Tuesday, May 28, 2013

(A)Typical LBR installation

Like I mentioned, today Paul Harden, da boss, and I installed two more low-band receivers today, but with only a 50% success rate.

The receiver is  dual banded for 74MHz/350MHz and is housed in a big and heavy 8"x12" metal box. for thermal stability. We call them LBRs or 4/P receivers -- 4 band is 58-80MHz (eg 4 meter wavelength) and P-band is 325-420 MHz or so. P comes from it's old military designation, like L, S, C, X, Ka, K, Ku, radar bands, etc.

She's mounted on a sturdy mount inside the "barrel" also known as the Focus Rotation Module, or simply the apex. It's also my second home. Inside the barrel is a central square steel through which the 4-band antenna and receiver output cables go through. The receiver mounts near the bottom of the apex, and plugs in to 8 different cables - two pairs of Heliax for the 4/p band crossed dipoles, one Heliax pair for the output, a power cable, and a calibration signal cable.

Each antenna input requires two pair because of the dipole design -- they're orthogonal crossed dipoles that are fed into the receiver as two linear polarizations. Some magic happens in the LO-IF rack that either sends them as a linear polarization to the correlator (mainly for ionospheric observations) or combines them in quadrature to provide left- and right-circular polarization for observations outside our own solar system.
The first installation went smoothly -- we removed a malfunctioning receiver from antenna 18 and replaced it with a fresh one -- it's pretty much plug and play. Somewhere through, we lost a two-way radio. No idea where it went. I think it always existed in some kind of quantum state, and now it's hiding.

The second was troublesome. Each output, left and right polarization (or X and Y) should show clearly the band passes of each band through a spectrum analyzer. What we saw was spikes caused by the 74MHz *something* oscillating like a runaway Hartley. (that was a pretty poor play on words)

So we hit it a few times, jiggled the cables, and poof, it was back to normal! We came back down to the vertex room to find it again went nuts. So we went back up, wiggled some more, and took the SA with us. Now the X side was gone! We took the cover off and poked around, but it was dead. Methinks the oscillation was strong enough to saturate an LNA, bust a cap or diode or two, and kill X.

Paul's gonna play with it tomorrow, and we'll see if we can make our pseudo-deadline of getting everything working by Tuesday night. Lots of pressure from the higher ups.

Tonight, I'mma gon' write a blurb about the SDR demonstration at the ARRL Youth Lounge at Dayton for Ward Silver, N0AX, upvote and argue things on reddit, and buy some wire and ladder line for field day. Need to make a 9:1 balun by then too.

View of South baldy and the Magdalena Ridge from the bus ride home

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