>
> ...it became apparent that although the radio worked perfectly at
> the lower end of the VHF band, when we selected frequencies
> above 125.0 ground stations were having difficulty in receiving us.
> I borrowed a Bird watt meter today and came up with some rather
> bizarre results. The radio (Walter Dittel) delivered 5 1/2 watts
> throughout the range into a dummy load (50ohms), but as soon as
> the Europa half wave dipole aerial was used very high readings were
> obtained at either end of the spectrum (11 watts at 118.0 and
> 9 watts at 136.0) but down to 3 watts between 126.0 and 130.0.
> ...I used a quality VSWR meter....
Hmmm. We have 5.5 watts flat (impressive on the flat part) out the
back of the box, into a dummy load. With antenna instead of the
dummy, we have whopping watts on extremes, and a sharp dip over just a
few MHz of the band, at which ATC can't read you. This sounds like
your antenna installation is a concoction of resistance, inductance,
and capacitance in a family squabble, depending on frequency. A VSWR
meter can't see this well at all. It sort of reads the total "complex
impedance," the total cacophony of the family squabble. But what you
want is 50 ohms resistance (R), the resistance of air for this type
antenna (roughly), to push out energy hither and yon. Well, inductors
and capacitors store energy, measurable in ohms too, so the energy
prefers not to go out to the party. Inductance + capacitance is
called total reactance, so you want reactance to be zero ohms, when R
reads 50 ohms -- a perfectly resonant antenna at a given freq (a
broadband setup like we need will be a compromise).
If you can hook up with a radio ham club, somebody might have the
popular "MFJ-259" Antenna Analyzer, and who thinks planes are cool,
and might bring it out to you and help. This neat puppy reads
resistance and reactance and hints quickly what's wrong, even though
its VSWR reading may be acceptable. From such a sharp dip, it sounds
like interference from nearby metal elements, but the 259 can't really
say such is so. So I'd cobble a crude dipole out of anything metal,
and how crude won't matter as long as the element lengths are correct
(259 says that too). Patch this to the radio and on the other end,
sample temporary relocations elsewhere within the fuselage using the
259, if present mount is in the tailfin -- apparently not working and
no likely no way to remedy.
W/o a 259, you can still do the same patching/sampling/flight testing.
All you need is a known point where distance to like a continuous ATIS
broadcast is very, very weak. Since interference metal can affect
360-deg radiation pattern, flying gentle 360s may tell much. Under
the Antenna Law of Reciprocity, it doesn't know receive from xmit,
only whether the electrons are going to or returning from the party.
---From long exp if not expertise, a box with only a measured 5W of
carrier can be marginal in real life, due to a bunch of things. Doing
only the math, even two tenths of a watt should be about usable for
VFR, but only if a perfect installation which can't be achieved.
Thus, the installation is more sensitive to gremlins.
Also, use a MFJ-259, VSWR meter, or field strength meter when the
aircraft is parked on concrete, not turf or asphalt. I know about a
dipole and inherent "ground reflections" within 8 feet of this stuff
(one wavelength), but what effect the surface material has I don't
know by theory, but have noticed real anomalies. Maybe black = carbon
and turf has water?
Reg,
Fred F.
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