Entirely correct, Fred. Common sources of very large losses like these are
bad connections, bad connectors, and bad coax. Lossy materials (i.e., poor
electrical conductors, like carbon fiber) in the vicinity of the antenna can
also ruin signal strength. VSWR doesn't always show problems of loss, as in
the case of the 50 Ohm resistor, which has a 1:1 VSWR, but doesn't radiate a
bit.
The other common problem is pattern distortion, in which the signal is weak
only in specific directions. This is caused by large or resonant (i.e.,
multiples of the length of the antenna) conductors nearby, such as a
vertically-oriented ELT antenna. The antenna's own feedline is often to
blame. If I can't run the coax perpendicular to the antenna for at least
the length of the antenna, I put an extra ferrite choke (like the one
installed at the end of the coax) on the cable, located half the length of
the antenna from the other one.
Performance of both antennas should be similar when properly installed, so
the main reason to choose one over the other is ease of mounting in the
desired location.
Dave DeFord
A135
----- Original Message -----
From: Fred Fillinger <fillinger@ameritech.net>
Subject: Re: Bob Archer - an endorsement
> I won't disagree that a mfr'd antenna may be superior to a cobbled
> copper-tape affair, but I suspect that the difference is substantially
> in defects in the copper tape setup. Did you by chance compare the
> VSWR on both antennas?
>
> On my copper tape antenna, and starting with Jim Weir's recommended
> length, the VSWR meter had me trimming a couple inches, probably due
> to some odd bends that Weir says are OK. It came out just slightly
> better in VSWR than a new, commercial, VHF comm broad-band whip that I
> compared it to.
>
> An engineer friend of mine (who designed antennas for the co. he
> worked for) tells me that antenna performance is strictly VSWR and
> optimum radiating pattern, which can produce some "gain," but not much
> available there if you need a 360-deg pattern. IOW, he says nothing
> in Archer's (or anybody's) design can produce an order-of-magnitude
> performance difference given proper tuning of each. He says also that
> the Archer sounds like just a common "J-matched dipole." These maybe
> have a more efficient balun, but he questioned its being made of
> aluminum (it is, right?), which has long-term performance problems
> over copper, whose surface corrosion is not detrimental.
>
> Regards,
> Fred F., A063, N3EU
>
> Tony Krzyzewski wrote:
> >
> > I just took ZK-UBD for its first flight after replacing the copper
dipole
> > aerial with a Bob Archer aerial. The difference in reception can only be
> > described as dramatic. The reception distance from out local field has
been
> > increased over 10 fold for a start. In addition, one of the southern
> > approaches along the coast is normally blanketed by the hills, for the
first
> > time in any aircraft I have flown I had full reception all the way in.
> >
> > I'd consider the Bob Archer com aerial a mandatory fit for the Europa
now.
> >
|