What are the functions of each of these?

Are they used on TX or RX circuits?

I know a cell site uses a duplexer so that it
has an extra RX antenna for diversity. How
does this work? Is the duplexer on the antenna
that transmit/receive combo antenna notching
out the TX frequency so the receiver doesn't get
overloaded by it?

"crb" wrote in message ...
What are the functions of each of these?

Are they used on TX or RX circuits?

I know a cell site uses a duplexer so that it
has an extra RX antenna for diversity. How
does this work? Is the duplexer on the antenna
that transmit/receive combo antenna notching
out the TX frequency so the receiver doesn't get
overloaded by it?


Crb,



These are basically filters. The terminology varies sometimes
for "diplexer" and "duplexer", and they are both used to describe the same
or different things. I'll give the way they are used in my field - two-way
radio and cell phone systems.




Duplexer:

When we transmit and receive at the same time, this is called duplex
operation.



A duplexer is designed to allow a transmitter and receiver (operating at
slightly differing frequencies) to use the one antenna at the same time.
There are several things that must be taken care of in this situation.



1 - The transmitter energy must be kept away from the receiver or it will
burn things up.



2 - The transmitter signal can still cause trouble if it is not lowered a
lot more than this "blow-up" level. The undesirable effects are commonly
intermodulation (mixing of the transmit signal with near-by transmitters
which then causes signals which interferes with other receivers) and
spurious responses (mixing of the transmitter signal with more distant
transmitters which cause signals that interfere with this receiver).



3 - Transmitters also put out noise as you move away from the carrier and
this must be reduced to where it does not cover up the desired signal in the
receiver.



As you probably understand, the difference between the
transmitted power and the received signal strength is very great and
typically around 170 to 190 dB.





To get #1 and #2, a filter is placed in the receive line which passes the
received frequency with little loss, but attenuates the transmitter power to
a level which keeps the IM products to an acceptable level. To get #3, a
filter is placed in the transmitter line which passes the transmit frequency
with little loss, but attenuates the noise (and any other spurious which may
be there) which the transmitter is producing on the receive frequency.
These filters can be band-pass or band-reject (notch) filters.







Diplexer / Combiner:

These two terms usually refer to devices that allow two
transmitters to use one antenna at the same time. However, the term
"diplexer" and "duplexer" are used differently in some circles. There are
several ways to do this.





The term "Combiner" is most common in cellular telephone systems, the term
"diplexer" is more common in the military.



Combiner:

One method (used in cellular transmitters) uses narrow band filters
(commonly called cavities). One filter is on the output of each transmitter
and, obviously tuned to that transmitter's frequency. Then, to allow these
to be all connected at one point so one antenna can be used, transmission
lines are used in a special way. These lines are made special lengths such
that the following happens.



We will consider only two transmitters on two different frequencies here "A"
and "B".



We look at the junction point where the two filter lines connect to go to
the antenna.



At frequency "A", the transmission line from filter "B", transforms (rotates
around the Smith chart) the impedance of filter "B" such that it is a very
high impedance at the junction point. This high impedance across the 50 ohm
transmission line accepts little power and therefore disturbs things very
little. This means that the power from transmitter "A" only goes to the
antenna and not back into transmitter "B".



For transmitter "B" just switch the A's and B's in the above.



The logical extension of this is for multi transmitter systems where the
transmission lines are made such that all filter / transmission line
combinations, other than the one frequency going through, appear as a high
impedance at the junction.



Systems use this technique to connect as many as 20 transmitters to one
antenna and the frequencies are quite close together - in the same band.



This is a narrow band system and must be re-tuned if the transmit
frequencies change.



The further apart the frequencies are, the easier it is to make this
device. With widely differing frequencies such as 144 MHz and 450 MHz, this
can be done with simple low pass and high pass filters.



Diplexer:



Again I remind you that some use the term "diplexer" and some
use the term "duplexer".



I believe diplexers are commonly band-pass filters. Using band-pass
filters, we can allow the transmitters to move around a bit. I have seen
things called diplexers used where the transmitters are in different bands
(like 150 MHz and 450 MHz.). There must be the same consideration that I
describe above for the impedance at the junction end of the transmission
lines coming from the filters where they connect to the common junction
point.



Finally, because these are filters, they can be used in receive as well as
transmit lines. With a two way radio that operates in two bands but only
uses one connector - say 144 MHz and 450 MHz. you can use these devices to
connect two antennas to the radio, one antenna for 144 MHz and one for 450
Hz.



You can also connect two radios, one operating at 144 MHz and one at 450 MHz
to an antenna that operates on either band.



You can also connect these two radios to a common feed line with a diplexer
and then use another diplexer (facing the "other way") to feed two separate
antennas for the two frequencies.







In the diversity situation you mention, the duplexer saves one antenna that
would be needed for transmit by using one of the receive antennas for both
receiving and transmitting (duplex). Some systems don't use a duplexer here
and do have three antennas. Two for dual diversity receive and one for
transmit. You may be able to see this as three vertical bars (antennas) on
each side of the triangular top of some cell towers.



Hope this helps.



73, Steve, K,9.D;C'I

Steve Nosko wrote:

"crb" wrote in message ...
What are the functions of each of these?

Are they used on TX or RX circuits?

I know a cell site uses a duplexer so that it
has an extra RX antenna for diversity. How
does this work? Is the duplexer on the antenna
that transmit/receive combo antenna notching
out the TX frequency so the receiver doesn't get
overloaded by it?

Crb,

These are basically filters. The terminology varies sometimes
for "diplexer" and "duplexer", and they are both used to describe the same
or different things. I'll give the way they are used in my field - two-way
radio and cell phone systems.

(Snipped for Brevity)

Hope this helps.

73, Steve, K,9.D;C'I

Nice answer Steve! I learned a few things myself!

Irv VE6BP
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Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

"Irv Finkleman" wrote in message
...
Steve Nosko wrote:

"crb" wrote in message
...
What are the functions of each of these?
[snippitty snip]

Crb,

These are basically filters. The terminology varies
sometimes
for "diplexer" and "duplexer", and they are both used to describe the
same
or different things. I'll give the way they are used in my field -
two-way
radio and cell phone systems. (Snipped for Brevity)
73, Steve, K,9.D;C'I

Nice answer Steve! I learned a few things myself!

Irv VE6BP


Thanks Irv. Just what I hoped to accomplish. Because my brain is full
and I am unable to get anything else in there, I am trying to get some empty
space and at the same time pass along some of the knowledge I have gained
over the many enjoyable years in Electronics. I do my best to get things
down in a simple, understandable way.
73, Steve, K'9;D.C,I

Anon wrote:
"What are the functions of each of these?"

My dictionary says:
"Diplexer - "A coupling unit which allows more than one transmitter to
operate together on one antenna."

The first AM broadcast plant I worked in did this with stations on 950
KHz and 1320 KHz. It was faultless.

Duplexer - "A radar device which, by using the transmitted pulse,
automatically switches the antenna from receive to transmit at the
proper timwee"

I`ve seen gas tubes used for this in radar sets in the navy and company
owned boats I worked on. Again, it worked like a charm.

Combiner - "A circuit for mixing video, trigger, and scan data from the
synchronizer for the modulation of the link."

Obviously definitions depend on applications. For example, I`m familiar
with combiners used to select the best signal from multiples in
diversity reception.

Best regards, Richard Harrison, KB5WZI