What about mismatch loss?
Assuming 50 ohm coax, the vee should be closer to 50 ohms.
The 73 ohms of the dipole would result in a small mismatch loss.
One could droop the dipole down about two-thirds along the way to get
close to 50 ohms - at least that's what my EZNEC says - been a user
since the first version.
Roy Lewallen wrote in message ...
Since this isn't true except under particular circumstances, I'd hate to
see it become an unqualified rule of thumb. For example, here are some
values for the total ground loss from an inverted vee and dipole, with
the apex of the inverted vee the same as the height of the dipole.
Modeling was done at 14 MHz over average ground.
Ht (ft) Inv Vee Dipole
12 5.77 2.05
20 2.08 1.18
30 1.47 1.18
40 1.65 1.4
45 1.72 1.39
50 1.63 1.27
As you can see, the difference is less than 1 dB except for the lowest
height.
At an apex height of 12 feet, the ends of the inverted vee were about 2
inches off the ground, so that was as low as it could have been
constructed with a 90 degree apex angle. Interestingly, the loss of the
inverted vee reached a maximum at 45 feet. The dipole did likewise, at
about
40 feet.
I mentioned that an "equivalent height" for a dipole is 1/3 of the way
out from the center. This is based on the current distribution, and when
a dipole is put at that height, it has about the same elevation pattern
as the inverted vee. Putting a dipole at the "equivalent height" of the
lowest inverted vee (i.e. 8 foot high dipole) shows 3.55 dB ground loss,
still 2 dB greater than the inverted vee at 12 feet.
Scaling for 3.5 MHz shows 4.5 dB for an inverted vee with apex at 48
feet and ends about 9 inches above the ground, vs 1.35 dB for a dipole
at 48 feet. So the difference is just slightly less than the
corresponding antennas at 14 MHz.
My conclusion is that the rule holds only for antennas very close to the
ground. But the actual difference will depend on the frequency and
ground charactersitics as well as the inverted vee's apex angle.
Anyone wanting to compare particular antennas can easily do so with
EZNEC. The free demo program is adequate. Simply specify Real, High
Accuracy ground and a 3D plot. Make sure wire loss is zero and there are
no lossy (resistive) loads in the model. Then run the 3D plot and look
at the "average gain". It'll represent the total ground loss. For the
values above, I used example files BYDipole.EZ and BYVee.EZ. In both
models, I changed the ground type to Real, High Accuracy, the ground
description to Average (0.005 S/m conductivity, 13 dielectric constant),
and the plot type to 3D. Height was then changed as required using the
Change Height feature in the Wires Window.
Although this is academically interesting, I don't see any immediate
practical use for the information. It doesn't tell you how strong a
signal your antenna will produce in a given direction, which is usually
what you want to know.
Roy Lewallen, W7EL
Reg Edwards wrote:
"Matthew and Wendy Plante" wrote -
Is there a significant difference in gain between an
inverted Vee and a horizontal dipole antenna?
===========================
Gain is not an appropriate term to use in a performance comparison.
The power radiating efficiency of an inverted-vee is typically 3 to 5
decibels worse than a horizontal half-wave dipole at the same height as the
apex of the vee. Less than an S-unit. This is due entirely to extra loss
induced in the soil under the antenna, the average height of the wire in a
vee being roughly half the height of the dipole.
. . .
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