What 50,000 watts of RF energy sounds like through a jumper cable • Hacker News Board

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#121

Post by upofadown » Thu, Feb 11, 2016, 3:32 PM UTC

jonahx wrote: post #120

upofadown wrote: post #114

Earlier quoted context omitted.

You sure can. The normal fair weather voltage gradient in the atmosphere is around 100V per metre. When there are close thunderstorms this increases. Unfortunately for energy collection air is quite insulative. That's also fortunate in that we don't get shocks all the time just from standing around in an environment where your head is 200V different than your feet. Popular Science once ran an article that showed how…

What's required for a tower to become electrified from the air? I feel like I've played on other metal towers in my life that were inert...

Most tall towers are aggressively grounded. You would need a tower insulated from the ground. A tower used as an element for a low frequency antenna for instance. I strongly suspect that those large black interlocking rings are used as a spark gap for lightning protection in the absence of a solid ground.

Once you were on the tower you wouldn't notice anything. You would have to get between the ungrounded tower and ground.

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#122

Post by sevensor » Thu, Feb 11, 2016, 3:40 PM UTC

StavrosK wrote: post #101

50CNT wrote: post #98

Earlier quoted context omitted.

I thought they worked by dielectric heating? Making polar molecules get all frantic and stuff. There is a safety mechanism to shut off the magnetron when the door is opened, and a screen to make the viewing window opaque to microwave radiation, but those can malfunction. [0][ https://en.wikipedia.org/wiki/Microwave#Effects_on_health ] [1][ https://en.wikipedia.org/wiki/Microwave_burn ]

As far as I know, that's the mechanism by which microwave radiation heats molecules, but the specific design of the oven is to produce standing waves so microwaves can become concentrated. I may be wrong, though.

Not wrong -- the safety screen is part of the cavity resonator. But if there's a hole in it, or if you defeat the door interlock, you can still cook yourself with microwaves. In general I'm not too concerned that I'm going to cook my eyeballs by looking at my spaghetti to make sure that it's not exploding all over the inside of the microwave.

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#123

Post by fluxquanta » Thu, Feb 11, 2016, 3:52 PM UTC

kabdib wrote: post #113

philh wrote: post #78

Does a tower like this only broadcast one radio station? That's not what I would have naively expected. But if not, why do they pick up that specific one?

I used to live in Fort Collins, Colorado, a couple of miles from the antenna for WWV, the broadcast time standard from the National Bureau of Standards (that'll date me, it's called NIST now). Electronics was one of my hobbies. Most of the noise in my circuits was that damned time signal. I could pick it up with just about any length of wire and anything that would act as a diode. A transistor. A pin on a chip -- I h…

Oh boy. WWV is great when testing out ham radio propagation for the same reasons it was a nightmare for you. Powerful signal being broadcast between about every major amateur band. 2.5, 5, 10, 15 and 20 MHz.

BEEEP dooo dooo dooo dooo

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#124

Post by jacquesm » Thu, Feb 11, 2016, 4:00 PM UTC

sathackr wrote: post #110

jacquesm wrote: post #33

Earlier quoted context omitted.

The bird would be dead. RF power will happily fry tissue if you get close enough to the radiator.

Yes and no. 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 hea…

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.

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#125

Post by jacquesm » Thu, Feb 11, 2016, 4:07 PM UTC

sathackr wrote: post #106

zurn wrote: post #42

Earlier quoted context omitted.

What's wrong with it?

I believe it's actually the arc producing the sound, otherwise the cable would make the noise regardless of the presence of the arc. Whether that's what jacquesm is referring to or not, idk.

Yes. The arc is what causes the AM modulation to be translated to moving air at the same frequency as the modulation because the intensity of the arc changes rapidly.

Since this is AM you end up with intelligable sound (for an FM station this trick wouldn't work).

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#126

Post by LesZedCB » Thu, Feb 11, 2016, 4:09 PM UTC

connorlee wrote: post #7

Amplitude Modulation, or AM, works by modulating the sound on the "peaks and valleys" of the frequency. If you were to look at the waveform of the carrier frequency, you would see it as variance in the "power levels" of the signal, going higher and lower depending on what's being sent. Because of how AM works, you can actually hear what's being transmitted by causing an electrical arc. It's the same concept of hearin…

ElectroBOOM has a cool video[1] on youtube talking about some of that! His videos are some of my favorite.

[1] https://www.youtube.com/watch?v=L5E4NiP4hpM

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#127

Post by gelo » Thu, Feb 11, 2016, 4:43 PM UTC

exabrial wrote: post #13

So dumb questions: 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)?

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.

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#128

Post by gelo » Thu, Feb 11, 2016, 4:48 PM UTC

gelo wrote: post #117

JohnDoe365 wrote: post #95

Earlier quoted context omitted.

The pictured parabola antenna likely has a gain around +20dB. Would that change your calculation?

i don't think it would change (probably wrong) since the active energy is only generated by the final PA's in the system. the Antenna's gain is only from the passively reactive design. Its like making a violin string resonate by only moving the bow and not physically the string.

i mis-read your comment - it has no affect at all as its not whats transmitting 50kW!

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#129

Post by JoeAltmaier » Thu, Feb 11, 2016, 4:51 PM UTC

connorlee wrote: post #7

Amplitude Modulation, or AM, works by modulating the sound on the "peaks and valleys" of the frequency. If you were to look at the waveform of the carrier frequency, you would see it as variance in the "power levels" of the signal, going higher and lower depending on what's being sent. Because of how AM works, you can actually hear what's being transmitted by causing an electrical arc. It's the same concept of hearin…

The carrier frequency is way above the hearing range. So it has to be shifted to the audible range. That happens by clipping and filtering. The arcing must be doing that somehow. Probably clipping; the low-pass filtering is done by the air/your ear.

Re: What 50,000 watts of RF energy sounds like through a jumper cable

#130

Post by cnvogel » Thu, Feb 11, 2016, 4:53 PM UTC

gelo wrote: post #82

50000W of RF power is approximately 355kV - if you work out based on that 1W dBm is 7.1 Volts.

No. Power generally is proportional to the voltage squared (P∝U²). P=U²/R → U=√(PR)

The numbers you quote (1W [not dBm!] is 7.1V) make me believe that you assume an impedance of 50Ω.

    In [1]: math.sqrt(1*50)     # P[in W] * R[in Ohm]
    Out[1]: 7.0710678118654755  # U[in V]

So, in an hypothetical cable of 50Ω impedance, 50kW of RF power is...

    In [2]: math.sqrt(50e3*50) # P[in W] * R[in Ohm]
    Out[2]: 1581.1388300841897 # U[in V]

about 1.6kV(RMS).

Note, though, that with large installations it is likely that there are a few individually tuned feeds. And the cable might not be a 50 Ohm cable, but maybe a chicken ladder (two strands separated by fixed spacers), large-area waveguide, ...