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
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