Fieldstrength and Chickens

Foundations of Amateur Radio

Yesterday during dinner I heard an interesting story. Apparently there was a farmer who had a chicken coop that he kept warm at night by using a 240 Volt light bulb hooked up to the chicken wire surrounding his chickens. The farmer's property was in the vicinity of a local AM broadcast transmitter. They only found him because the people in the shadow of the chicken coop had bad reception.

I'm sitting at the dining table, listening to this tale and wondering, could it be true? My gut feeling was no, but surrounded by food and friends it was hard to put my finger on precisely where this doesn't add up.

A little digging revealed that the transmitter in question was the local ABC 6WF transmitter at Hamersley.

So, what do we know about this transmitter?

First of all, it's a 50 kW AM transmitter. I must confess, I have operated this station. Imagine that, 50 kW AM, with an introductory Foundation License.

Anyway, back to the 6WF transmitter. It's located on a block of land, roughly 500 meters wide, 1000 meters long. Thanks to the ACMA we have a map that shows a contour line where the field strength of this transmitter is 1 Volt per meter. That means that there is 1 Volt difference between two points a meter apart. This contour varies in distance from the antenna from 2.1 km to 1.8 km. So in a sort-of-circle around this transmitter there is a circle of points at which you can step 1 m further away and measure a field strength decrease of 1 Volt.

We also know that electric fields decrease by the inverse square of the distance, or said in another way, if you double the distance, you decrease the field strength by 4, if you increase distance by 3, you decrease strength by 9.

Of course, the opposite is also true.

If you halve the distance, from 2 km to 1 km, you increase the field strength to 4 V/m. If you have it again to 500 m, the field strength becomes 16 V/m.

To power a 240 V lamp, we'd need to get within 100 meters or so of the transmitter. Of course, the block is 500 m wide, the closest you can get without going onto the property is about 250 m, where the field strength is about 64 V/m.

So, unless the farmer had a chicken coop on the transmission site itself, this is unlikely to have happened.

Before you ask, how can an f-call operate a 50 kW AM transmitter, on 720 kHz, easy, get invited to talk about Amateur Radio with Gillian O'Shaughnessy on ABC breakfast radio. Did you know, her grand-dad was an amateur?

Anyway, if you know the farmer in question, or if you're sure it's happened, I'd love to hear about it.

I'm Onno VK6FLAB