In article ,
"David Eduardo" wrote:

"Telamon" wrote in message
...
In article ,
"David Eduardo" wrote:

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.

Why would the terrain considerations, grounding requirements and such
be any different than in, say, Idaho or Wisconsin or Arizona or
Alabama?

They would not and since when is Idaho, Wisconsin, Arizona or Alabama
in south America?

And why would it be more expensive to do in Central and South
America? I can see no logic in any of this set of statements.

Where did I say it was more expensive in SA? The logic is why spend
money you don't have to spend. The problem looks to be reading
comprehension.

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.

Only for very short towers where a high capacitive reactance is
found. Otherwise, the bandwidth is much more the effect of the Q of
the ATU and the trandsmitter itself.

No.

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.

That is the only place where the unipole offers a distinct bandwidth
advantage. It also gets a better field strength at 1 km than a short
series fed tower. But the main reason anyone uses them is either due
to a bad ground or the need to put other antennas on the tower.

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.

Go back to Carl Smith's AM antenna and DA handbooks, Unless you find
a nice old Blaw Knox with a 24 foot center cross section, there is
not much gain except cost, maintenance, etc., in adding outriggers
insofar as broadbanding.

I don't have that book so I can't do that but general theory would
indicate otherwise and there are companies that offer tower kits to
improve bandwidth performance as I have described.

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.

A well tuned tower of 1/4 wave has less than 1.08 to 1 vizwar. And,
except for test situations, a tower measurement is usually done at
-10, licenced frequency and +10 kHz. This is what is often asked for
by outside fabricators of ATUs.

Again that is not my understanding of tower VSWR. The levels at 5, 10,
and 15 KHz are much higher than you indicate, which requires mitigation
efforts.

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.

A mismatch is generally considered to be a mismatch of impedance
output of the ATU with the tower itself at the fundamental. Since the
audio is brick-walled right under 10 kHz, there should be little or
no excursions beyond +/- 10 kHz.

For analog +/- 10 KHz sounds reasonable but it looks like IBOC is
going past that number. The testing recommendation I read suggests
testing to +/- 15 KHz.

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.

Not for the last 40 years or so. High Q was much more common pre-60's
when AMs mostly ran network showsthat came over 5 kHz lines from very
far away. When music took over AM, stations wanted better bandwidth.

I don't see how that can be improved. Series feeding the tower will
require a fairly high Q network that is inherently narrow band
compared to a shunt feed method.

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.

Since true shunts are no longer licensed, this point is moot. Most US
AMs, for economy, zoning, FAA, etc. use quarter wave series fed
towers. Since a huge percentage are directional, there are very, very
few Unipole directionals, so in that area series fed is the only way
to go.

I don't see where you keep on this tack about Unipoles as they can just
as easily be part of a directional network.

The FCC no longer authorizes shunt fed towers.

Why?

I don't know. This has been the case for many decades. I suspect part
has to do with the slight directionality the shunt itself introduces,
and the fact that shunts would be very difficult to do with
directional stations for this reason.

I don't see where a Unipole would present a problem in a directional
network.

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.

There is a lot about AM radio in the US you don't understand. The
fact is, shunt fed towers are very seldom used. I suspect that the
fact that they are not particulary easy to tune on short, 1/2 wave
towers may be part of it, but there must be more.

Yeah, there must be another reason.

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.

An ATU is not necessary if the tower is 52 ohms and not reactive,
found around about 100 to 110 degrees in electrical height.

The ATU is a matching circuit, to make the coax "see" 52 ohms (or
some other impedance) when, in fact, the tower is not of that
impedance. In some senses, a top hat or top loading does the same
thing... it makes the tower change the apparent electrical height.
You are trying to complicate something that is relatively simple...
cancelling the reactance and matching impedance.

That's fine if the coax and transmitter output impedance is 52 ohms. I
didn't realize I was making it more complicated I was just trying to
explain the dynamics of tuning the tower.

An ATU is usually necessary.

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.

That is not why the very few cases of transmitter damage have
occured. Most transmitters will simply shut down over reactive or
mismatched loads. The issues reported (and one that took out 80% of
the power modules at KTNQ) had to do with the control interface of
the HD exciter and the analog transmitter.... a design defect in
non-rf and non-af circuitry.

Wow, I guess the engineering of the IBOC working group really sucks. I
figured something in the way of the application in the field is what
would bring things down. This is far worse that I imagined.

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.

Or, in today's transmitters, it does neither. It detects VSWR and
shuts off.

And all my equipment has fuses or circuit breakers but I usually apply
effort toward making sure they are not used.

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.

Yes, I have worked in every nation in Latin America save Cuba and
Nicaragua, and visited stations in each one. There are pictures of
dozens and dozens stations I took, including some shunt fed ones, on
my web page, from trips I made visiting stations.

The fact is you have said that shunt feeding is common in Latin
America where it is highly uncommon and always has been. You made
statements about costs, land and towers in Latin America that make it
sound like you think we are talking about another planet, not the
same Hemisphere. In fact, the conditions and terrain in South Florida
are more difficult and hostile than in most places in Latin America.

I don't see the need to be so defensive about what I posted. I made it
clear it was an assumption and explained my logic for those
assumptions. The reasoning was technical and there is no need to try to
make it into something else.

--
Telamon
Ventura, California