EMC

[Panrock]

Thanks Kalee. OK, so the ladder would locally 'short out' the E field. Though I venture to suggest it might get rather hot placed right next to one of Crystal Palace's radiating arrays (and cause a horrible mismatch on the output?).

But what about the arm waving, when you've got several K's of body resistance to develop a voltage across? 'Interesting' too if, with your arms stretched out, you become a tuned dipole...

Steve

[Kalee20]

Just thinking about what I wrote in the spur of the moment last night - I don't think I was right. The ladder WILL have a PD between its ends, in a steady E-field. But you still won't get a shock, because so would your body...

And, I'm thinking about the arm-waving!

[Panrock]

Unless I'm looking for a novel method of suicide, I'd be best advised not to stretch my arms out at Wrotham, due to the likely Band II dipole tuning. I'd be more likely to get away with it at Crystal Palace.

Steve

[BusyBee]

Something I'm wondering. When an RF or AF electric field gradient exceeds (say) a hundred volts-per-metre, can you wave your arms about and receive a shock?  What about if you're on a metal ladder and touch two parts of it some way apart? Is the same potential picked up by the ladder, but this time with a low source impedance?

Steve

This reminds me of something an engineer who worked at the Rugby shortwave transmitters told me. When working on the feeder lines to the aerials they used earthing straps for safety even when the transmitter was supposibly off. He mentioned that one time he placed a strap across the line a short distance away from a shorted point and there was an almighty bang. I don't know any more details and it was a while ago I heard but it did illuminate the differences between low frequency / DC and radio frequencies. The point I would perhaps make here is that, at the lower frequencies, the effect would be small, but at RF it may be lethal. After all a ladder, at some nominally indeterminate frequency, would become a tuned line (effectively an aerial / tuned circuit?).

Pretty irrelevant except under normal circumstances but never underestimate the dangers of high RF powers and fields.

Tracy

[Murphyv310]

Hi.
An amateur using say 2 meters and 50W pep into a yagi with 13dbi gain will give 1kw eirp now at 8 meters high at ground level on the main lobe direction at 5 meters from the antenna you are just in the radiation zone that Ofcom states. Increasing the height of the yagi to 10 meters there is no issues. A foundation licence holder with 10 Watts ERP wouldn't have any issues with the same antenna (200 w eirp).

Just had a talk tonight on zoom from the RSGB on this, very informative and TBH most amateurs will be able to comply easily.

[Panrock]

Is there a helicopter ban near Crystal Palace? When I totted up the transmitter powers at CP, assumed they are erp's not eirp's and a mean frequency of 550 MHz, and fed this into the Ofcom calculator, I got a minimum safe distance of 434 metres. That's a 'forbidden zone' around the tower approaching a kilometre across. And what about the birds? They are around the right size to resonate at UHF.

Steve

[ppppenguin]

The birds probably avoid the high fields in the same way they don't squat on 400kV power lines. They presumably feel too uncomfortable as they approach.

[Refugee]

Birds do land on power lines and can overload them in extreme cases.
https://www.theguardian.com/environment/...er-outages

[ppppenguin]

That looks like 11kV which doesn't trouble birds at all.

[peter scott]

In attempt to stop seagulls nesting on my roof I resorted to ultra high frequency sound rather than ultra high voltage or ultra high frequency RF.

   

Peter