LF antenna gain]

[Andre Kesteloot]

Rik Strobbe wrote:

> Hello Walter,
>
> You are raising a question I have been thinking about for quite some time.
> Even after discussion with some antenna specialists I didn't come to a
> conclusion.
> You are right stating that the gain figures you mention for a short
> vertical (4.77dBi or 2.62dBd) is based on the directivity (as for any gain
> from any antenna). And this directivity is caused by interference of the
> signal radiated 'into the air' and reflected by the (perfect) ground.
> In fact a vertical monopole can not radiate without some form of
> 'counterweight', wether it is the ground, some form of radial system or
> even just the shielding of the coaxcable it is fed through.
>
> Some time ago I did a simple test : I built a small battery operated xtal
> oscillator, using a CMOS gate (7400) and a 3.579MHz xtal running on a 9V
> battery. I did put the whole thing a a small plastic (not metal !) box,
> connected a 1m piece of wire (= antenna) to the oscillator output and hung
> it on the brach of a tree (few m high) at about 100m from the shack. Using
> my TS440 and a 1m wire antenna I could barely hear the signal. Next I cut
> 50cm of wire from the oscillator's antenna and connected it to the minus of
> the battery and hanging down, as counterweight (in fact making a kind short
> vertical dipole).
> Now the signal was very clear, I assume at least 10-20dB stronger. Changing
> the position of the counterweight wire from vertical to horizontal made the
> signal sleightly weaker (few dB) and adding a number of additional
> horizontal counterweight wires increased the signal again.
>
> I believe this confirms the fact that a vertical monopole is not much of a
> radiator without a counterweight.
>
> Now returning to your question : what is the gain (directivity) of a 'real
> world' short vertical monopole ?
> As you mention only a very small part of the transmitter power is radiated
> (typical between 1% and 0.01% of the power for amateur sized antennas).
> But this means that the part that is radiated 'sees' some kind of
> counterweight. Unfortunately is it impossible to know the shape and
> dimension of this counterweight and I believe that the directivity of the
> antenna just depends on this.
> For example : assume you have a full size (half wave) vertical dipole. This
> antenna will have a gain of 0dBd (obvious) or 2.15dBi. Further assume that
> the lower half of the dipole is a multi-strand wire and you 'untwist' all
> the strands. Now you get a lot of parallel wires, but nothing will change
> on the antenna behaviour. Next you start to tilt all the parallel wires,
> creating a kind of Ground Plane antenna. Assuming the number of wires would
> be endless the antenna directivity would slowly increase from 0dBd/ 2.15dBi
> (all wires vertical) to 3dBd /5.15dBi (all wires horizontal). This last is
> the gain of a quarter wave monopole.
> So in practice the directivity of a short vertical monopole over an
> imperfect ground will be somewhere between the gain of a short vertical
> dipole (1.77dBi / -0.39dBd) and the gain of a short vertical monopole over
> perfect ground (4.77dBd / 2.62dBd).
>
> That's the way I see it, no claims to be 100% correct. But I hope it helps.
>
> 73, Rik  ON7YD
>
> At 10:04 25/01/02 +0000, you wrote:
> >Could one of you experts help me with the following please:
> >
> >A short vertical monopole antenna over perfect ground has a gain relative
> >to isotropic of 4.8 dB.
> >A half-wave dipole in free space has a gain rel. isotropic of 2.15 dB
> >Therefore, a monopole should have a gain of 2.65 dB over a dipole.
> >So the theory goes............
> >
> >But look at the qualifier on the short vertical gain - it has to be
> >operating over "perfect ground". No amateur has "perfect ground"; at least
> >not that I am aware of. I haven't heard of anyone laying out 36 radials 550
> >metres long under his antenna (not even G3KEV.......yet!)
> >
> >So nearly all the energy that goes into the ground is dissipated and does
> >not return to the feedpoint.  Therefore it cannot reinforce the radiation
> >pattern.  In that case, does the theoretical gain still hold?
> >
> >Gain is only obtained from directivity.  Directivity can be calculated from
> >physical considerations but the equation to obtain gain from directivity
> >is  G = e*D , where G = power gain, D = directivity, and e = radiated
> >power/total power.  The "gains" quoted above are actually theoretical
> >directivity figures but they assume that e = 1, that is, that there are no
> >ground losses (as the definition states) and that accordingly gain is the
> >same as directivity.
> >
> >Not so in an average amateur situation, where e = 1/1000 (1w radiated for
> >1000w input) so G = 0.001*4.8 = .0048 dB. In other words, the average
> >amateur LF antenna is no better than isotropic.
> >
> >Or should I be ignoring earth losses and only counting copper losses?
> >
> >Walter G3JKV.
> >
> >
> >
> >
> >
> >
> >
> >