On 7/26/22 4:03 PM, KENT BRITAIN wrote:
One must wonder if you ever owned a slide rule? hihi
(Still own my eye saving yellow one by Picket)
I recently saw an article for a 20 Meter beam with dimensions in 1/10,000ths of an inch.
Someone needs to slap that lad up side of the head with a K&E slide rule!Guess he also needed to establish the exact temp the Aluminum should be at for that measurement.
You may laugh, but sometimes, small changes in dimensions are important, and therefore so is temperature.
For Aluminum, one might see a length change of 0.2% (and corresponding resonant frequency change) for a temperature change of 100C. 100C sounds like a lot, but between a cold night in Winter, and a hot day in Summer (particularly if the aluminum is shiny), that's not an unreasonable temperature delta.
If your antenna is in space, a 100 degree change from full sun to full shade is pretty common.
But on the ground, maybe a 50C swing is more likely - that is a 0.1% length change, the gain doesn't change much (0.2%) but more important is the phase change of 0.2 degrees (due to the rapid change in reactance).
0.2 degrees isn't a lot, but if you're trying to form nulls, 0.2 degrees turns a perfect null into a -40dB null. Large arrays forming a radio telescope care about this kind of thing. You don't want the apparent source to be in a different place in the sky.
In a more extreme case - When measuring the range to a spacecraft, we send a signal to the spacecraft, which transmits it back, and we compare the phase of the outgoing and the incoming signal.
A properly designed system can make this measurement to about 1 part in 10^15. Let's put that into context - it's about 10^9 km to Jupiter (at opposition). So that's 10^12 meters. We're making that measurement to 1 mm. (it's about 36 degrees phase difference at Ka-band - 32 GHz).
So we need to know not only the temperature of the antennas at both ends, but the temperatures of the waveguide and feedlines. (and the gravitational and thermal distortion of the DSN antenna, etc.).
As for *why* - by making precise measurements of range (to cm) and velocity (to mm/2) we can very accurately measure the orbit/trajectory of a spacecraft, and from that, we can measure the gravitational field of Jupiter, and from that we can infer what Jupiter's internal structure might be.
But yeah, if you're throwing a dipole for 40m up in a tree, measurement to the nearest inch/cm is probably good enough.
