Fred,
A very good analysis and well said for the novice reader. The
conclusions done nicely.
Jim
<fillinger@ameritech.net> writes:
> 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.
> The Europa Forum is supported by Aviators Network UK
> <info@avnet.co.uk>
>
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