> I looked at your picture and from what I think I see I believe the top
loading coli will most likely act like a trap thus you will most likely not
have an 1/2 wave electrical antenna.
Actually a loading coil almost always acts like an inductive reactance
termination on the next band up, and it SHORTENS the antenna's electrical
length, increases loss, and raises the feed impedance.
It's a simple series L and shunt C problem. At resonance XL = Xc where Xc is
the effective capacitive reactance at the top of the antenna above the coil.
At twice the frequency XL is often about twice as large so the result is the
antenna is terminated in an inductance. This reflects back and appears as Xc
at the antenna base if the antenna is moderately tall on the next band up.
If you look at antennas like the Hustler verticals, they add a capacitance
hat below the coil to restore the system to normal operation.
If you have a parallel C across the coil you can make a trap, but without
that the effect of the coil hanging there is almost always deleterious. The
weird thing is some people actually advertise their antennas as "not having
lossy traps" because they use loading coils as isolators, yet the loading
coil is significantly more lossy than a trap would have been!!!
Those mobile antennas with multiple resonators? You probably guessed what
coming. The unused bands hanging there hurt performance even though they are
not traps. Fortunately the type of antenna they are used with are so poor no
one would notice an extra 20% loss.
This doesn't say it WON'T work, just don't delude yourself into thinking it
doesn't decrease performance some amount. It works the same in any antenna,
including those inductor loaded slopers that are sold as "trapless" designs
without trap loss.
You really need to know the amount of XL and the shunt C across the coil to
determine how much effect it has. Its a pretty complicated issue.
73 Tom
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