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.

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.

--
Rich W2RG

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 :-)

--
Rich W2RG

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.

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

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

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


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

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

"Rich Griffiths" wrote in message
communications...

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.

Antenna gain on both ends explains most of that.

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.

Shannon's equations provide most of the answers:
http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html

Some of that is hard to follow, but the net effect is that you need a
certain (minimum) amount of power to send a complex signal in a confined
bandwidth. With ATSC, they put about 20 Mbps into a 6 MHz channel. To get
a decent SNR (16 dB or better), they need MANY KW.

I did a little mickey-wave engineering, myself. Point-to-point is easier
than broadcast!

"Sal"

On Sun, 14 Jun 2009 00:46:50 -0700, Sal M. Onella wrote:

"Rich Griffiths" wrote in message
communications...

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.

Antenna gain on both ends explains most of that.

To me, it's a partial explanation, but not most of it. Microwavers often
resort to quasi-scientific explanations like "troposcattering", as do VHF-
ers. And sometimes the explanations get even fuzzier, like
"enhancement". They (and ducting) are somewhat predictable, but as a
rover the operating mantra was always just 'Let's try it".


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.

Shannon's equations provide most of the answers:
http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html

Shannon's equations don't actually tell you much that's useful, as a
Ham. And even in a lot of commercial situations, it can be hard to be
sure what assumptions to make about noise and signal strengths, for
example.


Some of that is hard to follow, but the net effect is that you need a
certain (minimum) amount of power to send a complex signal in a confined
bandwidth. With ATSC, they put about 20 Mbps into a 6 MHz channel. To
get a decent SNR (16 dB or better), they need MANY KW.

I did a little mickey-wave engineering, myself. Point-to-point is
easier than broadcast!

"Sal"

As an amateur rover, I generally found point-to-point to be more
difficult, because of simple real-world Ham issues that don't have a lot
to do with heavy theory. At 10 GHz, a 60-cm dish has a beamwidth of
about 3.5 degrees. The rovers can cope with that with some care, but it
seems many fixed stations are using rotators that don't provide that kind
of resolution. So it could be hard lining up. In contrast, over-the-
horizon signals smear out a lot horizontally. I once estimated another
rover who was 180 km away as having about a 45-degree signal width, and
found 20+ degrees to be common.

Besides that, 10 GHz, and sometimes 5.7 GHz, will occasionally do
downright freaky/cool things. Rainscatter is one fun example. And I
once worked a station (S9, SSB) over a 50+ km path with a 90-degree bend
in it, because the "line-of-sight" path was heavily obstructed by trees
etc. We told each other we were bouncing off downtown Cincinnati, which
was not visible over the horizon.

I guess I'd like to end this sales pitch with the note that the Ham
microwave bands can be great fun. The advantages, and Ham capabilities,
of playing around with "real radio" and the small antennas that involves
can be substantial.

50 MHz and up ... Everything else is DC.

--
Rich

On Sun, 14 Jun 2009 10:17:20 -0500, Rich Griffiths
wrote:

Shannon's equations provide most of the answers:
http://www.st-andrews.ac.uk/~www_pa/...rt8/page1.html

Shannon's equations don't actually tell you much that's useful, as a
Ham.

This would presume that the Ham is rather indifferent or incapable.
Shannon's work is exceedingly useful, and at the core, quite simple to
perceive which further illuminates those mediocre qualities of your
Ham. It is unfortunate that the link above offers no graphs by which
Shannon's points would become startling apparent.

For instance:
http://www.aero.org/publications/cro...ages/04_04.gif
shows how signal to noise ratio has a vast effect over bit error in
digital transmission. In the face of equal powers (noise and bit
level), you would run the odds of 1 bit in 10 being mistaken (pretty
good odds, actually). If you were to raise the power in the bit by
10dB, that would fall to 1 bit in a million being mistaken.

In the same graph, Shannon reveals how, if you code your bits (I will
leave it to the student to discover the meaning of that), you could
achieve the same 1:1000000 advantage with the addition of less than 1
dB of power boost.

73's
Richard Clark, KB7QHC

On Jun 14, 10:03*am, Richard Clark wrote:

In the same graph, Shannon reveals how, if you code your bits (I will
leave it to the student to discover the meaning of that), you could
achieve the same 1:1000000 advantage with the addition of less than 1
dB of power boost.

73's
Richard Clark, KB7QHC

Until fairly recently, hams didn't do much coding, for a variety of
reasons. Computational horsepower is probably a big reason. Coding's
easy, decoding not so easy, at least in a "parts readily available
from Radio Shack" sort of sense. Obviously, today, one could do all
sorts of coding on a laptop PC, particularly at low bit rates, but
you'd still need to have an unusual convergence of someone who knows
how to implement the coding algorithms who's also interested in
amateur microwave operating. It's not anything like a turnkey thing,
or even a "go get gnuradio" thing. Where you see coding in common ham
use, it's buried in an application (PSK31, JT65, and the like)

The other problem is the frequency control issue. If you want low
rates and ragged edge of Shannon, you need good frequency stability
and control (and to a lesser extent, good phase noise). Until
recently (with GPS disciplined oscillators and surplus Rb sources)
this was a real challenge. As Rich commented with respect to antenna
pointing, you also have to be right on for frequency, and that's hard,
especially in a field situation. Tuning to 10Hz accuracy at 10GHz
implies 1E-9 frequency accuracy, which is challenging. To a certain
extent, processing power in a PC helps (get close, do parallel
demodulation, find the signal), but just like for coding, it requires
finding a person (or small group) who can deal with building low phase
noise stable oscillators AND with developing software that is somewhat
complex, compared to the usual "whack it out in a weekend of coding"
stuff.

I suspect there ARE hams experimenting with this, but it's a long way
from critical mass wide acceptance. You need something that you can
write an article in QST, and offer $100 widgets to make that
happen. There's not much cheap surplus gear either, since commercial
equipment these days tends to be more specialized and isn't as
amenable to hackery.

There are also proprietary rights issues with some coding techniques
(e.g. Turbo) but I suspect that legal issues aren't what's holding
hams back. For things like LDPC, there are published software
implementations that are free to use. I haven't looked but I imagine
that various convolutional codes and decoders are also publicly
available, along with Viterbi soft-decision decoders.

On Sun, 14 Jun 2009 11:08:36 -0700 (PDT), Jim Lux
wrote:

Until fairly recently, hams didn't do much coding, for a variety of
reasons.

This neither negates the specific issues of signal to noise in their
relationship, a matter that is quite in the power of the Ham to
control to some extent; nor does it invalidate the simplicity of that
relationship revealed through one graphic that serves to reduce the
obscurity of a lot of math.

As for the variety of reasons, computation power would seem to be in
abundance (the first mythical Cray is a door stop today). That as an
excuse is a croak.
It's not anything like a turnkey thing,
Like I said, an indifferent or incapable individual in the guise of
"Ham." I am amazed how that Lid is raised on a pedestal.
or even a "go get gnuradio" thing. Where you see coding in common ham
use, it's buried in an application (PSK31, JT65, and the like)
So, let me get this straight, because it is available (a seeming
contradiction from the tenor of your response), it is not accessible?
Or it is not useful? Or it is not understood? Or Shannon has been
rendered obsolete?

Your objections are answered with your own solutions and yet the sense
of what you say is shove Shannon out the window and whine about the
noise.

73's
Richard Clark, KB7QHC