Since the tower is metal, it will inherently start to resonate to the transmissive power. (The same effect can be seen when you touch a phono connector to an active amplifier, the 50/60Hz hum can be heard because you are absorbing some of the mains voltage energy radiated around you.)
When you transmit, any metallic object around the antenna can affect the impedance of the system. Usually the system is tuned with respect to the antenna and mast structure. At 50kW the antenna mast structure is resonated so hard by the antenna that high RF induction is present in the mast structure itself. Mast maintenance is normally done with the transmitter turned down or off entirely. Since this is a broadcast mast, this is not so easy. Grounding the mast structure may imbalance the tuning slightly but not enough to be of a concern with damaging the PA's. But the safety of maintenance engineers is a mandatory requirement.
Why not ground the mast anyway?
If you are inductively resonating a gigantic mast, you are also assisting in improving the gain of the irradiated signal. Hence why in the video the bottom of the mast has a black block of plastic/rubber.
With most AM stations, the tower itself is the radiator. That's the reason for the insulation from ground. If the tower were grounded, it would not be able to transmit a signal at all. In the video, while that cable is attached, the tower cannot transmit.
I wasn't sure about the device they were shorting so I asked the chief engineer for several large radio stations in the area.
That device is a lighting transformer [1][2] -- it allows AC power to be coupled to the tower, to power the lights, without a direct connection, which would severely impact the tower's ability to radiate RF energy. The two little balls is called a lightning gap, it gives lightning a path to ground, other than the connection to the transmitter, since the tower itself is not grounded.
i can understand the need for the large towers to TX AM band signals but grounding a directly fed antenna like that would kill the PA. You'd really screw up the SWR.
I am also curious about the cable. At the start of the video, its upper end is clamped on to a part of the antenna. The guy appears to be about take the lower end off what I am guessing is a grounded part, then he decides to give the demonstration. So, in the beginning, is this a live antenna that was shorted to ground while still being powered? (perhaps because it was being worked on?) If so, it seems a rough way to handle the transmitter, though I suppose it must be robust enough to handle lightning strikes (you can see two rods with balls on the end that I guess make a spark gap.)
Alternatively, could this be an unpowered antenna that is picking up signal from a nearby powered one?
Alternatively, could this be an unpowered antenna that is picking up signal from a nearby powered one?
I'm pretty sure this is what's happening. From past experience with high voltage arcs (admittedly at 60hz, not ~1 MHz), I don't think we're seeing more than a few hundred watts of power in that arc. Also, if this tower was actually the one transmitting that signal (presumably 50kW is the transmitter power) they'd shut it off or switch to a different antenna for maintenance on this one. If you shorted the output of the transmitter at the point where it's normally driving a resonant load, the best possible case is that it would trigger some kind of automatic shutdown on the final amplifier.
How do you think the human standing right next to the tower is not getting fried?
Humans climb live radio towers routinely.
The tissue absorption of RF energy at common AM station frequencies(below 2mhz) is negligible. Maximum absorption happens around the frequencies commonly used by FM stations (about 100mhz)
I have climbed near an live FM antenna. You can feel the heat(well, radiation, converted to heat by your body tissue) from the antenna, it feels like a heat lamp. Your eyes are the most sensitive parts as they have the least ability to dissipate excess heat. The old rule-of-thumb is, if your eyes are watering, you're too close to the antenna. But like most things 'the way they use to do it' -- by that point damage is already being done. Cataracts are one of the first signs of RF induced tissue damage.
It would take many thousands watts, transmitted at frequencies well above common AM frequencies to cause any sort of instant damage. I have never seen a field of dead birds near even major FM stations broadcasting with 50kw+ power.
I've accidentally fried small birds at ~1KW around 100MHz. It all depends on where they sit and how the power is radiated. Near the end on a Yagi is not a good spot.
I made a point of swishing the rotor side-to-side a couple of degrees after I figured out where the dead sparrows came from. That caused them to fly off and once the transmitter was on they'd stay away from the antenna.
there is. it's very, very noticeable on micro wave towers. the drum like cover on microwave antennas is just a lame regulation to try to save birds. they still die just by passing by. the building were i lived had one that required constant "maintenance" (dead bird cleaning)
I have never seen this, and I work with microwave towers nearly every day.
The 'drum like cover' is called a radome, and it's primary purpose is to reduce the wind loading of the microwave dish. It also serves to protect the usually sensitive feedhorn inside the dish, keep out bird nests, ice, water, etc...
The common cause of bird deaths around radio towers is collision, not RF energy. In Florida it is very common to see osprey nests build on/in/around towers. Microwave dishes are a common location because they provide a large horizontal surface to support the nest. See Wikipedia article 'Towerkill'
uh. have a completely different experience. mostly with south america and 15+ years ago. i recall very well the impression of the row of dead birds in the roofs. granted, i only got near the towers 4 times in my life.
The physics of electricity propagation in a powerline circuit is fundamentally the same as the propagation of FM radio waves, or even the beam from a flashlight. All of these examples involve electromagnetic energy propagating at the speed of light. So why do we need wires for powerlines, but not for propagating radio signals or light beams?
The principle is that light (in a vacuum at least) travels at a constant but non-infinite velocity, c. Hence, the electrical wiggle received at an observer's location now i.e; (x,t), has been caused by some earlier wiggle conducted by the source (x',t') such that the t-t'=(x-x')/c. We should expect that the effect of this 'time delay' is more profound if the source is wiggling faster in time.
A more specific way to state this is that the electromagnetic power radiated into free space by a dipole increases as the square of the dipole moment and the fourth power of the frequency. Now compare a powerline (60 cycles/sec), with FM radio (100 Million cycles/sec), and with the light from a flashlight (500 Trillion cycles/sec). That's why even atomic dipoles can produce intense visible light, while it would take a very very large dipole to radiate a similar intensity at 60 cycles.
Feynman Lectures Vol.II is absolutely the best reference to learn this stuff.
That is not to say that a bird needs to make contact with two wires on a utility line, in order to suffer harm. If the line voltage is high enough (e.g; 110,000 Volts, as in high-tension power transmission), an electrical corona would form around the line from electrostatic effects. The corona is actually ionized air indicating the high electrostatic field strengths in the region, it can emit a bluish glow and growl at 60 cycles. Birds can no doubt sense this corona & stay away.
Why were they shorting out the tower? (What was the purpose of the cable)
What are those rubber things?
What would happen to a bird that landed on the tower (not grounded)
What about a large metal helicopter (if it didn't crash)?