Fire @ 1KW RF

[StuUK]

Be really interested in your thoughts on this.. You can watch the vid for a fuller explanation but if you don't have time... Essentially a person put 1KW RF using FT8 mode (15sec duration normally) into this cable (attachment) and it caught fire.... question is would we expect it to and why?

Video https://youtu.be/__zocm2Pib8

[bd139]

My 400W dipole wire is about twice as thick as that stuff.

1kw is approx 626 volts peak to peak in a 50 ohm system. But expect a lot more than that near the ends in a resonant antenna. Also the amount of current shooting though that is going to cause a lot of heating just due to the resistance of the wire at kw. RF currents flow mostly on the surface as well. Bye bye insulation followed shortly by copper.

[ConKbot]

As said in the video, the tip of the antenna is high voltage, high voltage high frequency applied to PVC is going to make it heat up a lot, as it is quite lossy compared to better plastics. (The PE-4 sheet mentioned in the video is UHMWPE, much less lossy) 
As mentioned above, skin depth plays into it too. About 17um deep at 14MHz, on 2.053mm diameter 12awg wire leaves 0.11mm^2 in use. A 5m long antenna would have 1.7 ohms of resistance then.  I^2 R losses in a resonant antenna become very not fun then.

Heat from the copper surface, heat in the plastic. Even if there wasn't a dielectric breakdown of the air at the tip to light everything off, it could have started to char the PVC (conductive, but very resistive) and had everything go to crap from there.

Unexpected? Probably not. Could I tell you at what power level the antenna would go up in flames? Probably not

[cncjerry]

I have a vertical loop, 260ft long IIRC, it is resonant on  80M and 40M.  I use a UNUN and a BALUN to go from coax to ladder line with a 1:4 impedance raising it from about 12ohms to 48 as I feed it on the corner.    I am using 12ga insulated stranded wire from Home Depot.  I have an internal tuner on my IC7610 but I use the load on my 1.5Kw amp and it tunes great on 80/40M and some other bands.

Remember, insulated wire.  Right after I tuned the amp the first time, the insulated wire was only about 1.25 inches apart at the corner where the ladderline feeds it.  I noticed the power output on the amp was dropping while tuning and I heard some noise outside.   I went out and the wire had arced through the insulation, melting the 12ga wire and welding it together after it dropped and touched. I've never seen anything like it.  Could have burned down the whole heighborhood.  I fixed the feed point and now all is well. 

Jerry

[mrf184]

I've had antennas burn up with far lower powers... recently I built a vhf magnetic loop (electrically small) tuned using a wire-twisted "capacitor" at the end, and it only took 20W for the twisted pair to melt and catch fire. It probably had many kilovolts across it and the dielectric is lossy plastic. I replaced this with a real variable capacitor (a small surface mount type) and that arced over too. I'm still looking for a variable cap that can handle the voltage and has decent Q at vhf...

Also this is an ordinary wifi antenna that melted with 20W input... that doesn't bode well for radiation efficiency...

[StuUK]

My 400W dipole wire is about twice as thick as that stuff.

1kw is approx 626 volts peak to peak in a 50 ohm system. But expect a lot more than that near the ends in a resonant antenna. Also the amount of current shooting though that is going to cause a lot of heating just due to the resistance of the wire at kw. RF currents flow mostly on the surface as well. Bye bye insulation followed shortly by copper.

What type of wire are you using?

[bd139]

Not sure what wire it is. It’s an M0CVO made DBD-2040 dipole. Very thick and rigid stranded wire.

http://www.m0cvoantennas.com/apps/webstore/products/show/4942142

Nice antenna.

[StuUK]

Turned out not to be the wire (although an obvious contributing factor) but the latest batch of the plastic discs used in the construction are conductive...

https://youtu.be/Vbjy_ch-xT4

[CJay]

He's all shook up because it turns out once you've gone black you can go back?

[xaxaxa]

I had a 900MHz tesla coil that will easily tell you how good a dielectric is; put ~10W of RF into it, touch the material to the tip of the coil, and if it starts smoking you know it is lossy. Last time I tried it with 10W even FR4 burns; the only materials I found that "pass" the test are teflon and glass...

[In Vacuo Veritas]

Nice to see

Blank Reg found a hobby.

[CJay]

I was thinking more Yondu but not as handsome.

https://en.wikipedia.org/wiki/Yondu#/media/File:Michael_Rooker_by_Gage_Skidmore_2.jpg

[bd139]

Yondu from Dartford.