Can anyone explain this?

I'm running a radio with a tuned loop antenna directly connected, plus a random wire of about 6metres on the antenna positive.

The set is earthed from chassis to the 240v AC power supply earth pin.

The other day, I plugged a 3-core extension power flex into another socket on the same AC switch. The other end of the flex was not connected to anything and the flex was just straggling along the ground for about 5 metres.

I noticed that when I plugged the extension cord in, the radio on that same power circuit exhibited a volume boost of about 3db, which returned to normal when I unplugged it.

I can't see the insulated 3-core flex acting as an extra earth connection, and it is itself connected to earth via the mains plug.

I speculated that it might be forming some sort of dipole with the random-wire antenna, via the chassis.

Any other experiences or informed views on this?

Maven

It wouldn't be the only radio where touching earth made it louder,
I have one here, touching earth or antenna makes it louder (though you're not saying you touched it).

Does it have to be connected to earth to be an earth?
isn't that what baluns do, and other cheating antennas?

There are only Hertz antennas and Marconi antennas as far as I know,
they all fall into either category.

A balun (BALanced to UNbalanced matching transformer) is to match balanced, such as the old tape TV lead, feed line to unbalanced feed line, such as coaxial cable. In the case of TV tape it was balanced 300 ohm; TV coax is unbalanced 75 ohm.

TV antennas which are almost always dipoles and hence balanced, also need a balun to match them to coax.

Early TV VHF antenna systems did not need baluns because the antennas were designed to match 300 ohm tape, as were the TV antenna inputs. Baluns for TV came in with coax needed for UHF.

For TVs, if a balun isn't used the impedance mismatch point reflects the signal causing ghosting and signal loss.

The balun accommodates the impedance difference (the ohms part) as well as the balanced/unbalanced part (tape/coax).

A rough equivalent of impedance matching is the gearing on a bike or car. A high gear going up a steep hill would be a poor impedance match.

For our old radios, a bipole antenna would need a balun to match it to coax, if you want to use coax to minimise noise pick up between the antenna and the radio. A balun might also be needed to impedance match the coax to the radio antenna input. Our old radios mostly have an unbalanced input suitable for a single length of wire, except some upmarket radios which will have A1 and A2 aerial inputs suitable for two wires ie a dipole.

Car radio antennas, being a monopole, are unbalanced and can be connected directly to coax. In this case I think the coax is 50ohm.

Hi Maven,

... forming some sort of dipole with the random wire antenna .... Yes, I think this is correct .... or in other words, forming a better ground plane for the random wire antenna than natural earth.

Do you have both antennas connected at the same time??
If so, it is possible for one of the antennas to radiate some of the signal. Would be interesting to try them separately ... and with and without your extension lead plugged in.

Cheers,
Ian

Point is all AM radios of the day I've seen are designed to be grounded (Marconi antenna),
and Dipoles are insultated from Earth on both elements (Hertz antenna),
so as long as it's connected to Earth,
and again, I'm only learning, It cannot be a dipole antenna.

I would expect it to just as easily causing the loop to resonate at different frequency, a change in overall capacitance has occurred,
and only a dipole antenna if indeed the Earth is disconnected behind the mains sockets.

Forgetting Loop antennas .....
Element type antenna provide the capacitance for the LCR 'antenna' circuit
.... capacitance requires 2 'elements'
.... 1 element is the log wire, the other element is natural earth.
Theoretically, there is really no difference between dipole or long wire antennas .... the wire being 1 element and natural earth is the other element, sometimes referred to as a ground plane or virtual pole depending on the physical structure.

Cheers,
Ian

Only observation I can make is that the earth of the set is via the earth of the power lead i.e the household earth.

How good is the household earth and how how much wiring is involved before the earth gets to the radio itself may be a factor.

Maybe try putting in a separate earth rod into the ground with a wire running to the earth terminal of the radio, and keep this earth wire as short as possible.

See if this makes a change one way or the other.

Some household earths can be a bit dodgey at times.

Thanks - a few ideas here to ponder.

Until recently, this radio never had an earth connection. the AC supply was 2-core, and the nominal earth wire wasn't connected. During other servicing I installed 3-cored power flex and earthed the chassis through it. The earth wire from the antenna coil doesn't connect to anything, though the coil does earth to the chassis. Earthing via the power plug did improve signal, so the household earth must be at least partially effective, unless it is just the length of wire between the power point and the main switchboard acting as a ground plane.

I'll try setting up a separate earth rod. I won't be very close, though, as the workshop sits on reinforced concrete and I'll have to run a wire some metres to reach a natural earth.

I'll still have an antenna wire basically vertical, and an earth wire basically horizontal - so some dipole effect should be present I guess. Transmission wave polarity relevant?

Maven

The soil does impact on the earthing, I was lucky at the previous dwelling albeit, there are opportunities here, the earth was tied to a bore casing that went down 123 feet into 80feet of brackish water.

Signal can come via the ground.

A bore as described would be an excellent earth, the best that could be gained.

At my last place of residence had a few feet of copper pipe bashed into the ground and this pipe was placed under the overflow outlet of a rainwater tank.

The ground was always moist. The best earth I have ever had.

The SEC when it put down the old single wire SWER here (now two phase) they placed a grid in the ground.

Fortunately they accidentally put it in an old creek bed. I have a natural shallow under-ground stream out side the shed & the main fuse box is on that shed.

Being able to divine water I will have no issues in driving a spike into it if there are earthing issues. The plumbers did help by digging into it when putting the septic tank in, so I know where the soft ground is.

Marc

I meant to say counterpoise, no BALUN.
I don't see why (if the mains Earth is far away),
the extra wire near the ground couldn't become a counterpoise.

If you want an Earth stake Maven, Bunnings have a copper coated steel stake for $11
ready made to drive into the ground with a hammer.
Then you can water the earth and bash it some more,
and finally if you cannot salt the earth because it's near a garden you can piss on it.
I got mine in nearly a metre.

All you need is a clamp for the wire connected to it.

Then you can water the earth and bash it some more, and finally if you cannot salt the earth because it's near a garden you can piss on it.

No need for water, salt or wee-wee, unless you live in the Great Sandy Desert. Those earth stakes are designed to provide a satisfactory earth for electrical systems. They'll do the job for a radio no worries.

All you need is a clamp for the wire connected to it.

If Bunnings doesn't have the correct clamps you can get these at any electrical wholesaler for a few bob.

I was lucky there is a well watered garden right outside my window.
I get about double the signal (as well as double noise).

€�... forming some sort of dipole with the random wire antenna ....”

This rang a bell and have found the reference at last in “Radio and Television Reference Book”, an English publication. Can’t do any better than quote verbatim.

“Aerial Earth Systems
Short vertical aerials with their lower ends at, or near ground level, behave towards the incoming wave as though a reflected extension to the aerial existed beneath the aerial. Connecting the input circuits of the receiver between the lower end of the aerial and an earth plate buried in the ground will allow the aerial earth system to develop the greatest signal potential.

It is important to recognise that in such an aerial-earth system the earth corresponds to the other half of a centre terminated doublet or short aerial. If the earth connection is omitted, the aerial can only function through the effect of the capacitance between the chassis and the ground; and this being relatively small, will offer an appreciable reactive impedance to the flow of the signal frequency currents. It (the aerial) will thus be less effective than if a low impedance, direct return were used (ie a good earth).

It will be seen that the receiver is virtually connected to the centre of a short aerial in all vertical aerial-earth systems and therefore at appoint of low impedance. It is necessary to keep not only the reactance but also the resistance of the earth lead and earth plate to the lowest possible value to avoid loss of signal. Short direct connections to a cold water pipe is generally effective, if it can be made where the pipe enters the ground. Hot-water system piping and electric wiring earth returns are less effective, and are to be avoided.

If a water pipe cannot be used, a copper earth rod or plate driven deeply into the ground which can be kept reasonably moist, will prove to be very satisfactory.”

So your long wire connected to the earth terminal provides enough capacitance between it and the aerial to improve the signal.
Other important points are that medium wave broadcast signals are vertically polarised and travel horizontally, parallel to the earth, and only the vertical section of an aerial picks up signal. All the horizontal section does is provide capacitance to earth to reduce impedance of the aerial. So no good earth, less capacitance, less signal; and the higher the aerial the better.
At night when signals are also reflected off the ionosphere, the signal comes in at an angle and the horizontal part of the aerial also contributes to signal pick up, but also results in fading as it competes out of phase with the signal coming in horizontally and picked up by the vertical part of the aerial. So if fade free reception is wanted a vertical aerial (like a car aerial but much higher) is best.