AM/FM radio & powered extension cord

[Cliffh]

The other day I was working on the truck.  Had the little battery powered AM/FM radio sitting on the elevated deck  playing an AM station.  Needed some electricity, so I ran an extension cord across the deck, over the radio and to the truck.

Soon as I set the cord across the top of the radio the volume and clarity on the radio picked up.  WTF?  Took the cord off the radio and the volume (and sound quality) fell off to normal (shitty).  I put the cord back over the radio for the rest of the time.

It'd be nice to be able to duplicate the results without having to use an energized extension cord.

[French G.]

Pffft. If 110 is good 440 ought to be stellar.

[WLJ]

Pffft. If 110 is good 440 ought to be stellar.

Actual photo of Nikola Tesla tuning his AM radio   

[Cliffh]

I'm somewhat familiar with eddy current, figured they played into this somehow.

Neighbors would most likely freak out if I was to use a coil to amp it up.

[TommyGunn]

Inductance,  I think.  But I ain't no Einstein.  Or Tesla.     That electric discharge is just too scary.  That's another reason I like digital tuners....... 

[Perd Hapley]

Been there many times.

[Bogie]

Now try coiling it...

[230RN]

Radio waves are subject to reflections, even down in the AM band. Could be your power line acted as a reflector.  Or acted as a larger antenna "coupled" to the radio antenna, thereby boosting the signal into the radio's circuits. I doubt the 110AC had anything to do with it, but that could be tested.

All radio waves are line-of-sight, it's just that they can be reflected by other things. The only reason you can hear distant radio stations is because the signals are being bounced back down to earth by layers in the ionosphere.  This bouncing back and forth, up and down, is why you can hear some radio signals around the whole planet.

You move the extension cord, you change the whole accidental antenna system.

People have reported "weird" things with their wireless mouses and keyboards, where moving the transmitter (mouse or keyboard)  a couple of inches might kill the signal picked up by the computer.  It could have been reflecting off another object in the room when it was working right, but that reflection gets lost when the "mirror" or the mouse gets moved too far.

I sometimes have trouble with my over-the-air television signals when my neighbor opens his steel front door, or when the trees outside get wet. (All our front doors are steel jacketed.)

Not only that, but signals can be "directed" as well as reflected.

In the famous Yagi antenna, the long element is the reflector, the middle element is the "driven" element, meaning it's what's connected to the radio, and the shorter element (up top in the pic below) is the "director."'  The signal is strongest in the direction of the "director" element and forms a beam.  Hence, "beam antenna."

There doesn't have to be an actual electrical connection between these elements.  This is a Yagi made out of steel measuring tape.



The dimensions and spacing of the elements is critical, but calculable.

Many pictures of the "Little Boy" Hiroshima atomic bomb shows the Yagi antenna for the radar fuse.  It's toward the nose, but the driven element (the one in the middle) is a folded driven element for impedance-matching purposes.  The detonation was set for a 600 meter airburst.  There was also a barometric fuse.



I always found it personally amusing that the Yagi antenna, as shown here on the Little Boy atomic bomb, was invented by the Japanese.  Tee-effing-hee-hee. 

Terry, 230RN

Pic credits in Properties

[K Frame]

The cord was acting as an antenna.

[230RN]

^ Ayup. 

[fifth_column]

I wonder if the same effect would be observed if the extension cord wasn't energized?

[230RN]

I wonder if the same effect would be observed if the extension cord wasn't energized?

As I stated above, I doubt it.  But there are a couple of possibilities that might allow that.

One is that plugging it in to the household wiring ("energizing" it) simply makes the "accidental antenna" bigger.  Now since the efficiency of an antenna is related to how well it matches the wavelengrth of the signal, plugging it into the house wiring might make a better length match (as well as a larger capture area.)  The wavelength of the AM band is around 300 meters, so it is possible that unplugging it at the house would make the signal at the radio go away, but I don't think having actual 110V 60Hz power on the line would make any difference.

Two is that somewhere along the outside line, the actual transmitting antenna is close enough to the power lines that a signal is induced in the power line wires and is conducted around in the power grid and re-radiated near the AM radio.  This is a "parasitic" antenna and it finds use in areas where the radio is at a dead spot in the bottom of a valley. One places an antenna at the top of the local slopes with a transmission line down the slope to an additional identical antenna, which re-radiates the signal to the equipment in the valley.  There is no additional powered equipment necessary in this case  There is just a high antenna, a transmission line (cable) down the slope, and a re-radiating low antenna in the valley.

Unfortunately, whether the extension was energized or not is not stated, but I would assume it was.  We also have no information as to where the radio station's transmitter is located: down the block or ten miles away.

Antenna theory ain't simple.  But for the original questioners, my two posts afford a reasonably complete answer which nobody is being forced to read.

There are other far-fetched possibilities besides the antenna speculation. Might be a question of dead batteries or poor connections within the radio being "tickled" by the 60Hz power nearby.

Terry, 230RN

[RocketMan]

One way to test whether the long extension cord needs to be energized or not is to repeat the accidental experiment.  This time, add to the original extension cord length by plugging one or two more long cords into it.  Drape it across the radio in the same way.
At first, do not plug the longer extension cord into a house outlet.  Turn the radio on and move the cord in and around the radio to see if there are any changes to the signal.
Then plug the longer cord into an outlet and repeat the test.  I predict you'll have an answer toot sweet.
My guess is that there was some coupling between the extension cord and the ferrite AM rod antenna inside the radio.  It probably would not have mattered whether the cord was energized or not.  It was likely just the added length of the house wiring, and maybe the cable feeding the house, too.  At those low HF frequencies, longer is generally better from a reception standpoint.
I doubt you'd get much added signal from the high voltage lines on the other side of the transformer feeding the house.  Given how they are wound, the transformer would act as a choke for the AM band if I remember correctly.

[MechAg94]

Or cut power to the entire house then use the plugged in cord.   

[RocketMan]

Another bit of information:  Local propagation at AM broadcast band frequencies is all ground wave.  There is no ionospheric reflection of signals in the low HF bands until after local sunset.  That's when the ionospheric layers that reflect those frequencies become active and bounce AM band signals across the country.
Long antennas are good for both local reception of ground wave and ionosphere propagated signals.

[Cliffh]

I wonder if the same effect would be observed if the extension cord wasn't energized?

The same effect was not present when the cord was de-energized.  I placed the energized cable, noticed the difference, removed the energized cable, the radio went back to the original condition.  Replaced the energized cable and the enhanced properties came back.  Left the energized cable in place and disconnected from the power supply, the radio returned to the original condition.  Reconnected to the power supply and the enhanced properties returned.

The transmitting station is in Dallas - about 50 miles by road, and pretty close to that as the crow flies.  In a car, this AM station can be heard to the east on the far side of Tyler TX (~90 mi) and can be picked up in Midland TX (~330 mi).

The terrain between the transmitter and my place is fairly flat, a few small hills.  My place is surrounded by tall-ish trees, >50'.

I use rechargeable batteries in the radio, they had maybe 1/2 hour on a fresh charge.  It is a very cheap radio.

I was working outside again today.  There's a 50' extension cord buried* underground running from the workshop to a nearby trailer, with the last 7' or so above ground.  Since the radio wasn't receiving well, as usual, I thought I try using the extension cord booster again.

It didn't work.

In the first case, the cord was exposed for it's entire length lying on the deck about 3' above ground, oriented basically east-west, with the radio at the edge of the deck, on the east side of the house.  In the second case, the cord was ~1" below ground level with the radio on the ground and the cord oriented nnw-sse, at the end of the house with no obstacles (except the trailer 3' to the south) within 10' or so.  The transmitting station is due west.  The electric current came from 2 different power supplies, the house has a drop from the power lines to one meter; the workshop has it's own drop with a separate meter.

I'm not surprised it didn't work in the second case, since the cord was underground.

*The buried cord is a temporary situation.  One of the mowers is stored in the trailer until the new shed is built, the cord is for a trickle charger.  It's underground to remove a tripping/mowing hazard.  Trust me, it isn't going to be there long.

ETA: To add the trailer as an obstacle.

[Bogie]

What is this AM radio you speak of?
 
 
I've been trying to get a jazz FM station about 50 miles away to come in clearly... And I'm got amps...

[fifth_column]

The same effect was not present when the cord was de-energized. 

That answers that. I would have been surprised by any other result. Personally, I explain these types of phenomena as "the vagaries of the electron."