[TowerTalk] Temp tower/antenna question

[Jim Lux]

On 2/2/13 8:30 AM, jcjacobsen at q.com wrote:
> How do,
>
> Steve is looking for ideas for temp field day style antennas/towers.
>
> I suppose it depends on what you intend to do. Casual ops: Nothing wrong with a ground rod to an auto tuner and a wire up in a tree.
>

I'd say either a (non resonant) dipole in the trees or a 20-30 foot long 
wire laying on the ground (rather than a rod) and the long wire in the 
tree.  Either with a good autotuner with wide range.

A ground rod isn't a very good "RF ground", especially if it's only a 
few feet deep. You'd wind up having "the rig and connected wires" being 
the RF ground.

OTOH, a few dozen feet of wire laying on the surface of the soil has a 
pretty low RF impedance.  Capacitance of a 1mm wire close (5mm)to a 
plane is around 20-30 pF/meter  (2*pi*epsilon*L/arccosh(d/a))  epsilon 
is 8.85pF/meter

So, if you throw out 20 feet (6 meters) of wire, that's about 150-200 
pF.  At 14 MHz, that's a reactance of about 70-80 ohms, but the tuner 
will take that out.  The resistance (loss) is much less, and dominated 
by the soil properties.

The wire loss is very small..

Skin depth in copper at 14MHz is 0.017 mm, diameter 1mm, so the cross 
sectional area is about 0.05 mm2...so resistance is about 0.35 ohms/meter.

Throw out three wires, each 2 meters long, and your ground resistance 
(due to the wires) is about 0.2 ohms.

1mm diameter is roughly AWG 18.5