IEEE Consultants' Network of Long Island

Testing Dipoles - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this steerable phased array of half-wave dipoles needing to be tested, not for its RF properties because there was high degree of confidence in that, but just to make sure that all of the dipoles were physically intact.

There were literally hundreds of these things distributed over a huge parcel of land, somewhere in Texas as I was told. The most vexing issue was how to determine quickly and accurately if each dipole was still properly attached to its individual feed line. The mounting positions were much too high to be seen from the ground, so how could the feed line attachments be accurately, simply, and with there being so many of them, quickly tested? 

The Sculptor - John Dunn, Consultant, Ambertec, P.E., P.C.

The company had hired an engineer for a microwave project and the fellow dug into the work with energy and gusto. He seemed genuinely happy, until......

The planned design was going to need some stripline structures, but an examination of several circuit boards convinced this engineer that the available geometric precision of circuit board artwork at that company was inadequate for the purpose at hand. Therefore, he brought in from home an utterly exquisite set of precision carving knives and working by hand, he actually carved the required stripline structures out of raw copperclad circuit boards without ever etching them.

His stripline carvings were wonders to behold. Engineer after engineer, upon seeing them, walked away awestruck.

Distance To The Horizon - John Dunn, Consultant, Ambertec, P.E., P.C.

The earth has a circumference of approximately 25000 miles and you're going to put up a structure of some particular height, perhaps an antenna. How far will it be from the top of that structure along the earth's surface to the horizon?

The calculation, or perhaps more properly, the estimation is as follows: 

 

While it's true that the algebra would be simpler using the arc cosine instead of the arc tangent, I wrote the code for this in GWBASIC and I only had the ATN function available to me, at least so far as I know.

 

Inadvertent Waveguide - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this amplifier constructed on a circuit board and mounted in a metal chassis which worked fine if the chassis cover wasn't attached, but which would go into parasitic oscillation at 1 GHz when the cover was put on.   

 
My assigned task was to fix it without changing anything. (My boss at the time may have suspected that I dabbled in the occult arts. I was never sure with that guy.)

Radiated Susceptibility Problem and A Windom - John Dunn, Consultant, Ambertec, P.E., P.C.

You can fail a radiated susceptibility EMI test if the physical dimensions of the unit under test (UUT) are just right. Such was the case with an aircraft fuel gauge in which the fuel probe's dimensions had been designed  to match up with the aircraft's fuel tank.

At almost any irradiation frequency, everything worked okay, but not at 120 MHz.

 

It turned out that the probe's length was a half-wavelength at 120 MHz and worse still, there was a right-angle joint at the 37% of length point as shown above.

Dimensionally, this fuel probe assembly was a classic Windom antenna at 120 MHz and when it was irradiated at that frequency, the supporting electronics was thrown into all kinds of malfunctions.

None of the probe's dimensions could be changed because of fuel tank restrictions. However, since the fuel tank was metallic and therefore a good RF shield, a waiver was granted because there would never be any 120 MHz irradiation of the fuel probe in actual service.

How do you spell "relief"?