[Cu-wireless] guessing how the 5dBi collinear works

[David Young]

Caveat: In this e-mail I rely on my dim recollection of college physics
to try to explain how the collinear antenna I built works.

The collinear consists of a "decoupler assembly" and four 1/2-wave
elements made from brass tube, one 1/2-wave element attached to another in
series by brass rods. The brass rods have a 4-turn coil twisted into them.
I have no idea how the decoupler assembly works, but I will take a stab
at explaining the rest of the antenna.

Magically, the decoupler assembly introduces a signal-carrying current
into the brass tube at the bottom of the antenna. =)

The signal propagates on the outside of the metal, because that is what
high-frequency signals do. Every component of the antenna radiates,
however, the 1/2-wave elements made of tubing radiate most strongly. The
reason is that the 1/2-wave elements made of 3/32" tubing have a lower
curvature than the elements made of 3/64" rod, so the E-field is less
on those elements than on the rod. (Think how you induce lightning to
prefer to discharge through a lightning rod; also, think how 10,000-Watt
antennas are kept from arcing.) The H-field is inversely proportional to
the E-field, so the H-field *peaks* on the 1/2-wave tubes.  The H-field
contributes primarily to the radiated electromagnetic waves---I do not
know why that is. Thus the 1/2-wave tubes are the principal radiators
in the antenna.

What this antenna is, then, is a series of 1/2-wave (dipole?) antennas. We
want for each antenna in the series to have the right phase with respect
to the others, so that at every point P some distance from the antenna,
the elements' fields sum to the right field strength. It is a problem to
get them in the right phase, since they are all fed from the bottom,
while putting them at the right distance from each other. That is
where the coils come in. They retard the propagation of the signal up
the antenna so that in as little space as we desire between 1/2-wave
elements, there is great enough change in phase that we get the "correct"
interference pattern. I do not know how the coils retard the propagation
of the signal. Mark Lenigan says that it is due to induction. Sounds
reasonable to me.

Dave

-- 
David Young             OJC Technologies
dyoung at onthejob.net     Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933