[TowerTalk] TA-33 Jr. SWR curve

[Jim Lux]

At 10:18 AM 6/14/2005, paul at 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.