The problem with a monopole at LF is that the effective height is only half
(approx) the physical height. For an inverted L where the top length is at
least twice the height, the effective height is close to the physical height
.....so 3dB more ERP. It may not matter so much on 160m (I used a 16foot
centre loaded "tank-whip" at one time :-)) ) but where ERP watts are
difficult to come by, it IS important.
I believe one of the US "Lowfers" used a "gamma matched tower" but I cant
remember who now.
One has to be careful with NEC at LF it only models the things that are
built into it. There is a QEX article this month purely simulation based. It
will be fine installed in a 2acre field but not for a vertical on a normal
"back lot" There are two major effects (for LF) it doesnt seem to consider,
ground strata/skin effect, enviromental losses, and the effect of
top-capacity. These are much more dominant at long wavelengths.
Alan
G3NYK
----- Original Message -----
From: "Clemens Paul" <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Friday, August 14, 2015 9:20 PM
Subject: LF: RE: Folded monopole - Food for thought
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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