Hi again,
With my vertical triple bander up on the roof deck, have noticed
that the vertical element and radials need to be a bit shorter
to resonate within the frequency ranges of interest.
For example, manual values for the 10 meter vertical,
when mounted on a pipe 10 feet above ground are:
Vertical element length: 8 feet, 6 inches
Resonant radial length: 8 feet, 2 inches
Presumably, since the radials are to be terminated about
1 foot above the ground, and are "loaded" via reactance
coupling from the ground, they are physically shorter, but
electrically the same length as the vertical element going
on to some 18 1/2 feet up at the tip end. Or is this just
backwards?
Now, when I move the entire affair onto the roof, with the upper
tip now about 33 feet high at the tip, the lengths must all be
shorter by a few inches to come back to the same resonance,
if I used my MFJ 259B correctly. I suppose because of the
decreased reactance impact with the soil ground further
away below.....correct?
Also noticed I now found two resonant frequencies at the outset:
one below the 10 meter band, and one above!! The one with
the broader 2:1 vswr band was the lower, so I assumed that
was the "real" one. Therefore shortened the elements, both
vertical and radials to bring the resonance into band. . I suspect that
the "other" resonance may have come from the push up pole
holding the whole affair up off the roof! The push up pole, sitting
on the shingles, measured up to the bottom of the plate to which
the radials attach is about 13 1/2 foot long, could this also
resonate with the vertical element at the higher frequency??
Now seeing X=0 at three frequencies: 27.85 (R=22 ohms);
at 28.58, R is 33 ohms, X is 26 ohms, swr 2.1:1;
at 29.24, X=0, R is 92 ohms, swr 1.7:1; and up at
30.52, X again is 0 ohms, with R=15 ohms, swr 3.1:1.
Probably need to fiddle with the element length and radial
length and angle down from the vertical. Angle is now
very steep; nearly, but not quite, straight down!!
Saw exactly the same phenomena on 20 meters, but not
on 15 meters! Again had to shorten the 20 meter element
and radials to get back up into the band for low vswr. Now the
20 meter band seems to behave as a double resonant
circuit; with roof mounting, the tip of the 17 foot long 20 meter
element is up at about 42 1/2 feet above the soil surrounding
our home.
At 14004, vswr=1.0:1, R=52, X=0
R increases gradually to 61 ohms, while X remains 0 ohms
all the way up to 14350, and even higher. R then gradually
decreases back to 54 ohms at 15 MHz, where X is 0;
above here both gradually increase as the resonant
area is left! The reactance term remains equal to 0 ohms all
the way from about 13.950 to 15.4 MHz, where R has reached
85 ohms and the vswr is 1.6:1.
Ten feet above ground mounting for the 20 unit called for a vertical
radiator 17 feet long, with the radials extending down at an angle
to a a tie off point about 5 feet above the ground; the resonant
radials being about 16 1/2 feet long, again a bit shorter because
of the ground proximity compared to the 27 foot height of the
vertical element tip.
With my roof top set up, these lengths are again shorter by some
inches (did not measure carefully, just got the thing up into
the band as numbers show above, where things sit now.)
I have no idea what the efficiency of this thing is going to be
with only the two elevated, roof top radials per band. Thus
my earlier note and question about added roof lying
copper radials, perhaps in place of, or in addition to
the set up described above.
Won't bother you with the 15 meter numbers, but it looks pretty
good vswr, etc. wise. Again, don't know about the efficiency.
73, Jim, KH7M
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