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Seeking examples of Directional Coupler usage - RF Cafe Forums

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Below are all of the forum threads, including all the responses to the original posts.


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.


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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?

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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.


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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.

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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. http://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.


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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.



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