[Argus25]

I could add some remarks about loops.

Firstly there are loops for receiving and loops for transmitting and there are two types of loops, large air cored loops and small loops on ferrite rods.

Then of course there is the frequency range over which the loop is operating. For air cored loops for > 20 to 30MHz, single turn loops of a good size like 1m diameter or larger can suffice, but for MW loops need at least 5 to 7 turns with a tap at one turn for receiving, or injecting a signal if transmitting.

In any case this is what I have learned over many years of making loops for transmitting and receiving:

Receiving loops respond to the magnetic component of the transmitted EM wave. Far off from the station (greater than a few wavelengths) the ratio of the electric to magnetic field is about 377 volts per amp. An air cored or ferrite cored loop antenna are about equally effective in receiving. Because they respond to the magnetic (H field)component of the EM wave, they tend to ignore noise voltages of the E field. This is why pocket transistor radios appear to work so well with a small ferrite rod.But the rod is very directional.

The noise immunity effect is improved for an air cored loop if it has an electrostatic shield to shield the E field (hence the value of the coax as suggested by RadioWrangler) But for this, for MW band, the 5 to 7 turns need to be close together to apply the electrostatic shield. This effect can be exploited for any radio including valve radios (see below)

For transmitting on the MW band, it is another story. With a loop transmitting antenna, the 377 doesn't apply in the near field. The electric component of the EM wave (H field) is very weak, the H field strong and the E:H ratio reversed .

In the near field, for pantry transmitters, say less than 100 meters from the transmitter , when the wavelength at one MHz is 300m, the air cored loop antenna is great if you are transmitting to a radio with a magnetic response, ie a radio with a ferrite rod or a frame aerial or loop antenna. If it is a wire antenna on a valve radio, forget it, the result is poor. The reason is, in the near field, the electric component (E field), from a loop antenna is very very low and the magnetic component (H field)high.

The above means that if you are transmitting from a loop antenna from a pantry transmitter, and you want a good result with a transistor radio with a ferrite antenna you are fine.

But if you have a valve radio receiver you are in fact better to rig up a tuned ferrite antenna (on the frequency of interest) with a Tap at about 20% of the earthy end to feed the radio's antenna input. This also gives excellent noise immunity in conjunction with the loop transmitting antenna.

If you don't do that you are better to couple the output of the pantry transmitter to a long wire and receive the electric component of the field, with a wire antenna on the valve radio, but there will be a lot of extra background noise / interference this way.

Finally, although ferrite cored loops make excellent receiving loops in the far field, as good as a medium sized (say 1m diameter) air cored 5 turn loop, compared to the 1m dia air cored loop they make hopeless transmitting antennas. The primary reason for this is the small cross sectional area of the ferrite cored loop (compared to a large air cored loop)making the radiation resistance very low.

But for receiving the ferrite rod antenna is excellent for either a near field signal from a loop transmitting antenna or a far field radio station, and you get the benefits of noise immunity. This is why even your average valve radio will benefit from a tuned ferrite rod (on the station of interest) with a tap feeding the radio's antenna input, in terms of the signal to noise ratio.