There was once a space probe called Sunblazer. I was part of the staff that was working with MIT to develop receiver apparatus for it. It was meant to be a probe for studying the sun. I said "was meant to be" because the project never came to fruition due to a funding cut-off by Congress. There is a description of the project at Gunter’s Space Page.

Decades later, we had the IRIS solar probe as discussed in NASA’s sun-observing IRIS mission and soon we will have the ESA's solar orbiter (see ESA’s Solar Orbiter preps for a high-temp mission).

Getting back to Sunblazer though, there was an antenna issue. The probe's operating frequency was to have been 40 MHz. The receiving antenna was an array of half-wave dipoles arranged in a matrix over a large area of land somewhere in Texas. Each dipole fed a 75-ohm twin-lead, each with a variable delay line. Signal propagation time from each dipole to its receiver connection was made variable. By choosing the individual delay times properly, the array of dipoles became a steerable array. The main lobe's direction of reception could be pointed differently by selecting the delay times for each feedline.


Figure 1
Steerable array for 40 MHz reception

One of the concerns the MIT folks had was how to test the physical integrity of each dipole antenna. The biggest fear was that a feedline connection might break away from its dipole element. Severe weather was the anticipated culprit.


Figure 2
Damaged feedline connection

This became a big issue. There were thoughts of using dip oscillators to confirm antenna resonance, of using time domain reflectometry and some other clever stuff, but with hundreds of dipoles to test, anything they could come up with seemed impractical.

I told them not to worry about resonant frequency getting shifted. Unless a wayward bird were to collide with a dipole element and break it, dipole tuning would not change. Any broken element would be easily visible from the ground.

To test feedline integrity, each dipole could be shunted with a high value resistance attached to the dipole elements above the feedline connections. Each resistor would be too physically small to affect RF performance and if each resistor were made 100K, the dipole impedance at 40 MHz would not be affected.

Feedline integrity could be quickly confirmed with an ohmmeter reading on the ground at the bottom of each line.



Figure 3
Testable dipole and feedline

Nobody at MIT had thought of that but sometimes simplest is best.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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