Miles McCallum wrote:
> ...perhaps we could get this on a slightly more scientific (?) basis:
> having pinned my fin bulkheads in place last night, I was also considering
> antenna line routing: this raised the question of whether having the initial
> part of the line leading away from the dipoles (I'm planning on a std Europa
> installation) should be perpendicular, or does it not matter if it's
> parallel...
>
> So: a quesionaire
>
> What type of antenna
> location
> lead routing
> was a VSWR calibration done
> what was the final length
> what sort of subsequent problems occured
>
> all the best
>
> Miles
I'm using copper tape, mostly cuz Jim Weir (rst-engr.com) lost his '85
Oshkosh mug
and has full collection otherwise, and I had one so we traded for a
tape kit. Scientific enough? :)
True, but seriously, I believe any antenna location anywhere inside
the Europa airframe is compromised, since you cannot avoid any metal
part 1/2 wavelength or longer, within 1/4 wavelength of the ends of a
dipole as stated by Jim Weir. However, the compromise is not
necessarily fatal, in that only radiation pattern is distorted, but to
an unpredictable degree, Weir says.
Sidenote - you can make up for this by stuffing watts into the RG-58.
E.g., Weir makes a good case in a Kitplanes article that the 4-watt
Microair comm is wholly adequate, but this implies a real good
antenna, as much less than 4 watts puts you down to handheld xmit
power. And he doesn't say how good it will be if you're way down low
over hilly terrain needing ATC communication in an emergency. Not
criticizing anyone's choice of a Microair, but personally 7-8 watts is
less of a gamble.
Anyway, I vetoed the fin location because on the Mk. 1 fin, the foam
prevents a 30-deg cant of the dipole, necessary per Weir. But even in
the XS, you're still too close to the big pitch push rod. In either
case, I noted where one can mount tape externally -- covering over
with 2-oz. finishing cloth and SuperFil -- and use bends of the last
third of the poles to simulate the 30-deg angle (again per Weir), but
little distance from the pushrod was gained, and it placed the bends
more parallel to it which is worse. In neither case can you route the
perpendicular RG-58 cable from the center of the dipole a sufficient
distance (1/4 wavelength bare minimum -- 20.3" for comm) before making
a turn to parallel.
So I placed the tape in the rudder trailing edge joggle, with bends at
the extremities, all under the SuperFil. The coax runs thru the
internal foam to a female BNC. I call it the world's most expensive
comm antenna, with the rudder's allocated cost of the Stage 1 kit! It
may be annoyed by static buildup at times, but I'll just have to find
out the hard way but I doubt it.
VSWR calibration was beneficial due to the bends of outer third of the
poles (to fake the 30-deg cant of the poles per Weir). But in doing
so, the calculated length is off (and the ARRL Antenna Book hints
why). But the change wasn't much, just a couple tenths VSWR at one
end and little effect in mid-band. So if a true 'V' antenna is made
at specified length, it should work.
Placement in the fin should work enough, as it's the highest part of
the airframe, and there just ain't a lot of other choices. Placement
inside the aft fuselage isn't much compromised by nearby metal, as the
dipole ends would be perpendicular to the rudder cables and the pitch
pushrod, the latter being almost harmlessly near the center of the
dipole. But the absorption in front of the antenna would be brutal.
People in the seats absorb RF, as does all the engine room metal, and
a whirling carbon fiber prop (CF absorbs RF quite well) wouldn't be
good either. Fuselage side is OK I figure for the ELT, as it should
radiate up (suggests maybe a 45-deg -- 90-deg corner -- 'V' pointed up
as in the letter V ???).
Bob Archer's antennas avoid tuning problems, but if one balks at the
cost for multiple needs and has no VSWR meter, one can fake it close
enough. Mock up what you'll do with tape and balun on cardboard or
foam, and hook up a receiver. Drive to a location where reception is
really bad from a transmitter (126-128 ATIS for comm, 112-114 for
VOR). Twiddle length to tune, and be careful of the effect of your
own body or nearby metal. You'll probably find it's difficult to
tune, cause the effect won't be that dramatic due to the 'Q' of the
antenna we're working with. In the U.S. at least, err on being too
long for comm, to center it more toward 118.0, as 133.0+ is mostly for
IFR enroute.
Regards,
Fred F., A063
|