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Anyone else notice that the reflections are not correct? They should be drawn to show the remaining hair side, but show the same as the un-reflected art.
The final section of 6" copper is in place, and we are now actually connected to the new West main combiner. I've highlighted sign signal path from the first-stage combiner (far left), through the antenna switch, and then out to the combiner (right).
I fired it up, for just a few seconds, at 150 watts TPO this afternoon. Even at that miniscule* power level, it easily knocked the station off the air while my old transmitter on the 98th floor was still running full-tilt-boogie into the old antenna, from the point of view of master control six miles away. I have a feeling we're going to see a substantial improvement in coverage
* = this was actually hard to do. This transmitter is rated to do 38,000 watts, so to get it down that far I had to power down 5 of the 6 amplifier cabinets, and even then, with the power control nearly all the way at the bottom it, was > really < unhappy about being asked to operate at such low power. Twitchy and unstable as hell. It honestly made me wonder if this was what the operators at Chernobyl #4 felt like on that fateful morning, trying to get their rig stable at a power level so far below what it was ever intended to do.
The point wasn't actually to knock the station off the air, it was to test that the safety interlock on RF switch #1 was working properly, in an environment in which you hopefully won't burn up the switch contacts if it isn't.
Long story short: the interlock works. I started the transmitter on the load, massaged it gently until it was stable (not happy, by any means, but stable), then rotated switch 1 to put it on the antenna. It muted for a few seconds, then came right back up putting out nearly as much power as the factory radio on a stock 2008 Lexus.
Mein Gott, that combiner is gorgeous. All shiny and new.
So is it a cable or a waveguide? Or is this the blurry zone in the middle where the two are kind of the same?
It transitions back and forth between coax and waveguide at different parts in the system.
All of the shiny copper is coax. The center conductors are hollow, but that's merely because of skin effect. High-frequency RF tends to ride the surface of the conductor, so you actually get more effective conductor area by making it hollow (so that it has two surfaces) than you would if it were solid.
Once it hits the top of the bandpass filter on the combiner (the barrel-shaped objects with lots of fins), it turns into waveguide. The energy travels down through the filter and into the spine at the base of the combiner, which is also waveguide. Then all the way to the left through the divider (the Y-shaped piece), which splits the energy out into two equal halves, and then it converts back into coax again and heads into the riser which takes it up to the pole.
I've noted in green the parts where it's waveguide:
Internally, our own cabinet combiner and mask filters are also effectively waveguide, with coax ports on both ends:
These pieces are kind of weirdly counter-intuitive. Even though they have coax ports on the input and output, if you measure between them with an ohmmeter, there's no connection. And if you measure between the center and the outer conductors at either end, it's a dead short. You'd think it'd be 50Ω or something like that, but it's not. Well, at 503 Mhz it is, but at DC, the entire system is one huge short circuit.
I just noticed that the image in my last post was broken.
Here's what the inside of the copper pipe looks like:
This photo is of the "flexible" outdoor-rated version, so it's corrugated and has a rubber jacket, and the inner support is a continuous spiral instead of just a ring every few feet. Same basic concept, though.
Why split the power at the Y? Any ducks in the copper pipe? Is the inside purged with Argon? How many estimated years until it is obsolete?
The antenna itself is divided into two halves. The two lines feed the upper and lower separately.
No ducks in the pipe. I have no idea what the impedance of a duck is at 503 Mhz, but it didn't seem worth taking a chance.
The pipes are initially purged with nitrogen, and then maintained at about 1.5 psi. The idea is simply to keep the inside perfectly dry, as moist air inside the line increases the chance of an internal arc. These things have dozens of joints, and while they're all sealed with O-rings, nothing's perfect, so they do leak a little. A tank that size typically lasts for several months.
Obsolete? Not even sure how to contemplate that. This style of RF plumbing has been in use since the 1950s. It was unaffected by the transition to color, or the transmission to digital, and it won't be affected by the transition to ATSC3 if that ever happens.
So... I guess it'll become obsolete when broadcast TV becomes obsolete.
This is the pictures thread, so here is a picture of a small dog:
The antenna itself is divided into two halves. The two lines feed the upper and lower separately.
why? Redundancy?
No ducks in the pipe. I have no idea what the impedance of a duck is at 503 Mhz, but it didn't seem worth taking a chance.
I understand. The internet is forever (except for the threads old whats-his-name purged).
The pipes are initially purged with nitrogen, and then maintained at about 1.5 psi. The idea is simply to keep the inside perfectly dry, as moist air inside the line increases the chance of an internal arc. These things have dozens of joints, and while they're all sealed with O-rings, nothing's perfect, so they do leak a little. A tank that size typically lasts for several months.
Argon should technically do a better job of that, but if you get several months out of that tank then it's better for staff to be like 'hey that tank's been there for longer than it should let me investigate" as opposed to "hey those tanks last years so i never have to look at them".
So... I guess it'll become obsolete when broadcast TV becomes obsolete.
That's what I was hinting at...how long you think broadcast TV will be a thing. I have an antenna in my attic for local (philly) broadcast, but the new firestick loads the current live TV streaming service quick enough that we don't use the antenna anymore.
01-26-2022, 07:02 PM
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