Eznec rarely get the ground effects right. You need Nec4.2 engine to model that correctly.
too many years of modeling in the office and then going out on the grass and finding out
they dont match other than generally.
If you didn't measure the ground for the frequency your using an educated guess.
I've found that create errors and also the myth of the magic combination when its specific
to a installation. Also it tends to vary with season (wet/dry).
Also did you actually test the unun with a real antenna?
As to the unun doing what you intend that's not in question. It looks good. What is
the questiondoes the wire really behave at 2450 or 2800 ohms (nec model) which
is less than 14% difference. What if the wire is really 3300 ohms obviously the
1:56 will be better but not perfect. Yet 3300/2450 is only a swr of 1.34:1 and an
acceptable match.
I also said even if your wrong, to get to 2:1 you need to be wrong by a factor of 2
and that's a huge miss. What I didn't add is if you tweak the wire length you can
get to an acceptable SWR in some cases (usually with a higher complex impedance
and more RF on the shield). The trick for endfeds is a transformer that is in the
middle of the likely range or best exact which is not easily done.
For testing I use a bridge, and other instruments or an adjustable L network
(the later can match the wire. Then I put a variable resistor from the load
port and ground and adjust the resistor for SWR of 1:1 and that was the
feed point resistance of the wire. Why do both? often I have to lower one
end to measure and the other I can use a adjust and elevate (step and repeat)
to see the difference and often height is a factor.
Its more for those that don't understand that an end fed wire like a dipole has the same
variation in feed point impedance with height above a real ground.
I've included a white paper on how to make a resistive SWR bridge. fairly stock design for 50 ohms
but if you want to measure a real antnena you may want the bridge to be adjustable to find
out the antennas actual impedance. the design is a GQRP design and R1 determines the
"characteristic" impedance. Making R1 variable (about 5k for EFHW) and using a RF source
you can both find the resonant frequency and the actual resistive portion of the antenna
at its feed point. Note R5 should be in series with a larger choke to get a RF resistance
of about 15k ohms. [its to get the metwe out of the RF circuit.] Failure to do that will have
1K across the antenna terminals by default and create errors. Power to test in in the less
than a 100 mW range and depending on meter sensitivity 10mW may be enough. First
set the R for about 2 to 3K then adjust the RF source for null response (very low indication)
then adjust the resistor to minimize the null while adjusting the RF for the best null. The
result will be the resistor is the load resistance and the null frequency is the actual
resonance of the wire. The pot needs to be non inductive.
Usually standing on a ladder doing that, less then fun if you like me and height adverse.
Reason is to get the antenna up and also a short length of coax. a decade of antenna
design work and testing left me with a large bag of tricks to get the data and confirm
the result.
The common Tayloe design using a led may work were the R1 resistor is adjustable but the
led only lights with a lot of RF and is insensitive for this use. A meter could be added for
better null sensitivity.
When measuring high impedance or low I use the HP4191A and a bridge like the
one described to get answers as accurate as the HP (to about 4 digits.). I also
have a 50mw DDS RF source as the way to drive measurements. A SA like the
Rigol 815T or any with tracking generator can be used to make that measurement
using a resistive bridge. Experience says better than 8357A PNA without a
calibrated matching transformer and math to get the real impedance (most PNAs
do poorly at high resistive loads without fixtures).
Allison
