[TowerTalk] Modeling Ladderline as an Antenna Element

[Tom Rauch]

> I am thinking about using ladderline for a folded dipole

In a folded dipole currents and voltages are essentially in
parallel. No need to model it near resonance because the
line is all common mode so the dielectric has little effect
except for making the dipole electrically a tad longer, just
as it does with a single conductor antenna.

or for linear
> loading a wire antenna.

I'd be careful about that. Linear loading has very low Q.
You can think of linear loading a very poor form factor
loading coil.

Assume we have a coil that uses five feet of #16 wire in an
air wound coil. We probably have a Q of around 200 or 300.
With 250 ohms reactance the series loss resistance would be
only around 1 ohm.

Using a folded stub of air insulated #16 wire, Q is
typically around 25 to 50. We would have about five to ten
ohms of loss resistance.

The thing that kills Q of a folded wire stub (linear
loading) is the stray capacitance between the conductors and
the fact overall wire length has to be significantly longer
for the same reactance. It's actually just like we took a
good high Q coil and stretched it out into a long single
turn, ruining the form factor. Adding a dielectric only
makes it worse because it increases the shunt capacitance.

73 Tom