[Antennaware] EZNEC modelling

[Terry Conboy]

At 11:27 AM 2006-11-20, David Gould G3UEG wrote:

>Thanks for the input so far.
>
>Ed K4SB asked how I determine the L-match components..  I use the
>following website to match what the model gives me to 50ohms.  The
>network I use is the second down on the right.  You do need to watch
>the units ie frequency as 10.125e6 and the results give inductance in
>nanohenries.
>http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.html
>
>The antenna is a 48ft vertical, 2.5inches at the bottom tapering to
>2inches at the top.
>For the 10MHz example I am working on, that gives me
>A coil of  4.3uh  (X=274) from bottom of antenna to earth.
>A series capacitor 90pf  (X= -175) from bottom of antenna (top of
>coil) to feed line
>
>I just went ahead and modelled it in EZNEC (V3) as I
>described...  And it gave absolute rubbish...  (I am NOT blaming the 
>software!)
>The feed impedance was given as 220 - j18,000.
>I think this proves that my model is far from correct, but I don't
>know what to change to make it a true representation of my real life antenna.
>
>I am happy with the physical implementation of the antenna,  I just
>want to play about with the model so I can get some idea of what
>effect different coils (of different Q) and different earth losses
>make to the antenna efficiency and SWR bandwidth.

Dave,

My EZNEC model may be a little different than yours.  I used 3 
sections of 16 feet each with 2.5, 2.25 and 2 inch  diameters (64 
segments each), then inserted a 0.25 foot section of 2.5 inch (1 
segment) at the base where I place the source.  With no matching I 
got 425.5 -j 536.7 ohms (MININEC ground).

I put that into TLW, from the ARRL antenna book.  It produced a shunt 
L of 2.93 uH (Q=200) and a series C of 68.7 pf (Q=1000).  TLW says 
the loss in this network is 0.15 dB.

I added a 0.25 foot piece of 2.5 inch conductor horizontal and a 0.25 
foot piece to ground, where I placed the inductor.  The series C went 
on the base segment along with the source.  The impedance was now 
47.4 -j 4.5 ohms.  The resonance occurs at 10.175 MHz, where the 
impedance is 48.9 ohms.

One issue is that this antenna is very close to 1/2 wavelength long, 
and the impedance is very sensitive to segmentation, even without the 
matching network.

Hope this helps.  I'll forward Dave a copy of my EZNEC file.

73, Terry N6RY