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.
|
