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Walter Snafu Post subject: Seeking examples of Directional Coupler
usage Posted: Wed Oct 28, 2009 4:37 pm Lieutenant Joined:
Wed Oct 28, 2009 4:16 pm Posts: 3 I'm looking for examples (hopefully
with explanation or theory) of Directional Coupler usage where a Transmitter
and Receiver are simultaneously using the same antenna at the same frequency.
I am aware of an instance where this is done, though I'm not
sure of the reasons. That is, the directional coupler comes between
the antenna on one side, and on the other side splits out into the transmitter
and the receiver-mixer. I presume it is done to reduce the amount of
Transmitter signal getting into the Receiver. Or perhaps it is done
to reduce the amount of RF mixer noise that gets back to the antenna.
I'm trying to verify the reasoning. Whether it is a good approach or
not. Benefits and disadvantages. Or whether there is a better means
to accomplish the same ends. If you could point me to suitable
webpages that would be especially helpful. Thanks for your help.
Top nubbage Post subject: Re: Seeking examples of
Directional Coupler usagePosted: Fri Nov 06, 2009 11:24 am General
Joined: Fri Feb 17, 2006 12:07 pm Posts: 218 Location:
London UK Hi Walter I was scratching my beard pondering this
one. They would have to be applications where the transmission/reception
loss is very much lower than say a traditional radio link. The reason
is the directional isolation between the transmit and receive arms of
any directional coupler on its own will only be about 30dB. Thus the
received level would have to be within say 20dB of the transmit level
(for a 10dB signal to noise ratio), which is a very small loss. At only
one wavelength away from the transmitter, the free-space path loss will
be 22dB due to spherical spreading, so the transmitter and receiver
must be very closely spaced, or confined within a cable or waveguide.
What prompted this question? _________________ At bottom,
life is all about Sucking in and blowing out. Top
Walter Snafu Post subject: Re: Seeking examples of Directional
Coupler usagePosted: Wed Nov 11, 2009 3:03 pm Lieutenant
Joined: Wed Oct 28, 2009 4:16 pm Posts: 3 Thanks Nubbage
for writing back. I've been asked to improve an existing design
that happens to use a directional coupler. The competitor products happen
to not use a directional coupler, and this hints that a directional
coupler is not essential. Also, my associates say the directional coupler
causes a 20 dB loss in the received signal. As a signal processing person,
I am very jealous of the received signal, and look dimly upon anything
that causes a loss to it. So I am looking suspiciously at the directional
coupler, wondering if an alternative design might bring an improvement.
In this particular application, the transmitter and receiver
are indeed very close together (on the same circuit board), and using
the same antenna at the same frequency. The transmitter is about 10
watts, CW (continuous wave). The received signal is a tiny, modulated
version at the same carrier frequency (with a wider bandwidth than the
CW transmitted signal). The directional coupler is arranged
as follows. The transmitter signal goes through the directional
coupler, and most of the signal goes to the antenna, and outward. A
tiny portion of the transmitter signal is coupled (by the directional
coupler) to a terminating load (a dummy load, with a matched impedance).
Most of the transmitter signal is isolated from the receiver (which
is a good thing, and perhaps the main reason for the use of the directional
coupler ???). The transmitted signal leaves the antenna, and
returns containing information we want. This signal received by the
antenna goes (through the directional coupler) mostly to the transmitter
where it is dissipated and lost (unfortunately!!!). And a tiny portion
of the received signal is coupled into the receiver. Again, most of
the received signal is lost. (NOT GOOD!) I am not very familiar
with directional couplers, but so far I am not fond of this design.
Perhaps I am missing something and do not understand. Thus my questions
here. I am contemplating eliminating the directional coupler,
and instead using a voltage follower (transistor or op-amp) to measure
and isolate the voltage from the antenna. The goal is to measure the
antenna voltage without screwing-up the impedance match between the
transmitter and antenna. Thus a voltage follower (high input impedance,
and low output impedance). There are likely other designs that would
have a similar effect (such as a transformer?). This measured voltage
will contain a huge transmitter signal, plus the full-strength of the
received signal (here undiminished by a directional coupler) -- the
received signal will be tiny relative to the transmitter signal but
will be about 20dB higher than signal derived from the directional coupler
design. I would then use various techniques to eliminate the huge transmitter
signal and amplify the received signal. Etc. Please comment
on the accuracy and wisdom of this. Thanks for any help you may provide.
Top nubbage Post subject: Re: Seeking examples of
Directional Coupler usagePosted: Thu Nov 12, 2009 6:44 am General
Joined: Fri Feb 17, 2006 12:07 pm Posts: 218 Location:
London UK Hi walter You are correct in saying the receive path
suffers the 20dB loss (or whatever the coupling is), but in the short-range
industrial application I had inferred, the signal is usually large,
as the path loss is only around 20dB. What is really needed is a
circulator, and the type will depend on the radio frequency involved.
This is a non-reciprocal device that passes the transmit power from
one port to the other, and any return power from the antenna on that
port is "circulated" around to a third port. At HF this can be implemented
in a transistor arrangement, but at UHF thru microwave frequencies it
would need ferrite circulator devices. The insertion loss both for transmit
and receive is about 0.2dB but the isolation is about 35dB or better.
_________________ At bottom, life is all about Sucking in
and blowing out. Top Walter Snafu Post subject:
Re: Seeking examples of Directional Coupler usagePosted: Thu Nov 12,
2009 3:03 pm Lieutenant Joined: Wed Oct 28, 2009 4:16
pm Posts: 3 Nubbage, Thank you yet again for your helpful advice.
Your idea, a circulator (or isolator), is a great idea for my application.
Though I haven't been able to find one that operates near 14 MHz (as
in my application). That is too low a frequency for the usual circulator
designs. I found a design for a low frequency circulator, using
three op-amps arranged in a circular fashion. https://www.wenzel.com/pdffiles1/pdfs/RFDesign3.pdf
I interpret it as a "circulator" that doubles as the output stage of
the transmitter -- since an op-amp has to actively deliver the full
transmit power. I'm looking at modifying this design for my application.
Referring to the above-linked pdf file -- The transmit signal
comes in on Port 1; the antenna is connected to Port 2; and the output
to the receiver is Port 3. So the op-amp shown below Port 3 is unneeded
in my application. Moreover, the op-amp outputing to Port 1 is really
just the output stage of the transmitter. Sooo, in my application, the
op-amp outputting to Port 2 is the only op-amp needed. I'll take a look
at this. Thanks again Nubbage. Top nubbage
Post subject: Re: Seeking examples of Directional Coupler usagePosted:
Fri Nov 13, 2009 7:12 am General Joined: Fri Feb
17, 2006 12:07 pm Posts: 218 Location: London UK Hi Walter
That is the circuit I was referring to as being suitable for HF, but
I have also seen a UHF transistor version that might operate well up
to 50MHz. Caution about modifying this OpAmp circuit: note the wierd
resistor values? I have a feeling if you remove opamps or change
values the isolation will get shot. I will try to find the 3x UHF
transistor version, but it will be a couple of weeks before I am back
at base. _________________ At bottom, life is all about
Sucking in and blowing out.
Posted 11/12/2012
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