[Lowfer] Question

[Lyle Koehler]

Some quick and dirty modeling results yield four 137 kHz antenna "designs"
that will give approximately -20dBi gain at 100 kHz, which will result in 1
watt EIRP with 100 watts applied:

1) A 100 foot square loop of 1/2 inch diameter copper tubing, located far
enough from the ground and other objects so that there is only 0.2 ohms of
"excess" loss (beyond the Rac of the copper).

2) Reduce the wire size of the loop to #12, and increase the length per side
to 150 feet, somehow keeping the excess loss at 0.2 ohms. Remember that the
loop is in the vertical plane, so you'll need some fairly tall trees.

3) A 50 foot high vertical in a clear area, with a 6-radial top hat and
single #8 skirt wire. The top hat radius is 17 feet, making it comparable in
wind loading to a 3-element triband HF beam. Loading coil inductance is
about 4000 uH with a Q of 500, and the assumed ground loss is 5 ohms. Note
that this type of antenna could meet the "15 meter cylinder" interpretation
of the Part 15 rules :-)

4) If you can't get your ground losses (including losses from nearby trees,
etc) below 20 ohms, use the same top hat but increase the overall height to
80 feet. The required loading coil inductance is now about 3800 uH.

My assumption of 0.2 ohms excess loss in the loops is a rough estimate based
on Bill Ashlock's measured antenna current in a 50-foot square loop. It is
likely to be higher in larger loops unless the lower leg is raised well
above the ground. In practical installations, the loop will be rectangular
rather than square, which will increase both the Rac and ground losses for a
given enclosed area.

LowFERs TEXAS and OK achieved ground losses of approximately 5 ohms with
vertical antennas in open fields and very good ground systems. I believe OK
used 120 100-foot radials. 20 ohms is about what I used to get with the LEK
antenna system, using the metal garage roof as a ground plane.

The 1-watt EIRP limit requires a pretty substantial antenna, but it pales
into insignificance when compared to the antenna systems of hard-core HF
DXers and contesters. Rotating 200-foot towers with stacked monoband beams
are needed if you want full bragging rights among that crowd!

Lyle, K0LR