David Combs wrote:

In article ,
Michael A. Terrell wrote:

micky wrote:

On Fri, 09 Nov 2012 22:18:39 -0500, "Michael A. Terrell"
wrote:

...
I don't think so. And if that were the case, then a
radio transmitter broadcasting at 600 KHz would require an antenna
that was a third of a mile long.


No, it doesn't. AM BC band broadcast antennas are not a full
wavelength, and the aren't resonant at the transmit frequency.

Please say more about that. Thanks!


Early radio stated the wavelength in meters, rather than frequency so
it was rather obvious The wavelength at 600 KHz is 300,000,000/600,000
or 500 meters (1640 feet)


60 Hz is 5,000,000 meters for one cycle in free space. Roughly
16400000 feet, or 3106 miles. That means that you aren't going to have
much AC power radiated in a few feet of conductors. For instance, if
there is 20 feet of heating element, it is roughly 20/16,400,000 or
1.2195e-6 of a wavelength


http://www.csgnetwork.com/freqwavelengthcalc.html is one of many free
Javascript based wavelength calculators you can play with.


They all use use L/C networks to make the tower work at the
allocated frequency.

What, some kind of impedance matching?


Yes, and every installation is a custom design.


Some ATU are as large as the AM transmitter when the tower is
difficult to configure.

ATU -- what's that?


ATU= 'Antenna Tuning Unit'. In AM broadcast, it is a custom
impedance matching network designed to match the antenna to the
transmitter's output impedance of 50 Ohms. They have to handle the
transmitters full rated power, and be fairly tolerant of lightning
induced flash over conditions.


The old Voice Of America transmitter farm at Bethany Ohio had huge
underground vaults for the antenna switches, and remote tuned ATUs. Of
course, they could put out up to 500 KW at any frequency from 1.5 to 30
MHz into the directional array to target any point on earth. The WLW
site a few miles away could do 500 KW, but since it was designed for a
single frequency it was much smaller. Both of those sites date back to
the early days of radio, and the VOA site is now a golf course.