Throughout the years that radio broadcasting has existed there has
not been a strong need to operate more than one AM transmitter into a
single antenna. However, during the past several years vertical real
estate has become increasingly valuable due to economic factors
affecting large tracts of open land. The frantic efforts of the
anti-tower consortium to stop the erection of as many towers as
possible, together with FAA restrictions, have also added to the value
of existing transmitting antennas and their sites. Consequently, the
idea of feeding two or more transmitters into a single tower is
becoming increasingly attractive and more stations are finding it
necessary to diplex to stay viable, or even find a suitable site.
Figure 1. Block diagram of a common,
diplexed non-directional installation.
The roadblocks put in the way of new tower construction seem to be
increasing exponentially. Businesses that fall under the heading of
public utilities can usually force acceptance of a new tower in places
where they are banned. Unfortunately, despite the FCC's publicized
“law” that is supposed to supersede local zoning
regulations, the Commission has given little assistance to broadcasters
coming up against zoning roadblocks. New permittees have to find a
willing competitor who will rent radiating space on an existing
Diplexing FM transmitters, or even triplexing is not unusual today.
The same precautions have to be taken as for diplexing AM transmitters
but to a greater extent. The major differences are that while almost
any radiator will perform for AM, sometimes with minor modifications,
FM antennas have to be specially broadbanded to cover all the desired
transmitter frequencies. FM costs are usually higher unless the
originally installed antenna was designed for this purpose. Another
major consideration is that a distance of as little as one foot can be
critical in FM, and usually is far less critical in AM operations.
For AM non-directional operations, radiating two frequencies from a
single antenna is easy to do. In the case of directional antenna
systems dual frequency operation is possible, just more complex.
Where to begin
First confirm that the desired pattern for the new transmitter can
be obtained using the existing towers, orientation and layout. The next
step is similar to that for a single antenna but considerably more
complex. After the directional array is designed it becomes a matter of
designing isolating filters that provide clean signal passage to the
desired signals, and effective acceptance networks to bypass the
undesired signals to ground.
Figure 2. A modified diplexer. The added
networks and antenna matcher overcome undesired electrical
Sometimes antenna efficiency affects diplexing. If the existing
station operates on a higher frequency than the new one, the existing
antenna may not provide the required radiation efficiency on the lower
frequency. This was a problem about 40 years ago when CBS allowed WNBC
(now WFAN) on the new WCBS tower. The site is on High Island and
surrounded by seawater, which gives excellent ground conductivity and
coverage of New York City.
WCBS operates on 880kHz and WFAN is on 660kHz. Both are rated at
50kW. The shorter WCBS antenna did not provide the required FCC
radiation efficiency on 660kHz. It took considerable work to bring the
lower frequency efficiency up to the required level. In some cases,
folded unipole techniques or some form of top loading may be required
to increase the electrical length of the antenna for the lower
frequency station, while not greatly increasing the radiation
efficiency on the higher frequency unless such increased radiation can
be approved with a new CP. Consider all the critical electrical length
factors that are involved in constructing an antenna designed for
multifrequency operation. The possibility of significant sum or
difference frequencies being produced must be borne in mind in the
Figure 1 diagrams diplexed nondirectional operation. The antenna
circuit for each transmitter is similar. Between each ATU and the
antenna is an accept/reject filter and an acceptance filter to bypass
to ground the other transmitter's signal. This is the bare bones
circuit and would probably work well but it does not provide the
desired attenuation of the other station's signal. Audio quality may
suffer from the effects of Hi Q circuits and bandwidth restrictions. A
few modifications to the basic circuit will provide the best operating
conditions. Sometimes it is possible to incorporate components of an
ATU into various filters. However, this is not a good idea because it
can make future adjustments difficult and can result in performance
Figure 2 shows a diplex system modified to avoid problems of
bandwidth restriction, impedance matching and excessive Q conditions.
The higher-frequency side will be similar except that the frequencies
of the accept/reject filters will be reversed.
The design of the main filter is important, but will be saved for a
later column. The bandwidth of high and low frequencies will be
affected by the design of this filter. Sometimes auxiliary filters are
used and their presence affects the final main filter design.
Component stress has to be taken into account, especially 100
percent modulation current. Unsuspected high circulating currents can
develop as well as unexpected high RF voltages.
Sometimes an antenna resonator is required when using an auxiliary
trap. This resonator is actually a reactive device that may be positive
or negative, depending on the impedance at the input to the main
About 60dB attenuation between the transmitters is adequate. The
amount of attenuation generally determines whether an auxiliary filter
is required. If it's less than 60dB the auxiliary filter will be
needed. Anything less than 15 percent frequency separation will require
an auxiliary filter. With more than 25 percent frequency separation an
auxiliary filter will not be required.
E-mail Battison at email@example.com.