[BC] Long Wire Antennas

[RichardBJohnson at comcast.net]

What do you mean by, "Lost my tower site?" Did a tower
come down and you still have a ground-system or were
you renting something and lost the lease?

This is important because at 620 kHz, you need a good
(meaning large) ground-system to drive anything against
if you want vertical polarization.

Lambda = 299,792,458  /  F = 483.5 meters
Thin wire propagation is about .95
483.5 x 0.95 = 459. meters
For 1/2 wave 459 / 2 = 230 meters
230 meters * 39.37 / 12 = 1036 feet!

For horizontal polarization, you can create a 1/2 wave
dipole, drive it in the center, and not worry about a ground
system (assuming a STA would be granted). You
would need about 230 meters of wire (about 1000 ft). at
least 30 meters in the air (about 90 ft). Its feed-point
impedance will be about 150 ohms, but you need to
feed it balanced (a 1/2 wave-length of coax in series
with one side will do. That will bring the feed-point
impedance down to about 75 ohms, which is a
reasonable match to a transmitter if it is not
solid-state.

So you can see that with such a low frequency, even
a 1/2 wave dipole is huge! Now, if you have a ground-
system that you can feed it against, the length gets
divided in two, i.e., 500 feet gets you 1/4 wave with
a thin wire at 640 kHz. Its impedance will be about
70 ohms, also a reasonable direct match. Of course
an antenna tuning unit, if you have the correct
components will get you to 50 ohms +/- j0. Shorter
wires will become more capacitive, causing a
lot of displacement current to flow in your thin
wire, reducing efficiency.


--
Cheers,
Richard B. Johnson