitters and each uses
2 x 4CX35000 tetrodes. The Anode voltage is 17000 volts dc at
high current. The audio is applied to the tube screen grids
for modulation. These Doherty transmitters are made by the Marconi
Company, model B6034. They were installed in 1978 and still
look brand new because they use valves instead of transistors.
BBC RADIO 5
The TIE bay is mainly made up of the Optimod 9200
processor, the AM6/18 (delay type) limiter (BBC Designed), the
HCD signal sources and the reserve programme feed (Radio 5 from
Astra Digital), main programme feed is from Broadcasting House
via BT Musicline. Another reserve is ISDN which is rarely used.
AM TRANSMITTER (50 kW)
The London Asian station Sunrise Radio uses 1 x
50 KW , Nautel XL60 transmitter and 1 x 12 kW Reserve transmitter,
these are modular and use Pulse Width Modulation into a RC circuit
to gain the AM signal. They use a Optimod 9200 processor with
an Optimod 9100 as backup. Kilostream is the main programme feed
with Astra Satellite backup.
TRANSMITTER (100 kW)
Here is a fairly up to date AM transmitter design
from the Harris company, recognized by many as the leaders in
transmitter design and manufacture. There are no tubes here because
TalkSport uses 2 x 50 kW Harris DX50 transmitters which are solid
state using 128 modules, 48 active at Carrier power. These two
transmitter outputs are combined into a Duplexer to realise a
full 100 kW. Talksport process their audio at the studio, while
a basic Talksport supplied limiter keeps the modulation in 100%
modulation level. An AM6/18 limiter is used as backup. Programme
feed is via BT Kilostream with ISDN backup.
The distribution board takes care of the power
supply to the transmitters at the site. In the event of a mains
electricity power failure, the distribution board provides auxiliary
power from the on site generator. The distribution board is split
into two, one half maintained and the other non-maintained, the
maintained half is supported by an 800 kW generator. In effect,
the transmitters and station are split into two. What this means
in transmitter terms is that, for example, Talksport (100kW) goes
to half power during a mains electricity failure, as one of the
50 kW transmitters is turned off, but the service remains on air
due to the other 50 kW transmitter running as normal. There is
a radiated power drop from 100 kW to 25 kW (6 dB) due to the diplexer,
this is due to the way the diplexer has to divert some power into
a load resistor in order to keep the on air transmitter correctly
loaded. Radio 5 looses transmitter C, this means the radiated
power goes from 150 kW to 66.66 kW due to the triplexer. Sunrise
radio however, fortunately remain at 50 kW during a power failure.
So the 800 kW generator ensures that all the services stay on
air, albeit at low power for some. In broadcasting it is more
important to remain on air than achieve full power, so for a few
hours the radio stations put up with reduced power while the electricity
supply is off.
TECHNICAL IN BRIEF
Carrier Power is the quiescent
output power without any modulation. In fact the carrier power
remains constant regardless of modulation. At full modulation,
two thirds of the power are in the carrier, the other third in
the information carrying sidebands. The 150 kW Radio 5 transmitter
will output 150 kW rms with no modulation and will output 225
kW rms at 100% modulation.
Peak Envelope Power (PEP)
is 4 x carrier power when a transmitter is at 100 % modulation
so the 150kW (carrier power) Radio 5 system is 600 kW PEP. That
is four times the carrier power!
Peak Mean Power (Pm) 1.5 x
Carrier power at 100 % modulation so Radio 5 is 225 kW.
Voltage going down the feeders at PEP
is:- (Remember the aerials are 50 ohms) The 150 kW
Radio 5 = 5477 volts rms. This is simply calculated from ohms
law. The peak current through the feeder cable therefore will
be 109.5 amps, at PEP.
AM and FM transmitters are
different! FM transmitters radiate at peak power all of the time
regardless of modulation, but AM transmitters only radiate at
peak power on envelope crests at 100% modulation.