On Wed, 15 Aug 2007 14:55:00 +0000, Stephanie Weil wrote:
Another local station is gearing up to go IBOC: 1010 WINS, which has
been signing off on Sunday nights and plans on doing so for the next
couple of months.

Their antenna array is not "flat" enough for HD (whatever that means,
I'm not a transmitter person).

"flat" refers to the impedance (and possibly the directivity) of their
antenna system on frequencies near but not equal to their 1010KHz carrier.

WINS's operating frequency is quoted as 1010KHz, but that's just their
center "carrier" frequency. If WINS were to transmit "dead air", that's
the only frequency they'd radiate a signal on. Once they start
transmitting voice, they begin to have signal on adjacent frequencies,
5-7KHz either side of 1010. They might occupy a range between 1003 and
1017KHz.

For the transmitter to operate properly, the "impedance" of the antenna
system must be reasonably close to 50 ohms (and must have little or no
reactance) at any frequency at which the transmitter might radiate a
signal. A non-radiating "dummy load" is the only antenna that will be 50
ohms at *all* frequencies; any useful antenna for WINS will vary in
impedance and reactance with frequency. Since WINS has been successfully
using its existing antenna system for some time, one can reasonably assume
its antenna system exhibits an impedance reasonably close to 50 ohms
across the 1003-1017KHz range.

IBOC adds digital carriers to the outer edges of the channel. For WINS it
would extend from 995-1025KHz. If their antenna system is "not flat
enough" that means the impedance in the 995-1003 and/or 1017-1025KHz
ranges isn't close enough to 50 ohms, and/or there is too much reactance.

A related problem would be with the directivity of the antenna. WINS is
required to use a directional antenna system at all hours. The
directivity of its antenna will also vary with frequency. It is possible
the WINS system is close enough to 50 ohms across the IBOC band for the
transmitter to be happy, but for its directivity to not be adequately
maintained in 995-1003 and/or 1017-1025KHz. That would result in
increased interference to other stations on 990, 1000, 1020, and 1030KHz.
(beyond that which arguably would result simply from WINS's use of IBOC)

There are ways of fixing this problem. They seem to involve computer
modeling the antenna matching networks and finding new network designs
that solve the issue. The process is neither fast nor cheap, and once the
necessary changes are determined the adjustments must be made - the
networks may require modification - and then measurements must be made to
ensure things actually are working properly.

This will be an issue for many directional stations - it's why I don't see
September 14th bringing IBOC to a huge stampede of AM stations. WINS can
afford the work. WKRP in tiny Podunk maybe can't.