On Fri, 12 Jun 2009 16:13:40 -0700, Jim Lux wrote:

Rich Griffiths wrote:
On Fri, 12 Jun 2009 17:33:55 -0500, Rich Griffiths wrote:

On Fri, 12 Jun 2009 23:25:40 +0200, Gordon wrote:

AS expected, local channels 8, 10 and 12 moved their digital
broadcasts back to their VHF assignments last night. They had been
temporarily broadcasting digital in the UHF band. I was expecting to
have problems with my dual bowtie antenna. But this morning I
rescaned the channels (and verified that the move had taken place).
Then checked reception on the affected channels. It was great. No
problems. All that worrying for nothing. Thanks for listening.
One situation that cropped up in my area (Cincinnati) is that channel
9, which was broadcasting digital on channel 10 VHF, had to wait until
just before the conversion to raise their DTV antenna from 100 ft down
to the top of their tower. We're also using a dual bowtie, and their
change made a difference.

Maybe not THE factor in your case, but glad things worked out for you.

I should have mentioned:

what especially interested me about this is how relatively unimportant
antenna height has seemed to be in my microwave work.

Until this past year (when rotator cuff surgery took me out), I had
been doing quite a bit of microwave work as a rover (903 MHz - 10 GHz).
I was often impressed by how far over the horizon it would work with
an antenna only about 5 ft off the ground and about 1 W of power.

Granted we were working with MUCH lower signal quality requirements
than the TV stations, but I still am surprised by how poor our
reception is of DTV channel 12 (and earlier, ch9), which is
transmitting MANY kW from a multihundred-ft tower only about 16 km
away.

Engineers may argue otherwise, but it still seems to have a certain
element of black magic to me :-)

I dare say you weren't transmitting digital data at 20 Mbps in a 6MHz
wide channel as a rover.

Nope

CW or SSB with a bandwidth of a few hundred Hz or maybe a couple kHz at
a SNR of 0dB vs 6MHz BW and a SNR of 20dB

Yup

30-40 dB of increased noise bandwidth 20dB more signal power relative to
that noise for decoding

Yup. Which is why I acknowledged that "we were working with MUCH lower
signal quality requirements than the TV stations, but ..."

There's your 100kW right there (100kW = 50dB over 1W)

And, transmit end of the link has lower gain than you probably use as a
microwave rover, because it's omni (in the horizontal plane, at least).
A 1-2 degree beamwidth works out to about 30dB in gain

Yup. 1 W to a 60-cm (2-ft) dish at 10 GHz yields about 2 kW ERP with a
3.5 degree beamwidth.

This kind of thing is why working the world with 10W on PSK31 is pretty
straightforward, compared to SSB.

I know how to do the system and path loss calculations. Yet I'm still
impressed when I can work another rover at 300+ km, sometimes with S9
signals and then go home and see the choppy signals I sometimes get
between ch.12 at 300 m HAAT and my antenna at more than 10 m HAAT.

Amateur microwave work is pretty cool stuff :-)

BTW, using roughly 2-m dishes and only 100 mW, amateurs have transmitted
802.11b (WiFi) and higher-rate signals around 300 km several times. This
allows digital video and audio. I think the current record is 389 km.

--
Rich W2RG