Can a 19.8 kHz, 1 megawatt VLF tower melt metal?

In summary: Exmouth WA. He says that the room is all wood, and that a hammer was melted by the radio waves. He also says that the tower and fence aren't effected by the waves.
  • #1
Johnm35
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Not sure if this is the right place to post, but I am wondering, can this VLF station melt metal (details below post)?

I recently did a scuba dive with a company in exmouth, WA, who provided some info on this station. They had been told that the control room is made entirely of wood, and even had a wooden bolt provided by someone who allegedly worked there to prove it. They had been told that it had to be all wood as any metal would melt in the control room due to the high power of the radio tower. Apparently a hammer had been left in the room once and the next day it was melted..
this seemed a little odd, firstly I didn’t think radio waves would generate such heat when near metal, also how does any of the systems run in there with no metal for conducting electricity? Also if a metal hammer melted it would have been super hot, and likely burned the wooden room... and why doesn’t the tower melt? Or the fence? I think the dive company may have been told a good sounding story unfortunately that lacks facts. Thanks for any help you can provide!

From wiki: Naval Communication Station Harold E. Holt
is located on the northwest coast of Australia, 6 kilometres (4 mi) north of the town of Exmouth, Western Australia. The town of Exmouth was built at the same time as the communications station to provide support to the base and to house dependent families of U.S. Navy personnel.

The station provides very low frequency (VLF) radio transmission to United States Navy and Royal Australian Navy ships and submarines in the western Pacific Ocean and eastern Indian Ocean. The frequency is 19.8 kHz. With a transmission power of 1 megawatt, it is the most powerful transmission station in the Southern Hemisphere.[1]
 
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  • #2
Sounds like nonsense. The towers are supported by wires - if even something small like a hammer would melt then a long wire would pick up much more power (growing quadratic with the length) and melt much more easily.
 
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  • #3
Johnm35 said:
Apparently a hammer had been left in the room once and the next day it was melted..
no ... and I'm trying hard not to laugh ... sorry ... but you have been told a story that probably is told by locals to scare visitors for a laugh

just think for a moment, about what mfb said and also, do you think the radio operators in the control room wouldn't have
very bad health effects from such a high RF field ? let alone the damage that would be caused to all the metal in the
electronics of the transmitter etc ?

They use such hi transmitter powers to help overcome the big inefficiencies of antennas that are nowhere near
long enough to have good resonance at the required frequencies. The radiated power level is substantially
less than the 1MW ... probably more in the order of 100 kW or lessThe transmitter site from Google Earth

upload_2018-12-19_18-20-12.png
Actually, I plan to be over there in a few years time for an eclipse of the sunDave
 

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  • #4
some of the metal masts

upload_2018-12-19_18-32-52.png
 

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  • #5
Gold teeth melting? I think not.
 
  • #6
A quote from a history one of my local transmitting stations, which was putting out 400 kW at 200 kHz:
"An interesting phenomenom that the local householders, especially those living adjacent to the transmitter site, became used to was the sound of programmes mysteriously emanating from objects such as electric cookers and iron cooking ranges. In some cases electric lights continued to glow dimly after they were switched off.

This was due long lengths of house wiring and large metal objects acting as receiving aerials and picking up energy from the transmitting aerials. Poor electrical contact between panels and fittings causes rectification to take place and this gives rise to an audible, rather "tinny" signal. One lady claimed to be able to hear a programme every time she touched the poker on the firebars of her metal fireplace.

A similar effect occurs with rusty cast-iron drain-pipes and these also cause a mixture of the transmissions to be re-radiated as electrical interference. This is known as "the drain-pipe effect" and was particularly troublesome in the early days of V.H.F. television as viewers in Wychbold were well aware, frequently telephoning the transmitter to complain. On the transmitter site however the problem was due, not to drain-pipes but to the steel halyards used for hoisting the aerials and mast lift cages. Although the halyards were kept taut, a strong wind would cause them to move and make intermittent contact with the masts, giving rise to slight arcing resulting in electrical interference. This was corrected by the station riggers bonding (electrically connecting) the halyards to the masts at various places."
 
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  • #7
+1

My farther worked for the BBC before joining the RAF in WW2.

At the time the BBC broadcast some programs in German. My father told me several stories about people living near transmitters becoming alarmed at hearing "voices in German coming from the attic".
 
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  • #8
A Vertically polarised antenna (Monopole) for such a low frequency will produce significant (mirror) currents in the ground, way out from its base. Because the mast would be 'electrically' very short, there is often an 'umbrella' arrangement of wires, radiating out from the top of the mast (or sometimes an L or a T) which has the effect of increasing the current in the mast itself. This amplitude of current will also flow out on a large Earth Mat of radial wires on the ground. But it would require a mat of the best part of 1km diameter for 19kHz. Buildings within that sort of range or even more can expect to have currents flowing through any mains wiring. fences etc., certainly large enough to produce some audible effects. Hardly enough to melt things though.
There is a tale of a farmer who laid wires under his greenhouses near Droitwich 200kHz radio transmitter and helped to keep the chill off his plants. Story goes that he was prosecuted for it but it could be just another 'hammer' story.
 
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  • #9
Johnm35 said:
They had been told that the control room is made entirely of wood, and even had a wooden bolt provided by someone who allegedly worked there to prove it. They had been told that it had to be all wood as any metal would melt in the control room due to the high power of the radio tower. Apparently a hammer had been left in the room once and the next day it was melted..
t
I think this comment is based on fact but has become exaggerated.
Such stations are found in most counties, and typically operate with a transmitter power of around 1MW and frequencies in the VLF band.

The antenna consists of an elevated network of wires called a flat top, extending over perhaps a square mile. The flat top has an uplead at the centre which connects to the transmitter building and does the radiating. As the antenna looks like a capacitor, there is an Aerial Tuning Inductor in the loft of the building which cancels out the capacitive effect and allows a very large current to flow in the uplead. The inductor usually consists of spiders of cable about 20 feet diameter.

Within the vicinity of the tuning coils, the electric and magnetic fields are very strong, and a fluorescent tube will light with nothing attached. In terms of human health, however,. my recollection is that the permissible electric field is about 2000 V/m and this is not exceeded where people are working.
However, the fields within the coil are even larger, and resulted in an unfortunate fire at Rugby Radio Station during WW2. The station had been rebuilt and the spider for the coils was supposed to be entirely of wood, as the fields would be so strong that metal would become red hot. Unfortunately, the carpenter split one of the pieces of wood, and inserted a small metal tack to repair it. When the station was switched on, the tack must have become red hot and it burnt down the entire building.

As far as people off site are concerned, the ERP (actually Effective Monopole Radiated Power) of 50kW mentioned is quite typical and is not in any way hazardous.

It might be interesting to know that Rugby and one of the USA stations were built in about 1925 to allow public telephone calls across the Atlantic, which was not then possible by any other means.
 
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  • #10
The loss of any piece of length much lower than the wavelength is:
$$Loss(dB)=10*log(\frac{4*\pi*d}{\lambda })^2-G_T(dBi)-Gr(dBi)$$
Accordingly http://www.qsl.net/n1bwt/chap1.pdf
Where lambda is the wavelength, Gt is the gain of the transmitting antenna and Gr is the receiving one, if the wavelenth of the receiving antenna is much less than wavelength the Gr=1.76.
d is the length of the piece of metal (or person)
Accordingly that a 0.2m metal piece would receive 0.15W at 100m of the 1MW 28khz antenna
But if the hammer gets close the power increases a lot, to melt it must be at less than 2 meters accordingly the formula but any metal shape close to the antenna modify the radiation pattern so it can be changed
I should not recommend to be in the 400m from the antenna when transmitting. At 20m or less you should be toasted as a chicken in a microwave oven at 300 watts power :)
You could use nylon bolts, glass fiber wires to support the tower
 
  • #11
Javier Lopez said:
The loss of any piece of length much lower than the wavelength is:
$$Loss(dB)=10*log(\frac{4*\pi*d}{\lambda })^2-G_T(dBi)-Gr(dBi)$$
Accordingly http://www.qsl.net/n1bwt/chap1.pdf
Where lambda is the wavelength, Gt is the gain of the transmitting antenna and Gr is the receiving one, if the wavelenth of the receiving antenna is much less than wavelength the Gr=1.76.
d is the length of the piece of metal (or person)
Accordingly that a 0.2m metal piece would receive 0.15W at 100m of the 1MW 28khz antenna
But if the hammer gets close the power increases a lot, to melt it must be at less than 2 meters accordingly the formula but any metal shape close to the antenna modify the radiation pattern so it can be changed
I should not recommend to be in the 400m from the antenna when transmitting. At 20m or less you should be toasted as a chicken in a microwave oven at 300 watts power :)
You could use nylon bolts, glass fiber wires to support the tower
The formula quoted is for path loss between antennas. It does not tell us how much power a small piece of metal would receive.
When we approach a transmitting antenna closely, we find that there are two sorts of fields present. The radiation field consists of traveling waves moving away from the antenna. At distances further than about lambda/6 the intensity falls with the inverse square law, but closer than this it remains constant. So we do not routinely see small objects being fried by the radiation field. However, many antennas also have strong induction fields, electric and magnetic, which can (but not always) increase rapidly close to the antenna. It is these fields which cause local corona effects (from spectacles for instance, when climbing a mast) and heat up small objects near the tuning coil.
If we assume the voltage on the flat top is 100kV, and the antenna is 200m high, then the electric field strength under the antenna will be 100,000/200 = 500 V/m and this is within safety recommendations. There is no need to use glass fibre bolts and nylon mast stays; galvanised steel serves perfectly, but insulators are needed where antenna conductors are attached. It is interesting that some masts have a base insulator and others do not; this is one of those esoteric questions for antenna engineers. In the UK, Rugby had an insulator whilst Droitwich did not.
At 20 kHz the wavelength is about 15km, so we need to be careful when making calculations of received power in the vicinity of the site.
 
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  • #13
SeanS6 said:
Yes of course it such a transmitter can melt metal by induction from the magnetic field. This is very well known process in industry known as induction heating.

yes but under very different conditions
 
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  • #14
Whoever said they can't have metal near a VLF system might be telling the truth, or it might be a lie. The issue would be the local magnetic field.

A transmitter tank coil or an antenna matching or loading inductor, in even a modest power transmitter, can have enough flux density to heat small metal objects quite significantly. A metal object, if large enough cross section or with enough local flux density, can act like a "shorted turn" and have some fairly high circulating currents.

When I was an early teen my friends would stick the blades of their pocket knifes flat side to the tank coil in my 35 watt Ham transmitter while I held the key down. It could heat a blade red hot in the matter of a few minutes. They were amazed because I would hold my hand there and act like it was burning my hand, but the resistivity of my hand's meat was so high there weren't worrisome current levels in my hand. They thought it was magic.

The electric field at the high voltage end of that tank, or near the open end of an antenna, would easily light a large florescent or neon lamp to full brilliance without physical contact. I remember walking along with a short wire tied to my belt when I got near the counterpoise end of a 50 kW transmitter, and the wire arced through my pant and burned a big hole right through my pocket into my cheek. This is a case of voltage, not current.

If the wooden building were near or contained an open loading coil, a tuning network inductor, or a tank circuit inductor (as was often the case for VLF) it is quite possible the magnetic field levels could be high enough to heat an "iron slug" through eddy currents and the resulting I^2R heating. This could easily happen from the metal object acting like a shorted single turn secondary. The eddy currents could be very large with a high turns ratio from the primary to the single "shorted turn".

With actual ground current or antenna currents in the radiator or counterpoise system, there isn't a chance in the world for something like a floating metal object to heat. If something like a hammer was near a large open coil, especially when in a resonant tank of some sort, it could easily happen.
 
  • #15
The issue here is not whether or not induction heating works; it clearly does and it produced part of my Christmas Dinner today. The issue is how well you can couple a small metal object to a transmitting antenna that's specifically designed to radiate as much of its transmitter power as possible out into free space. If a hammer actually melted then so would all the other odds and ends in the ground in surrounding (farmers') fields. The radiation resistance of an object that is a squillionth of a wavelength is a squillionth of an Ohm so it can't be matched without connecting it to a conducting structure that is a significant fraction of a wavelength.
 
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  • #16
sophiecentaur said:
The issue here is not whether or not induction heating works; it clearly does and it produced part of my Christmas Dinner today. The issue is how well you can couple a small metal object to a transmitting antenna that's specifically designed to radiate as much of its transmitter power as possible out into free space. If a hammer actually melted then so would all the other odds and ends in the ground in surrounding (farmers') fields. The radiation resistance of an object that is a squillionth of a wavelength is a squillionth of an Ohm so it can't be matched without connecting it to a conducting structure that is a significant fraction of a wavelength.

Loading or tuning inductors and tank coils in older high power transmitters were mostly air core. While the antenna system (excluding loading or tuning coils) itself does not have have enough magnetic field in a small area, certainly a large open core inductor could. There could easily be a thread of truth leading to the misunderstanding it was the antenna radiation at work when there might be areas in a tuning house where any "shorted turn" is a bad idea, especially with iron at very low frequencies.
 
  • #17
Tom Rauji said:
where any "shorted turn"
The area of the shorted turn will determine how much Power can be coupled in and a (even Thor's) hammer is pretty small. The radiation resistance is a relevant quantity here.
 
  • #18
Radiation resistance has little to do with flux density in the axis of a tank coil or resonator in a resonant circuit.
 
  • #19
Tom Rauji said:
Radiation resistance has little to do with flux density in the axis of a tank coil or resonator in a resonant circuit.
You're right, as the local Impedance is not 377Ω and it's not free space but the area still counts. If the hammer effect is real, where would it all end? The ground would be hot from the effect on conducting ores or even ions in ground water.
 

1. Can a 19.8 kHz, 1 megawatt VLF tower actually melt metal?

Yes, it is possible for a 19.8 kHz, 1 megawatt VLF (Very Low Frequency) tower to melt metal. VLF towers are typically used for communication and navigation purposes, but they can also generate intense electromagnetic radiation that can heat and melt metal objects within close proximity.

2. What materials can be melted by a 19.8 kHz, 1 megawatt VLF tower?

The specific materials that can be melted by a 19.8 kHz, 1 megawatt VLF tower will depend on various factors, such as the distance from the tower, the power output, and the composition of the material. Generally, metals with lower melting points such as aluminum, copper, and zinc are more susceptible to melting from VLF radiation compared to steel or titanium.

3. How close do you have to be to a 19.8 kHz, 1 megawatt VLF tower for it to melt metal?

The distance from the VLF tower for metal to be melted will depend on the power output of the tower and the composition of the metal. However, it is generally recommended to stay at least a few hundred feet away from a VLF tower to avoid potential health risks from the radiation.

4. Can a 19.8 kHz, 1 megawatt VLF tower cause harm to humans or animals?

Yes, a 19.8 kHz, 1 megawatt VLF tower can cause harm to humans and animals. The intense electromagnetic radiation emitted by VLF towers can have detrimental effects on living organisms, including DNA damage, disruption of cell function, and potential long-term health risks.

5. Are there any safety regulations in place for the use of 19.8 kHz, 1 megawatt VLF towers?

Yes, there are safety regulations in place for the use of VLF towers. In most countries, VLF towers are heavily regulated and require proper permits and safety measures to be in place. This includes keeping a safe distance from the tower and ensuring that the radiation levels emitted are within acceptable limits to protect human health and the environment.

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