up in the roof and attached directly to it. Using a MFJ
analyzer we found
out that the antenna tunned correctly buy we got a very low
impedance (about
20 ohmns) so we assumed that the performance wasn´t as
expected. Tried a
couple of quarter wave radials that didn´t have any effect
on the antenna
(not on SWR neither on impedance...strange, isn´t it?). We
decided to take
the antenna down and mounting at ground level. Installed the
2 quarter wave
verticals (on of them elevated) and we got the perfect 1.1
with an impedance
close to 35 ohmns and an efficiency of 99% (acording to the
antenna
analyzer)>>
When we did the MFJ 259 analyzer I was very much opposed to
including "match efficiency". I was strongly opposed to
including it because most people would not have any idea
what it meant. My fear was they would assume very wrongly it
was the efficiency of the system or the feedline.
Match efficiency is nothing more than a way of expressing
SWR. The match efficiency can be 1% and the antenna and
feedline efficiency still be 99%. It is a useless number by
itself in the systems we use, and you should just ignore it
unless you understand what it really means.
As a matter of fact when I wrote the manual I made a great
effort to tell people very clearly it just is another
expression for SWR and does not indicate system or feedline
efficiency at all.
Base resistance also does not have much to do with antenna
efficiency. That is another old Ham myth everyone believes
works every time. I can have a ground system with terrible
efficiency and it might make the antenna impedance look
perfect at the feedpoint. I can have a very good system and
it might look very unusual at the feedpoint, either high or
low. I've seen this effect dozens if not hundreds of times
over the years.
I can build a ground system with terrible efficiency that
has low impedance, and build a good one with high base
impedance. For example W7EL and myself recently carefully
measured the base impedance of a 40 meter 1/4 wave vertical
and measured field strength. It was 35 ohms or so. We then
changed ONLY the ground system and had a new base impedance
over 50 ohms, and the field strength was much better for the
same exact power delivered into the antenna.
If we want to know the field strength or efficiency of an
antenna we must somehow measure field strength. There really
is no other choice. We cannot look at our DXCC total, our
contest score total, the feedpoint impedance, if we won the
pileup, or state of our physical stimulation to determine
efficiency.
You did a good job by comparing two antennas if they are
physically far enough apart and if you are comparing them at
the distance and directions you want to work. You should
have done that with the antenna in the first location also.
All the impedance meters really won't tell you what you want
to know, other than if the SWR is acceptable for the
equipment.
73 Tom
. After this, we connected the coax to the radio to make a
side by
side comparision against my half wave dipole for 80 at 33
ft. The
propagation wasn´t quite good so I didn´t have any reply
from outside of
Europe. Comparision?...well the dipole beated the vertical
in about 2 S
units every time. In those cases when the signal got over 9,
I had a 59+10
on the vertical and 59+20 on the dipole. The only advantage
I have seen is
that the vertical is less noisy than the dipole. OTOH I´m
still trying to
figure out how the hell the vertical doesn´t work out over
the metallic
roof. It could be that we got so much ground
plane?....Wondering if
isulating the antenna from the metallic roof and adding a
bunch or radials
would let it work out. It would be a perfect location far
from the
neighbour´s view...Any suggestions?. 73s all
EC1CT Fernando
E-mail: ec1ct@ure.es
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