Yes, it's quite possible to measure the return loss with a directional
coupler. Yes, if you're dealing with a microwatt or so, it's quite
possible to detect it with a simple diode detector. (Actually not
terribly difficult to get down below 10 nanowatts.) Yes, it's even
reasonable to make your own directional coupler. For example, if you
download and install the free "RFSim99", you'll find in the pulldown
menu under "toolscomponentcoupler" a little window that will show
you the dimensions needed in microstrip or stripline (I believe the
coupling shown assumes 1/4 wavelength of line...), or three other ways
to do it that are more practical for lower frequencies. If you were
really careful, you might even be able to mechanically cut a PC board
to make the microstrip version.

Cheers,
Tom


Allan Butler wrote in message news:lCSEb.601435$Tr4.1574972@attbi_s03...
Ok here is my problem.

I would like to build an antenna for the S band. That is near 2500 MHz for
those of us that are not knowing of the letter to frequency correlation. :-)

The difficulty is in testing the antenna to make certain that it will accept
antenna power and not cause the transmitter grief while it is doing it. I
know that I can't go to Radio Shack and buy their SWR bridge to do this
testing. I have a friend that could help me in the 1200 MHz range but he
does not have a spectrum analyzer that will go to 2500 MHz.

He suggested the possibility of taking a signal generator and feeding the
antenna through a directional coupler. The return signal from the antenna
would then be sent to the spectrum analyzer so that it could be monitored
for a relative level and as the antenna was tuned closer to the proper
impedance and the return loss went up the spectrum analyzer would read a
lower and lower level.

Unfortunately we lack a directional coupler and a spectrum analyzer for
these frequencies.

I just got an idea. We still need to come up with a directional coupler
but if we can get a crystal detector that works up there we can monitor
the relative power with an oscilloscope or a DVM.

I have a few more people in the area that I can ask about the directional
coupler.

Does this look like a possible solution to anyone here? Or should I get
a piece of waveguide suitable for this frequency and a crystal detector
and calculate the VSWR by making the detector slide from one end of the
waveguide to the other. I remember in tech school doing this at microwave
frequencies to measure VSWR.

Any suggestions or ideas will be investigated to make this work and will
be greatly appreciated.

Thanks in advance.

Al Butler
ka0ies