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Re: R: 3m dish applications




At 08:25 AM 4/5/2004 -0400, Robert Bruninga wrote:
>I would like to do 2.4 GHz microwave oven CW for EME.
>Yes, I konw that the signal from an oven is very poor and
>very broad and full of 60 Hz noise.  This  makes it not receive
>well on a narrowband receiver.
>
>BUT!  Why cannot someone with a DSP soundcard, invent
>a wideband detector for 60 Hz microwave oven.  Taking the
>broadband  CW "signal" energy out of a broadband receiver
>with DSP should give "processing gain" shouldnt it?


Hi Bob,

A typical oven magnetron has an output of around 800 watts
(+59 dBm) and a bandwidth of over 2 MHz. If you put the magnetron
right at the focus of a 10' dish (Gain = +36 dB), you would get
an impressive +95dBm EIRP.

For the receiver, lets assume a preamp with .5 dB noise figure,
also mounted right at the focus of another 10' dish. However,
this receiver needs to be 2 MHz (or more) wide so the resulting
receiver sensitivity is around -153dBm.

The eme path loss to the moon and back is around 280 dB so
the resulting return signal is +95 -280 = -185dBm and
the signal to noise ratio is -185 - (-153) = -32 dB

It would be very tough to pick this signal out of the noise
even with a really good DSP since the oven signal is so unstable
and it is over 2 MHz wide so sound cards are out.
You can't get the same "processing gain" that you would with
spread-spectrum because the TX signal is not predictable.

Now, compare the same setup with 10 dB less output (i.e. 80 watts)
and a 100 Hz bandwidth. The TX EIRP is then only +85 dBm but the
receiver sensitivity is now -196 dB giving a SNR of around 1 dB.
Heck, you can hear that in your headphones.

Or try 8 watts (+75dBm) and a simple sound card program to pick out
the -9 dB SNR return signal and you are in business.

73,
Tony AA2TX (Back from vacation!)





  
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