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From the amount of questions we have received on the web page, I saw a need for an article to
demystify the subject of duplexers and cavity filters. Duplexers and their cousin the Diplexer (note they
are not the same thing) are electrically simple filters. They allow us to transmit and receive on the same
antenna at the same time, reject unwanted signals and in the case of the Diplexer feedtwo different
signals to the same antenna.
Electrically a duplexer is a device using sharply tuned resonate circuits to isolate a transmitter from
a receiver. This allows both of them to operate on the same antenna at the same time without the
transmitter RF frying the receiver. Note that there must be a separation of the transmit and receive
frequency. This is called the “split.” On two metersthe split is 600 kHZ. On 70cm the split is a much
easier to do 5 Mhz.
Diplexers are ofen mistakenly called duplxers. The common appliication for a diplexer is to connect
a dual band mobile radio’s two antenna connections to a common feedline and antenna. Diplexers
are completely different and much simpler to build devices then a repeater duplexer. While duplexers use
narrow passbands andnotches to work their magic, a diplexer is a simple high and low pass filter connected
There are several different ways to build a duplexer. The hybrid ring, cavity notch and bandpass/
bandreject design are all found commercially. Each design has its advantages. The hybrid ring is rarely
seen in Amateur service. It uses a combination of both cavities and phasing lines. The ARRL Handbookhas a excellent explanation of how it works if your interested.
For many years the Handbook also had plans for a six cavity notch duplexer. The Handbook has
an excellent explanation of the theory behind this design. I have built these and they do work. I have also
found this design to be extremely difficult to tune, noisy and not all that stable. I do not recommend it.
Yes, I know some havebeen built, but there are better and easier to tune designs available now.
My all time favorite duplexer
design is the Wacom four cavity Band
Pass/Band Reject model with 8" cavities.
Their only flaw is their expense
(close to $900 at this writing). By using
larger high Q cavities and a better
design, Wacom was able to get the
performance of the six cavity notch
duplexer using only fourcavities.
Examination shows that two of
the cavities are in series with the transmitter
output, and two in series with
the receiver input. The two “halves”
are joined together with a “T” connector
and connected to the coax line to
the antenna (see fig. 1).
Each cavity has two functions. First it must pass the desired signal (the “bandpass” or pass band).
SEITS Duplexer Theory and TuningDave Metz WA0AUQ
South East Iowa Technical Society
Second, it must stop as much as possible of the undesired signal (the band reject or notch). In figure two
I show the ideal response curve of a typical transmitter cavity. Note that is passes almost all of the signal
on the transmit frequency of 145.37. And that it has a deep (30dB+) narrow notch at the receive
frequency 600 kHZ. lower.
Thetwo receiver
cavities are exactly the
same except that they
have their pass band
tuned to 144.77 the receive
frequency. Their
notch or Band Reject
frequency is on the
transmitter frequency of
145.37. Thus the transmitter
cavities keep the
wide band RF noise
from the transmitter out
of the receiver, and the
receiver cavities keep
the RF power from the
transmitter out of the
receiver.Note at this point that transmitters do not put out all of their energy on just the desired frequency!
They also put out wide band white noise (hiss) for a considerable distance from the center frequency. If
your transmitter (this is a rare problem) puts out excessive white noise, you may not be able to notch out
it all out. Some transmitters radiate less white noise then others. A few very...
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