LF: Folded monopole - Food for thought

[Andy Talbot <andy.g4jnt@gmail.com>]

On the RSGBTech group recently G3RZP described his folded monopole topband antenna.  he runs an insulated wire up the side of the lattice mast to which it is bonded at the top.  The bottom of this wire becomes the feed point and requires a matching unit to tune out the residual inductance of the resulting loop - plus impedance transformation..  The bottom of the mast is grounded, and connected to a radial mat.

The advantage of this arrangement is it allows a conventional mast to be used as a radiator.  The feed is nominally inductive, being a think squashed loop, but the height of the mast stays what it would have been if fed conventionally.  An HF beam and sundry other stuff on the top acts as a capacity hat.

Disadvantages relate to isolation of cabling for equipment for other bands' use, and needn't concern us with its use here.

I wonder if anyone has tried this scheme on MF or LF over the years/ (I know topband is MF, but you all know what I mean :-)   

Being able to feed a short monopole using capacitive tuning instead of a conventional bulky, complex-to-build adjustable loading coil must have its advantages.

It may also make for a quicker to put together portable of fast up- fast down system, or ad-hoc antennas?

Some time ago one of the well thought out more rational arguments about magnetic loop antennas was that when they become larger than "very very small" , the linear dimension across the loop then starts to behave as a dipole with appreciable addition to the true loop operation.    here we have a loop, but deliberately elongated to enhance the dipole (or here the monopole) behaviour.   

Consider the ultimate limit, a tall twin feeder, shorted at the top and fed at the bottom, with one side grounded.

An idle thought...

Andy  G4JNT