At 10:18 AM 6/14/2005, paul@w8aef.com wrote:
>For all intent and purposes the antenna is blind to everything behind the
>reflector. All you gotta do is get the reflector off the ground.
>
>de Paul, W8AEF
This is not true. The reflector has RF current flowing in it. Put that
element close to a dielectric and the current distribution and magnitude
will change because of the dielectric loading. It's no different,
conceptually, than using insulated wire or bare wire for a dipole. There's
an easily measureable difference.
Running a quick NEC4 model of a 3 element 20 yagi, over 13/.005 ground
at 20 m off the ground, the feedpoint impedance runs like this:
14.0 18.57-j3.01
14.02 18.2-j1.05
14.04 17.84+j.953
14.06 17.48+j2.99
14.08 17.12+j5.06
At 30 cm (1 ft) off the ground, it's like this
14 20.02 -j3.93
14.02 19.56 -j1.98
14.04 19.09 -j0
14.06 18.63 +j2.01
14.08 18.16 +j4.06
Sure.. there isn't a huge change in the SWR, and the resonance shifts by a
whoppin 10 kHz, but you are changing the impedance by several ohms out of
17-18. From a sending power from the transmitter standpoint, you'll
probably not see a huge change. A 3 element Yagi just isn't all that high
Q a device anyway.
However, if you're trying to adjust element lengths by looking for the
resonant frequency of the elment. so that you get a particular F/B, then
those small half ohm changes in the reactive component are important.
Running the same simulation, but looking for where the resonance on the
back element is.
at 20 m off the ground, the back element has a fairly noticeable resonance
at around 13.21 MHz
But, close to the ground (1 ft), the resonance isn't very pronounced, and
is somewhere around 12.5 MHz.
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