Hello Andy,
I seem to remember that NEC modelling done by a real expert in this area,
W4RNL (sk), years ago revealed that a 'linear' (ungrounded) monopole of the
same height as a folded
monopole has a significant higher gain if the height is below 60 degrees (0.17
lambda)
(excluding losses in the matching circuitry).
But I've never modeled these antennas myself.
73
Clemens
DL4RAJ
>-----Original Message-----
>From: [email protected]
>[mailto:[email protected]] On Behalf Of Andy Talbot
>Sent: Friday, August 14, 2015 5:52 PM
>To: [email protected]; [email protected]
>Subject: LF: Folded monopole - Food for thought
>
>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
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