Topband: Inverted L

Dudley Chapman chief at thechief.com
Tue Dec 20 11:36:47 EST 2005

Cris,
When radials are on or very near the ground, they act more like
distributed capacitance to ground rather than like a tuned element.  So it's
not necessary to tune them to any particular length.  Instead, the radial
length is more determined by the law of diminishing returns than anything
else.

If your inverted L is a 1/4 wave, radial lengths reach their diminishing
returns at around 0.2 wavelengths long.  The number of radials to be used is
also determined by diminishing returns.  A general rule of thumb is that for
radials of the same length, their number should not exceed that where the
chordal distance between the ends of two adjacent radials does not exceed
0.025 wavelengths of so.  (However, ground radials really don't have to be
all the same length).

The current distribution along a given radial is a mirror image of the
radiating element.  So it's high near the element, and decreases
sinusoidally as you move out towards the end, going to zero at a quarter
wavelength.  So running radials longer than 0.2 wavelengths or so doesn't
help you unless you are running them a few miles out along the fresnel zone
of the antenna.

There is nothing critical about any of these numbers. You are free to put
more and longer radials then these rules of thumb suggest, but it will not
be worth the trouble.  Also, I would be inclined to not connect radials to
the fence.

Many hams install their radials on top of the ground.  They mow the grass
short, lay the radials down on the grass and pin them down with a "staple"
made from some stiff wire bent into a U shape.  Over time, the grass and
root system grow over the radials and they tend to just disappear into the
vegetation.  If you are more ambitious, you can use a garden edger to make a
long "slot" in the ground.  The depth is really not important.  Hams bury
their radials to get them out from underfoot, but it's not important
electrically.

The radial wire can be any cheap thing you can acquire.  When you have 20
- 60 radials, the current in each one is low, so #14 wire is fine.  The wire
can be insulated or not.  Insulated wire might be better since it may cut
down on the corrosion from galvanic action with other grounded metals in the
system.

Good luck on topband, OM.

Dudley - WA1X