I'm going to do a short presentation on small transmitting antennas at the
Topband dinner. After the Hamfest, I'll put an expanded article up on my
website.
I'm not sure how much we can cover about efficiency, bandwidth, and so
on...but here are some important points to think about. It just happens
that most of the things we should do to improve efficiency in a mobile
antenna can also improve bandwidth!
1.) The bandwidth of a compact antenna is mostly controlled by reactance of
the system. Resistance is normally pretty high and won't change much unless
you have a BIG ground system or make extreme changes in system element
resistances.
This means any changes in a mobile antenna's bandwidth has more to do with
how much inductance is required (assuming the coil has very little internal
shunt capacitance) than how much loss the coil or system has.
2.) If we add capacitance ACROSS a coil or between turns in a loading coil
by adding a dielectric (or large metal end-caps), circulating currents in
the wire increase. Extra capacitance increases loss while tending to make
overall system bandwidth NARROWER, because the slope of inductor reactance
with frequency occurs at a higher rate.
This is why dielectric constant and where we place the dielectric is almost
always more important than the dissipation factor or the quality of the
insulating media, assuming it isn't just a horrible loss tangent material.
The effect of additional stray capacitance reducing bandwidth while
increasing loss is largest when the ground system is poor (like in a mobile)
and coil reactance requirements are large. We can actually change nothing
but the coil in a given system, and reduce efficiency while decreasing
bandwidth.
3.) Efficiency in an antenna with fixed and large losses, like the high
ground loss resistance of a mobile, are directly influenced by current
distribution in the system. While current at each end of a normal loading
coil is almost equal, the taper in current above the coil is very rapid. Any
current taper reduces radiation resistance, and directly reduces efficiency
even if the feedpoint impedance doesn't change a noticeable amount. The
feedpoint impedance won't change a noticeable amount in a mobile because of
the swamping effect of the ground and coil losses.
I measure about 20 ohms or so ground loss resistance in a Supercab full-size
long-bed pick up truck on 1.8MHz. You can imagine what it is in a compact
car.
73, Tom W8JI
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