Following this discussion re antennas, I rechecked my copper foil
setup (comm) a bit more scientifically. It's installed on the rudder
trailing edge, with coax run fwd to a BNC connector in the LE
closeout. The rationale was placement as far as possible from noise
sources and interfering metal elements, and not installed on the A/C
is conducive to comparative test. It was compared to a 1/4 wave whip
antenna on a ground plane.
VSWR of both is 1.2:1 at 122.8 mHz (re below), and a using handheld
tuned to an Approach Control 60 statute miles away, smaller A/C at
2,000-3,000 AGL with probable sub-10W comms are weak, starting to
drive into AGC (auto gain control). This distance is the threshold of
line-of-sight, and the estimated rec'd signal strength per chart in
the "book" was probably a few millivolts. The performance difference
of the antennas was undetectable, but not really a good test. Though
inside an aluminum-sided house, this is very good.
So a VHF comm test rig that among other things will broadcast a
few-milliwatt signal on 122.8, with antenna collapsed to 4", was
placed in the far end of the house, making rec'd signal barely audible
in the noise. Again the difference was more clearly but basically
undetectable, favoring the dipole if anything, unless the 1/4 wave
whip was pointed downward (as predictable per its natural radiating
angle - xmitter in lower level of house - some antenna gain).
The foil in the rudder is in a sort of "C" shape, to approximate the
15-30 degree "V" as recommended by RST Engineering, and keep the lower
tip away from the rudder actuating metallurgy. It's radiating pattern
is not omni, and favored either side vs. fore-aft, but not by much.
Placing lengths of metal up to 6 feet long in various orientations and
distances from the tips of the antenna, it's effect is unpredictable -
either improving or degrading both VSWR and audible signal vs. noise.
They say it moves the resonant frequency and/or changes the radiating
pattern. Most significant was whether one's body is placed between
xmitter and antenna, causing total loss of the signal up to about 10'
---From antenna, an unavoidable situation in aft fuselage mounts - along
with engine and prop, affecting reception straight ahead, whether
comm, nav or xponder. Second was the routing of the coax. If it does
not run perpendicular from the center straight ahead for at least 1/4
wavelength before taking a more parallel route, there's problems.
Bob Archer says his "E" antenna is a "continuous conductor" dipole,
using inductance/capacitance for a balun at the center feed point,
otherwise a dead short. He claims in comparison that ferrite beads as
a balun (as in a two-conductor dipole) are both "useless" and cost "up
to 12db loss." That's over 90%, and it was clearly not evident in
above test. Whether the "E" shape is supposed to be beneficial beyond
compactness (Ferg??), I don't know. However, a quick mock-up of an
"E" shaped 2-conductor dipole did not produce a omni radiating pattern
either, but a satisfactory antenna also. Also, ferrite beads did
reduce VSWR, tested both ways, but not by much, though radiating
pattern appeared affected. It appears an otherwise long marker beacon
antenna can be "E'd" to make it more compact.
Made from 14 ga. solid wire, the mock-up was very "peaky" at one
freq. Archer's is wider than with 1/2" tape, and thus more
broadband. Though VSWR on mine runs from 1.4 to 1.9 at the high end,
intentionally so, to peak it nearer the most common VFR and tower
freqs here. Making it wider like Archer's will make it more
broadband, but little benefit at these VSWR's.
I attempted to test them in xmit, using a field strength meter on a
tether for equal distances. There appeared a bit of null fore or
aft. On the rudder, it was behind, the back end of the "C." On the
mockup, it was in front of the open end of the "E." However, the FS
meter wasn't sensitive enough for sufficient distance, and it was
obvious that my body was doing bad things. Another lesson in how a
dipole must be in "free space" as much as possible.
A dipole is a dipole, unity gain by nature, and omnidirectional
(depending). Likely then that Archer's is as good as a compact dipole
can be made, but copper tape can be equivalent, IMO. Especially if
you spend $1,000 like mine (estimated allocated cost of rudder in the
Stage 1 kit). :-) Either one can be greatly affected by installation
issues, though.
Excuse the long post and any technical inaccuracies, but I thought
this might be useful info.
Best,
Fred F.
|