Wednesday, January 04, 2006

Preliminary Orion "Radar" data

I was working with the Orion, trying to figure out its T/R timing on QSK CW. It turns out the dead time, from end of "key down" to receiver audio output is about 28 msec, by my measurement. The ideal QSK rig has zero dead time, so that the entire interval between key-down times is available for listening. The Orion has lots of DSP things to turn around, or so I suspect.

I did some of these timing checks on the air (low power) just to be sure we had realistic conditions. After finishing, I tuned around to what I could see on the different bands. The bands were dead -- no signals heard on 20 meters and up. Were there echoes tonight? There was nothing audible. But when I set up on 20, this is what I saw.




The horizontal time axis is 10 msec. per division. The frequency is 14.124 MHz, receive BW = 400 Hz, beam heading 110 degrees. Power is 100 W. We are looking at the AUX audio output. AGC is off. We are using the internal keyer set at 20 wpm. The bright horizontal line is the T/R dead time. The time is 0500Z, 4 Jan. 2006.



To confirm that we are seeing something from "the sky", switch the SteppIR to 180 degrees and transmit toward bearing 290 degrees. The return pulse is absent.



To see a little better what is going on, back at bearing 110, decrease the time resolution to 20 msec. per division. Here we see the two dits and the received interval between them. There is a suggestion, after looking at these waveforms for a while, that we are seeing multiple return pulses that are confused with each other after the first visible pulse. (The slope of the "dead zone" is due to AC coupling in the 'scope.)



For comparison, do the same experiment with the same bearing on 15 meters, 21.127 MHz. No effect is visible.

These photos are like a radar "A scope" view. Range increases to the right. What is the minimum detectable range? This is determined by the relatively slow T/R changeover time. The speed of light is about 5.3 msec per 1000 miles. Therefore the minimum range return echo that can be seen is roughly 5300 miles round trip, or 2600 miles down range. The delay we see here is about 35 msec (falling edge to falling edge), or perhaps 3000 miles range.

This echo return was not audible to the ear, and the range is apparently quite a bit shorter than with the pronounced returns I experienced recently with my 21 MHz New Year's contact with ZS6SIG.

Will some radio science emerge here? Stay tuned.

p.s. WWV is reporting the following:
:Product: Geophysical Alert Message wwv.txt
:Issued: 2006 Jan 04 0603 UTC
# Prepared by the US Dept. of Commerce, NOAA, Space Environment Center
#
#          Geophysical Alert Message
#
Solar-terrestrial indices for 03 January follow.
Solar flux 85 and mid-latitude A-index 2.
The mid-latitude K-index at 0600 UTC on 04 January was 1 (06 nT).

No space weather storms were observed for the past 24 hours.

No space weather storms are expected for the next 24 hours.
I.e., very low activity.

Note added (1/8/2006): Radar interpretation requires understanding the systematic delays of the equipment. Further study shows that the Ten-Tec Orion, as a DSP transceiver, has significant "processing delay" in its receive path. In modes that use the SSB demodulation, including CW, the audio output is delayed by up to 14 msec from the RF input. This will alter the range calculation, above.

(5/22/2008): Figures restored.
Post a Comment

ShareThis