Hello Alberto and group,
A short vertical monopole has a gain of 4.77dBi / 2.62dBd because of its
radiation pattern, especially the rather 'sharp' vertical pattern.
This gain of 4.77dBi assumes a perfect ground, so no losses at all.
I believe that there is a lot of confusion between the terms gain (or
directivity) and efficiency :
Gain (directivity) is what a certain antenna 'wins' because of the
directivity in the radiation pattern. The reference antenna is a so-called
isotropic radiator, a non existing antenna that will equally radiate in all
directions. Any 'real world' antenna will have a certain directivity and
thus a gain over this isotropic radiator (so you cannot have a negative
gain in dBi).
Efficiency is another thing, it is the ratio between the power you put into
an antenna system and the power that is radiated. Assume you have a short
vertical with a radiation resistance of 0.05 Ohm and the antenna system has
a loss of 50 Ohm then the efficiency is 0.1% or -30dB. Any real world
antenna will have an efficiency of less than 100% (or less than 0dB).
Yet another thing is the effective radiated power, this is the product of
gain, efficiency and the power you put into the antenna system. So if you
put 200W (= 23dBW) in an antenna with a gain of 4.77dBi and an efficiency
of -30dB the EIRP will be -2.23dBW (600mW), the ERP will be -4.38dBW (365mW).
This is the theoretical (calculated) ERP, as mentioned in previous mails
the real ERP will be most likely 3-6dB lower. For small antennas in an
urban or forrested environement the difference might be even more.
In for the above expamle one could expect a real ERP of 180mW in the best
case, 50mW or less in the worst case.
73, Rik ON7YD
At 10:32 1/10/01 +0200, you wrote:
Rik Strobbe wrote:
[...]
The gain of a short vertical is 4.77dBi or 2.62dBd.
[...]
Hi Rik and the group,
maybe today I am denser than the usual, but, how can a short vertical
monopole have a gain when compared to a dipole (I suppose here a lambda/2
dipole) ?
I would have intuitively thought that a short vertical monopole has a loss
wrt to a full size dipole. Please enlighten me, thanks.
73 Alberto I2PHD
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