On 12/26/2011 5:52 PM, DEFCON 88 wrote:
I've seen pictures (which I can't seem to find on the internet) of old
AM
broadcast antennas consisting of multiple parallel wires strung
between
two towers on the rooftop of a building. These were typically located
on
top of a hotel, with the broadcast studio in the hotel.

Were these wires truly parallel, or were they strung in a zig-zag
pattern
(which I'll try to illustrate below)? If parallel, what's the theory
and
advantage of having them parallel?
[...]


Another poster referred to cage dipoles, and it is possible that a
few early broadcast arrays were cage dipoles, just as some were
inverted Ls, but the vast majority of these early parallel wire
antennas were "T" antennas. What looks like the feeder in these
antennas is actually the vertical radiating element, and the
symmetrical horizontal wires (yes, typically all in parallel) are a
non-radiating capacity hat to lower the system's resonant frequency.
Multiple wires have somewhat more capacity than single wires, and
also make the antenna more broadbanded and thus easier to tune, but
I think their use had more to do with early semi-mythical antenna
lore than it did with real-world benefits.

These antennas weren't understood too well at the time they were
first developed, but early spark transmitter and LF alternator
operators knew that they could tune an antenna system's reactance
out and make it "take power" on the desired frequency by varying the
width and length of the capacity hat.

When mathematical models began to be used to design antennas, in the
late 20s / early 30s I think, it was found that the same or better
performance could be had with simpler designs that survived the
weather better.

T antennas are seldom used on the Standard Broadcast Band today, but
amateurs still use them fairly often on 160 meters, where the
requisite height for a quarter wave vertical is hard to achieve.


With all good wishes,


Kevin, WB4AIO.
--
http://nationalvanguard.org/
http://kevinalfredstrom.com/