On Sat, 5 Jan 2013 03:43:45 +0000 (UTC),
dkc...@panix.com (David
Combs) wrote:
>In article <
CcCdnVraIvTgdADN...@earthlink.com>,
>Michael A. Terrell <
mike.t...@earthlink.net> wrote:
>>
>>micky wrote:
>>>
>>> On Fri, 09 Nov 2012 22:18:39 -0500, "Michael A. Terrell"
>>> <
mike.t...@earthlink.net> 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!
Since the radiation from a horizontal wire close (in terms of
wavelength) to the ground will cancel out due to the ground "mirror"
reflection, only vertical polarization makes sense at frequencies
below about 1 MHz (300 m wavelength).
There are two practical ways to generate vertical signals, either by
using a 1/2 wave radiator, isolated from ground and fed from the
middle or using a 1/4 radiator and a *good* ground plane.
An example of the first approach was the LW tower in Poland
http://en.wikipedia.org/wiki/Warszawa_Radio_Mast
with a mast 646 m tall, fed by 227 kHz (1320 m wavelength) thus a 1/2
wavelength structure (considering the end capacitances). The advantage
was no ground losses, but unfortunately the hight was huge (tallest in
the world when constructed).
If you are not willing to build such high structures, the alternative
is to use 1/4 antennas (requiring *good* ground planes). In order to
reduce the masts even further, various top capacitance hat structures,
such as T and Z constructs could be used, but this reduced the
radiation resistance. Look for instance of the antenna structures of
the Titanic, the 500 kHz (600 m) would have required 150 m masts, but
in reality, there were several antenna conductors along the ship at
lower altitude, forming a top hat capacitance as a T-antenna. The
actual radiation occurred from the vertical line going from the radio
hut to these horizontal wires.
>They all
>>use use L/C networks to make the tower work at the allocated frequency.
There are two issues.
In order to radiate effectively, the antenna_system_ must be resonant
(i.e. purely resistive) at the transmission frequency with top hat
capacitances and loading coils at the transmitter end.
Monopole antennas that are much smaller than the 1/4 wavelength. have
a radiation resistance well below the text book 37 ohms, perhaps an
ohm or even a milliohm.
>What, some kind of impedance matching?
>
>
>>Some ATU are as large as the AM transmitter when the tower is difficult
>>to configure.
>
>ATU -- what's that?
To be usable, a transmitter+antenna combination must
1.) The antenna _system_ must be resistive at the design frequency
2.) the transmitter output impedance (sometimes kilo-ohms) must be
matched to the antenna milliohm to ohm radiation resistance.
In some (most) cases this can be done with a 2-3 component antenna
tuner unit (ATU), but the losses may be huge and flashovers and smoke
may occur.