Re: [amsat-bb] 10GHz experiment question

["KC6UQH" <kc6uqh@xxxxxxx>]

William,
The antenna gain must be considered and testing in the near field can give
the results you describe. It is expected that the size of the echos will
dimimnish at the rate of distance raised to the 4th power. that can be
calculated. The second effect comes from the transmitted signal combined
with the incoming reflections that will provide for the sum and difference
of the two signals in your detector. The TX frequency and the delayed target
signal will mix as the side bands from the pulse will not frequency match
the target echos being displaced in time. Providing better shielding between
the Tx/Rx antennas will reduce this problen and a dc bias on the receiver
didoe will help make up for the lost R.F. bias. The third effect is mutipath
reflections in the near field that follow no predictable responce.
I did some experiments with directional K Band units, these are Dopler type
but have two detectors spaced 1/4 wave from each other. I found that by
combining both outputs I could obtain a consistant output that the amplitude
did reflect the the expected distance to 4th power return. Also by using two
audio phase shift networks, I could gain a large sensitivity increase for
objects traveling one direction.

Art, KC6UQH

----- Original Message -----
From: "William Leijenaar" <pe1rah@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, November 25, 2004 10:55 PM
Subject: [amsat-bb] 10GHz experiment question


> 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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