Digital Journal: Bill Hayes
DTV Testing and Building Blues
This month I would like to start with the initial power on tests
of IPTV's first solid-state DTV transmitter for facility KRIN in
the Cedar Rapids/Waterloo, Iowa, market.
Unfortunately I cannot, because we were unable to get the transmitter
connected to the dummy load for testing. The reason is that the
precision directional coupler was manufactured using Myat line,
which is incompatible with the Dielectric transmission line used
throughout the rest of the installation, and line layout didn't
allow for the use of adapters. The part shipped March 22 and we
continued the install March 25.
The transmitter install otherwise has gone much more smoothly
compared to our first install in Des Moines. However, that is not
to say we haven't had a few issues come up and I would like to spend
some time on a few of those issues. Possibly the most perplexing
problem we have had so far was not particularly technical. In fact,
it was more biological. Who would have thought that the construction
of a rather small addition to our existing transmitter building
would necessitate the relocation of the septic system? I know what
you're thinking: Bill, why would you build your addition on top
of the septic system?
THE SMELL TEST
While it is true the DTV conversion has been mired and at times
things haven't smelled quite right, there must have been another
location. Strangely enough, we didn't build on the septic system,
but since it wasn't where the prints showed it to be, the new addition
was close enough that there was concern over the footings. Although
not initially a major problem, it did become significant when the
contractors dragged their feet on the process because it wasn't
part of their initial bid. The old system was filled in so that
work on the building addition could move on, but then there was
much discussion and consternation over where the new septic system
could be installed.
By the time the contractors were ready to start, winter had set
in and the ground was frozen. The resultant folly necessitated the
use of an outhouse through an Iowa winter. Historically, I think
Iowa winters were the driving force behind the invention of indoor
toilets.
IPTV has selected Harris as the manufacturer of our DTV transmitters.
Our network consists of eight stations and although two of our facilities
(Des Moines and Iowa City) use IOT-based systems, the remainder
use solid-state systems. KRIN is our first installation of the Harris
DiamondCD series transmitter. In this case we are installing a DHD90P3
rated at 20.5kW average DTV power. Our construction permit calls
for an ERP of 250 kW. From our perspective, solid state was the
best way to go for IPTV, given the number and locations of our stations.
FINDING GOOD HELP
When the network was originally built each site was manned by
at least one full-time RF engineer and many had two engineers. Over
time, like most facilities, we have seen budgets reduced and fewer
engineers interested in RF. As a result we have had a difficult
time filling vacant RF positions. After a significant period of
vacancy it is difficult to hold an open position when we've been
able to keep the facilities running with the remaining personnel.
It is especially difficult when the choice may come down to eliminating
a vacant position or letting an employee go.
By constructing solid-state transmission facilities, in theory
we'll increase the reliability of the operations in an environment
where there will be fewer and fewer engineers available to service
the systems. By standardizing on a single manufacture and model
of transmitter we can better service the system with fewer people.
It is certainly not the ideal solution but under the given conditions
it seems to be the most sensible way to proceed.
Another particularly interesting challenge with this project is
that we are working with a commercial partner on the facility. I
described this partnership before and I won't go into any real detail
other than to say that IPTV has purchased a panel antenna and transmission
line that will be shared with KCRG for their DTV service. In exchange
for letting them use the antenna and transmission line, they are
letting us use their tower. We're splitting the costs of the tower
modifications.
This is a pretty straightforward arrangement; however, recently
a third station in the market has approached us about also using
this antenna for their DTV service. As of this writing, there is
still no final agreement on the third station's participation, and
with construction slated to begin in less than a month, time is
running short. The output of our transmitter must go from our building
to KCRG's building, where the combiner will be located and then
out to the antenna. Theoretically, the third station will also have
to send their feed into the KCRG building for combining. Since KCRG
and the third station have adjacent DTV channels, the combining
gets a little tricky and the hardware is quite large, so the quicker
the decision occurs, the better.
LIKE JELL-O
Although the tower work hasn't started yet, I'm already getting
concerned about how things are proceeding. The plan from the beginning
has been for KCRG's existing antenna to be mounted on top of IPTV's
new panel antenna. That stack would then be mounted on the KCRG
tower. To prevent any need to change KCRG's operating parameters,
the center of radiation for its antenna has been treated as a fixed
point in vertical space.
Knowing that point and the length of the stacked antenna, a certain
amount of the existing tower would be removed, a new top plate manufactured
to mate to the stack and - voila! - project complete. Yes, I know
there is more to it, like changing guy points and strengthening
the remainder of the tower, but the basic concept is pretty simple.
The first proposal for this was made in May 2000. About a month
ago, I was speaking with a rep for the tower company and he informed
me that IPTV would need to have our existing analog antenna removed
and put up a temporary antenna. This is because our antenna was
up higher than the tower company thought it was - which, of course,
comes to light after we have already signed an agreement for the
work including pricing.
The confusion over the location of our antenna really surprised
me because it hadn't moved since this company had built the tower
and installed it. One of the reasons for going with the company
was that they were intimately familiar with the tower, there would
be less room for error. It turns out error is like Jell-O and there's
always room for it.
More next month.
Bill Hayes is director of engineering and technology for Iowa
Public Television. You can contact him via TV
Technology.
|
