Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

facebook.com

Post reply on HN
Print view View original

151–160 of 163 posts

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

#151

Post by gelo »

sathackr wrote: post #135
gelo wrote: post #127

Earlier quoted context omitted.

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. Us…

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…

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.

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

#153

Post by sedachv »

50CNT wrote: post #60
derefr wrote: post #51

Earlier quoted context omitted.

You mean they conduct from the entry-point to ground, similar to a lightning strike or electrical burn? Or is this more like a radiation burn? Actually, now that I think about it, it'd actually be most similar to how a microwave heats meat, wouldn't it? Microwaves being within the RF part of the EM spectrum and all. (Though not quite the same, given the lack of dipole interaction with water molecules outside of the p…

That reminds me of an urban legend regarding microwaves, which doesn't seem entirely infeasible. It involves a man working at a small fast food stand, who worked next to his microwave. Now his microwave was used often, and it had a small hole in the metal screen allowing microwave radiation to leak out. The man slowly starts to feel abdominal pains after a while, goes to his doctor, who diagnoses it as liver problems…

https://en.wikipedia.org/wiki/Microwave_burn#Injury_cases

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

#154

Post by sathackr »

gelo wrote: post #151
sathackr wrote: post #135

Earlier quoted context omitted.

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…

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

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

#155

Post by mannykannot »

mmagin wrote: post #145
mannykannot wrote: post #99

Earlier quoted context omitted.

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…

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 sh…

gelo's reply (https://news.ycombinator.com/item?id=11081227) suggests that the 'nearby powered antenna' is actually supported by the mast, and the mast itself is not an antenna.

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

#156

Post by jacquesm »

HeyLaughingBoy wrote: post #143
scruple wrote: post #56

Earlier quoted context omitted.

Yep, I have a RF burns on 2 of my finger tips that I sustained between 5 and 10 years ago... They are dead little bits of flesh. Discolored a deep yellow, there is no feeling left and they feel, I guess I'd describe it as, quasi-numb when touched. I was an embedded engineer at an RF company and it sort of comes with the territory. We had technicians who had lasted 4+ decades in RF (same company, actually) with scars…

What was the power level? We do a lot of RF stuff here and it would be good to know that we're safe.

I got mine at about 100W from a Tronser trimmer.

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

#157

Post by jacquesm »

sathackr wrote: post #136
jacquesm wrote: post #124

Earlier quoted context omitted.

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.

Why are you pointing your yagi at small birds? :-)

I know, it's sad.

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.

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

#158

Post by Florin_Andrei »

sathackr wrote: post #135
gelo wrote: post #127

Earlier quoted context omitted.

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. Us…

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…

> 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.

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

#159

Post by topspin »

cocoablazing wrote: post #141
topspin wrote: post #45

Earlier quoted context omitted.

Disclaimer: I am not a doctor or burn expert of any sort. I am an extra class ham. An RF burn will penetrate more deeply than a "conventional" burn of similar degree, possibly burning nerve cells, circulatory and other structures that an external burn would not. RF burns typically heal very slowly. I can't compare RF burns to burns caused by electrical current, except to say that electrical current will follow some p…

The skin effect should result in a high-frequency AC current propagating at lower tissue depths than a lower-frequency AC current. Why would RF AC propagating in flesh not follow this general rule?

We're getting beyond my depth here, but a few things occur to me. First, I know microwaves `work'; the optimal frequency at which water exhibits the skin effect is about 10GHz, so at 2.45GHz the energy from a microwave oven penetrates meat effectively instead of only frying the surface, as one would expect if the skin effect dominated. Also, I suspect the skin effect could actually cause internal burns; an internal organ that exhibits a strong skin effect at some frequency will cause localized heating on its surface, burning tissue internally.

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

#160

Post by jimmaswell »

xwintermutex wrote: post #8
ronreiter wrote: post #4

Wow, can anyone explain this?

I guess it is because of the use of amplitude modulation [1], where the envelope ("strength") of the carrier (that is some higher frequency signal) follows the signal that is modulated onto it (speech, in this video). With say FM it wouldn't work, I presume. [1]: https://en.wikipedia.org/wiki/Amplitude_modulation

If you listen to an FM signal interpreted as AM you can often still make out the audio, it'll just be very clipped and noisy. That was the result when I did this with an RTLSDR, at least
Post reply on HN