[BC] Jeff's back! Various comments and thoughts, or beating dead horses...

[Jeff Welton]

Wow, take three or four weeks away from email and everybody wants me!
Sorry to take so long responding or commenting on various queries, but I
took a two week camping vacation with the family sans laptop and
cellphone - it was fantastic! (first time I've been that far out of
range in 15 years).  Then a week in Mexico and a week recovering from
Mexico (E. coli is just not all that much fun) and I'm back in the
saddle again!

Jason asked:

> Anybody seen or heard from Jeff Welton lately?
> I've got a question for him re: a Nautel ND1...

Regarding your question, Jason, your problem is ground resistance is
changing and, like the guys said, you need to check to see how much
(bridge the transmitter output).  That said, the ND1 has more than
enough headroom to do what you need it to do (assuming you haven't
already called here and spoken to one of the other guys - although I'm
the best there is, I'm not the only guy in the room <g>).  Just crank
the power adjust pot fully clockwise, then peel the cover off the POWER
LIMIT pot on the control board and set it until you're at about
1050-1100W.  Then reduce the power adjust pot (the front panel one)
until you're at the desired power level.  

You still need to assess the ground system if the resistance is changing
that much - and if the radials are more than a couple of years old, I'd
be surprised if some of them haven't been pulled apart.  That won't
affect transmitter operation, but it sure will affect coverage,
depending on how many get broken and how close into the tower the
break(s) is(are).  Definitely though, stabilizing the ground resistance
would be an improvement over continually having to adjust output power -
couple of ChemRods and some copper strap should help that, as long as
you put them in the right places <g>.

Other topics I just have to reference (you folks have been busy while I
was gone!):

Curt was talking about scopes and mod monitors and mentioned...

>I was given one of the first Scope-meter's to evaluate for broadcast
uses by >Fluke years ago. Unless they've greatly improved since then ( I
suspect they >have )  it was nearly useless for mod levels.

That must have been one of the 90 series (we have a 97) and you're right
on both counts - they have improved and the first generation was useless
<g>.  The newer 190 series (we have a 196 and I'm pushing for a 199)
work very well for measuring AM modulation - both for negative pinch-off
and positive peaks.  For example, I was using ours to check mod on a
station in Mexico last week.  Set a reference, then used the cursors to
measure the reference peak voltage.  I then set the persistence to
infinity, went for a coffee and nicotine break, and when I came back,
set the top cursor to the mod peaks.  From that I was able to calculate
the dV between the peak reference and the peak modulated carrier -
modulation was 137.5% positive peak.  The cursor resolution is
sufficient to provide better than 1% accuracy, depending on setup and
how recently the scope was calibrated.  Once mod passes 130%, those
pesky Belars just won't tell you how much you've got <g>.

On a different topic, Harvey S. mentioned:

>tubes can take lightning, transistors cannot.

Anybody that's talked to me would know I just couldn't resist this one
<g>.

As far as lightning survival of transistors vs. tubes, yes, it takes
much less peak voltage down the pipe to blow a transistor (or even a
whole bunch of transistors.  However, assuming decent grounding and
proper engineering practices are followed during site layout, or
previous deficiencies are corrected during installation, there's no
reason a transistor based transmitter can't survive lightning just fine.


Remember, the system only needs to pass enough peak voltage (for AM) to
allow carrier plus mod to get through - any voltage above that can
mostly be shunted to ground through properly set up spark gaps... which
I almost never see.  As an example, at 50 ohms, spark gap setting for a
5kW transmitter should be in the order of 0.040 inches (that's 40
thousandths).  Not 0.4" or more, which is probably a more common
setting, based on field experience.  Note that this is variable, based
on spark ball diameter and several other parameters, but it gives an
idea of the one of the main reasons people complain that transistors
fail too easily.  

If the site is set up to allow only the desired voltage to pass through
the system without being shunted to ground then transistors won't see
any unacceptable voltages (and the quality of the ground connection is
another issue - a properly set spark gap is useless if one end isn't
connected to earth).  There are a bunch of other variables in the
equation, but the spark gap is the "first defense" - without that,
everything else is just places for the sparks to happen.

I've got 100kW transmitters in Paraguay (which receives the most
lightning strikes anywhere in the Americas - in excess of 200 strikes
per square km per year) which have never lost a component due to
lightning.  I've got several in Florida (most lightning in North
America) and a bunch in Sudan (highest lightning incidence in the
world). Similar results all way around - lightning failures are
typically not an issue.  Engineers in these areas know the risks from
lightning and, _for the most part_ (emphasize this phrase), lay out
transmitter sites accordingly.  

We see more damage from lightning at sites in less susceptible areas,
where less attention tends to be paid to grounding and lightning
protection.  The funny thing is that a good lightning protection layout
can be implemented at install time for a fairly low price (relative to
the price of repairs down the road).  I'd agree with Robert and Grady on
transmitter reliability vs. site layout - if cost of repairs approaches
even 20% of transmitter cost over the first 4-5 years, it's really time
to assess the site layout. Obviously, I'm speaking only about
transmitters with our logo on the front of them.  Also, there's no
question that some models can be more susceptible to damage than others
- I don't claim that all of our transmitters are 100% bulletproof (or
lightning proof), even when good practices are followed. However, the
vast majority of the issues we run into could have been avoided by
proper practices during installation.

All of our guys are taught that after we get the second call from any
one customer in less than a year with lightning damage, it's time to
start digging for information on site layout.  You'd be surprised how
often we get customers fighting us on this ("no, it can't be the site
layout, we did that just fine, it has to be your transmitter, quit
asking about the grounding!"), as if we're trying to demean their
abilities.  The fact that we have, in some instances, over 800 units of
the same model transmitter all over the world and they aren't blowing up
in lightning storms seems to be irrelevant.  I'm not saying we're right
all the time either, but with 8000+ transmitters on-air, we've learned a
few things here and there, we may have some ideas that could help.
<Okay, I'm starting to sound cynical, rant mode off now>

Best,

Jeff (don't drink the water, don't drink the water, don't....) Welton
Customer Service Department
Nautel 
+1.902.823.3900 x 127
www.nautel.com
MAKING DIGITAL RADIO WORK