No, Chris, you are quite correct. The monopole over a
perfectly conducting ground has 5.15dBi gain. Both
antennas patterns have the same shape in the elevation
plane, only in the case of the monopole, half the pattern
is missing. Assuming 1 watt input power, the 1/2 watt
of radiated power that illuminated the lower hemisphere
of the free space dipole in the case of the monopole
is imaged by the ground plane into the upper hemisphere.
Thus, the field intensity in every direction in the upper
hemisphere of the monopole is twice that of free space
dipole, hence the 3dB increase in gain. This 3dB gain
manifests itself in the radiation resistance of the monopole
which is 35 ohms or 1/2 of the 70 ohm radiation
resistance found with the free space dipole. Thus
for the same input power, the monopole exhibits
1.41 the base current, 1.41 times the field strength,
and twice the field intensity of the free space dipole.
Kinda cool how it all works out.
73 de Mike, W4EF.............
 Original Message 
From: "Chris Adams" <n4vi@arrl.net>
To: <towertalk@contesting.com>
Sent: Monday, March 11, 2002 9:14 PM
Subject: [Towertalk] reflection gain addendum
> The more I thought about it, the less I liked a vertical in free space
> as there is no ground in free space to drive the vertical "against".
> Also in my previous comparison the power delivered to the antennas was
> not adjusted to be equal.
>
> I redid the experiment with a vertical over perfect ground compared to a
> vertical dipole in free space. Both antennas were adjusted to resonance
> at apx 7MHz and the vertical was fed over perfect ground with 1amp
> current. The current in the vertical dipole was adjusted such that the
> power delivered to both antennas was the same (0.715amps, probably
> should be 0.707!). This yields the following results:
>
>
> 1. max gain of vertical with perfectly conducting ground (which
> occurs at zero degree azimuth) = 5.15 dbi
>
> 2. max gain of vertical dipole in free space (which also occurs at
> zero degrees aximuth) = 2.13 dbi
>
>
> I believe there's a weakness in this argument... my suspiscion is that
> it's related to setting the power delivered to both antennas as equal.
> That said, I can't figure it out at the moment and it sure seems the
> power should be equal, after all your transmitter doesn't know what
> antenna is out there!
>
> Oh well, when I get it figured out, if anyone still cares I'll post it!
> This thread may have gotten a little esoteric anyway.
>
>
> 73's
>
> chris, n4vi
>
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