"Telamon" wrote in message
...

I would say bandwidth. Large aspect ratio antenna elements have a narrow
band of resonance. It seems to me that there are some companies out
there that have tower kits that run 3 to 4 wires on spreaders so the
electrical diameter of the tower is increased. This will allow the tower
to have lower VSWR over the +/-15KHz required.

That is not the purpose of folded dipoles, called unipoles, which is what
you are describing. A unipole requires no base insulator, so in this day of
using towers as a revenue center, it allows other antennae to be mounted on
the tower with no need of isocouplers. The feed is "found" at some point up
the outrigger wires, and the base is at ground potential. The folded dipole
is also appropriate, like a Franklin, when there is a bad ground system,
such as antennas in marshes and salt flats where they corrode, or where
there are structures on the property. A unipois also useful with a shorter
than 1/4 wave tower because the tuning network needed to tune out the
capacitive reactance often narrowbands the antenna (not the tower itself).
So a unipole is mostly used to compensate for bad ground systems and the
need to multitask the tower, not to reduce the noxious effects of a less
than conforming tower, as the FCC requires a very special showing to allow
low antennas.

A broader cross section will broadband the tower a bit, but the difference
in a 24" to 30" cross section and a folded dipole is minimal.

The bandwidth for AM is, by NRSC, 10 kHz in each sideband... actually, a
little less. This is to avoid 10 kHz harmonics with adjacent channels.

In any event, the intent of broader bandwith would be to improve quality by
decreasing the differnces in impedance and reactance at plus or minus 9.99
kHz from carrier. A well tuned ATU, whether high Q or broadbanded, does not
create a significant amount of reflected power. A tower that is mismatched
at carrier does.

Next up I would look at the transmission line to tower coupling. Many AM
broadcast towers are series coupled with the tower isolated from ground.

The FCC no longer authorizes shunt fed towers. The unipole is the closest
you get to this; one manufacturer, Kintronics, who makes kits to order,
compares them with shunt fed systems. So, except for the unipoles, all US
towers for AM are insulated from ground.

Transmission lines are never couple to the tower (with maybe one or two
exceptions... more later) because so few towers are a perfect impedance
match with the coax and devoid of +j or -j. An antenna coupling unit is
placed between the coax and the tower, using a network to match the tower to
line impedance and to bring reactance to zero at the carrier. The ATU is
typically attached to the tower with a copper strap, copper tubing or
sometimes even braid. In any case, it is silver soldered to a connector,
which is usually pressure bolted to the output of the ATU and to a leg or
the base plate of the tower.

There are a couple of licensed US stations that have towers in the 100 to
110 degree height range and direct couple to a series fed tower without
tuning because in that degree range, occasionally a perfect match is already
present. This is very rare.

The tower would tune a little more broadly if it was grounded and shunt
coupled. I believe this is the preferred method in central and south
America.

This method was used by a few high power AMs in Latin America in decades
past, ones like XEB and XEW. The rest, if they have a tower (many use
inverted L's of wire) use series fed towers. Since many towers are diplexed
and even triplexed, a rejection network is required and that requires an
ATU. Shunt fed towers are generally half wave or similar, and shunt feeding
is not and has never been common with quarter wave or less towers.

I have visited every AM in Mexico City, and only 3 had shunt fed towers in
1963... today, I believe only XEW has one. In Colombia, I have visited about
20 50 kw or higher sites, and none was shunt fed. In Ecuador, today and in
the past, no station was shunt fed. Of the several hundred stations I
visited in Central America, none was shunt fed. The most powerful AM in
Argentina, Radio 10 on 710 with 100 kw, with a nice half wave tower, is
series fed.

The only other Latin American shunt fed I know about was CB106 Radio Mineria
in Santiago. That station, saying AM was no longer viable in Chile, turned
in its license and turned off the 100 kw transmitter nearly a decade ago.