bang goes the theory on bbc

[markymark2406]

watchin the programme the other night, and the guy had a Led which lit when he had a CB radio transmiiting near it, he did mention the wires had to be the correct lengh so what is the lengh?

 

[properleckie]

Hi, as CB is @27MHz it has a wave length of just under 10Mtrs so a sub division of that would match to it, there used to be sold a small light that went on tip of antenna which lit on transmit but I am sure they were neons not Led's.
If you hold a small fluorescent tube near antenna Voltage max. point it will glow, this may be at different points on different types of antennae, eg near top or around 1/3 way up.
Do not touch antenna when Transmitting! RF burns are very nasty, should not be a problem @ legal 4W but .....
My transmitter in van runs 65W - 100W & will burn badly! (not CB), take care,

 

[ban-all-sheds]

Try the burns from a 12cm wave....


Or a 10.6µm.

 

[Taylortwocities]

Or about 600 nm clicky

 

[ericmark]

From Ipswich with Orford Ness so close and you ask that!
See Nikola Tesla it seems pre-war he was able to radiate huge amounts of power using aerials

Wireless transmission of power and energy demonstration during his high frequency and potential lecture of 1891

Closer to home Sir Richard Davis whom I met in Leaston did some research into radar and is seems they did look into the whole idea of a death ray during second world war and this resulted in our microwave cooker.

Seems seagulls were landing around Orford Ness ready cooked!

 

[markymark2406]

so how long?

 

[Taylortwocities]

2 hours in an oven at 190ºC

 

[ericmark]

It is 9 meter band so half wave would be 4.5 meters so two lengths of 4.5 meters should do.

However you would be receiving AC and the LED does not have a high threshold voltage so unless you used some other diodes and limiting resistors then likely you will blow the LED.

 

[ban-all-sheds]

Or about 600 nm clicky

Nah - you can see that one - takes all the fun away.

 

[Stoday]

For BAS make it 100nm.

Posters on this forum have threatened him with ultraviolence.

 

[mattylad]

Going back to my old CB days, there was always little LED lights that you could put on top of your DV27 aerial etc that lit up when you were transmitting.

If the chavs with the blue lights & undercar fluorescent tubes had CBs now then they would certainly have them too.

 

[davelx]

This effect is the basis of absorption wavemeters. Add a calibrated variable tuned circuit, diode and a meter and you can determine the frequency of the signal. We used to use them when setting up the aerial tuner units (ATU) in HF transmitters.

There is a story that a guy living close to the BBC 200Khz LW transmitter once put a giant tuned coil in his loft and tapped off power to light his house. It is said that they caught him by using said wavemeters to plot the field strength in the surround area and the resulting map showed them converging on his house.

 

[tim west]

If he had a licence i'd be surprised if he didnt get off

 

[Stoday]

I once visited GBR, the 16kHz transmitter at Rugby before it was dismantled.

You could hold a 5' fluorescent tube in the room with the tank coil and the tube would light to full brightness.

You can just see a bit of the tank room in the background of the second photo here: Click