Re: [amsat-bb] 10GHz experiment question

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

William,

What you are describing sounds like a variation of what is called backfire
antenna or "boomerang".  The original boomerang was a diode detector
usually mounted in a short section of w/g or horn and modulated with an ac
signal.  The early days of amateur mw it was used with gunnplexers to check
for signal output irrespective of mw freq.  What happened was the mw signal
mixed with the low freq signal (say 30-MHz) to produce a reflected AM
signal with F+30 and F-30 sidebands.

An interesting artifact was that the signal received back at the Gunnplexer
mixer would go thru peaks and nulls as the distance changed between them.
This was caused by the varying phase between the two sideband frequencies.
Very accurate distance measuring can be done using a boomerang.  Also small
shifts in the mw freq. can be seen in the change of the standing wave
distance allowing accurate freq adjustments to be made.

I suspect that you will see a similar effect if you face the detector with
your gunn unit and vary distance.  My theory is the pulse is chopping the
mw signal and impressing a pulse modulation.  It is the sidebands that are
setting up standing waves at differing phases that are causing peaks and
nulls to be observed.  

I do not have the formula that one can use to predict the peaks/nulls but
this has been a old favorite method of the San Bernardino Microwave Soc.
http://www.ham-radio.com/sbms/

Have fun with your experiments!
73's Ed - AL7EB

At 07:55 AM 11/26/2004 +0100, William Leijenaar wrote:
>Hi AMSATs,
>
>Yesterday after work I have done some new tests again with my X-band test 
>setup :o)
>I managed to get a pulsed 10GHz output and I wanted to do some radar tests 
>by measuring the pulse phase difference between the tx and rx :)
>
>I take the 10GHz gunn-diode transmitter and next to it I placed the diode 
>detector (both waveguides) It was working great, I could detect reflections 
>very well, but I find out something I am not sure about it. What I find out 
>is that the output amplitude changes with the distance to the object (object 
>that reflects the tx energy to the rx input).
>It changes very rapidly...
>
>I suspect it is a mixture of several reflections what gives nulls and 
>maximums, but it also occurs at very small distances when the object is just 
>a few mm away from the in/output. At a few mm there are not many reflections 
>from other objects what makes me believe less in this theory.
>
>The gunn-diode is followed by a directional coupler, so it is not the load 
>that changes the gunn-diode behaviour or matching. Behind the directional 
>coupler I placed a pin diode switch to pulse the output, maybe this is the 
>one that is effected ?
>The detector I use is a simple 1N23 in a waveguide, where I read the DC 
>output on an oscilloscope.
>
>Maybe its somewhere it the books, but I couldn't find it out yet...
>
>73 de PE1RAH,
>William
>---
>
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