In article ,
"David Eduardo" wrote:

"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.

I wasn't going after the commercial reasons but methods of causing
existing towers not to be so narrowband resonant. You will find using a
shunt feed method will cause the tower to be dampened compared to a
series feed method. With a shunt feed the resonant peak will be a little
lower and broader than with a series method.

The feed is "found" at some point up the outrigger wires, and the
base is at ground potential.

Yes.

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.

Yeah. That's why I thought it would be more popular in central and south
America where expensive or difficult to install, due to terrain,
grounding systems requirements could be reduced.

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).

That my point. The shunt type coupling is more broadband.

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.

I don't know what you mean by "the FCC requires a very special showing
to allow low antennas" but I wasn't after the shorter tower aspect.

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.

That's not my understanding.

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.

You usually make measurements beyond spec to show the trend over the
entire specification. Here the spec is VSWR.

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.

Here that results in a lower VSWR over the IBOC operating bandwidth
using a shunt fed tower.

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.

Yes that is my point. The tower itself will have a narrow resonant
bandwidth so a method is needed for the tower to not present a high VSWR
+/- 15 KHz around the carrier frequency.

ATU's that couple the coax transmission line to the tower using a series
connection to an insulated tower have a stronger tendency to be
narrowband in and of themselves. An ATU designed to couple the coax
transmission line to a shunt fed tower tends to tune a little less
sharply and the useful resonance range is broader.

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.

Why?

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.

I don't understand why the FCC would care how transmission lines are
coupled to the tower or tuned / matched by an ATU.

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.

That's all interesting information but we are mixing and matching
terminology. The ATU doesn't just tune the tower to resonance it is also
a part of circuitry that couples the transmission line to the tower.

Reactance is near zero at resonance and the antenna looks like a
resistive load to the transmission line. The resistive load value also
needs to match the line impedance so there is little or no reflective
power. This is one reason why transmitters are getting damaged when IBOC
is turned on. The reflective power goes up over the increased power
bandwidth IBOC requires.

Several things happen when the reflected power goes up. Mainly the power
does not go into the antenna to be radiated and instead heats the
transmission line and transmitter finals. Another bad effect is the
reflected power can make the transmitter unstable and generate spurious
energy.

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.

Perfectly understandable.

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.

You sure get around.

--
Telamon
Ventura, California