[gelo]

Why were they shorting the tower?

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.

[sathackr]

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.

[1] http://www.austin-insulators.com/radio/xfmr.html

[2] http://www.sonifex.com.au/?media_dl=3300 (PDF)

[Florin_Andrei]

> With most AM stations, the tower itself is the radiator.

Right. At those low frequencies, yeah, you need a pretty huge chunk of metal to radiate efficiently.

[gelo]

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.

[sathackr]

I believe there is some sort of protection circuit that keeps such an event from destroying the PA.

[exabrial]

So you're telling me that giant ass arc was caused by induction??? Holy crap!