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Fw: 434 or 439 LVSB ATV



Hi Everyone.

It seems about once a year the old issue of ATV Vs weak signal and
satellites comes up on this reflector.  I forwarded a few of the messages to
Tom O'Hara, who is the most knowledgeable person on ATV that I am aware of.
In addition to owning/operating PC Electronics, Tom is the ARRL Technical
Advisor for ATV.

As someone pointed out, most of the ATV operators are also weak signal and,
or satellite operators.  Therefore, it is not We Vs. Them.  It is US.

Tom's response is attached below.

Merry Christmas,

Woody
KJ4SO

> --------
> 434.0 and 439.25 MHz ATV Technical Band Plan Considerations
> by Tom O'Hara W6ORG
>
> As more and more FM voice repeaters come on the air in any given area
below
> 444 MHz, ATVers get more interference and consider moving down the band.
> Cross band repeat with 426.25 input and 33 or 23 cm output is a good
> alternative if most have good line of sight and close enough to the
repeater
> site.  However the 70 cm band still goes much farther than the higher
bands
> given the same power and antenna gains - 900 MHz is 1/2 and 1200 band is
1/3
> - not to mention the higher coax loss and cost.
>
> Forget the ARRL 70 cm band plan, it was never a technical band plan but
was
> adopted back in 1979 after a survey of what some people happened to be
using
> at that time by the VUAC.  It has never stood technical review since then
and
> since there are so many different local band plans now, I doubt one could
be
> agreed upon.  One of the glaring errors is the overlap of the 439.25 MHz
ATV
> passband to 444.0 MHz and FM voice repeaters down to 442 MHz.
>
> I suggest each area have a band plan meeting with 1 or 2 of the best
> technically qualified representative from each amateur mode to work out an
> agreed upon technical band plan as we did here under SCRRBA (So. Calif.
> frequency coordination group) sponsorship many years ago.  SCRRBA does
> repeater and link coordination as well as maintains the band plans but the
> band plans are engineered and determined by consensus of a technical
> committee representing weak signal, satellite, digital, experimental as
well
> as FM voice and ATV.
>
> Using 434.0 or 439.25 lvsb for ATV can be a win win situation for FM
> repeaters as well as ATVers.   Both will have room to operate with minimal
> interference.  FM repeaters or what ever modes the local band plan works
out
> can go down to 441.0 if 439.25 is used for ATV or 438.7 if 434.0 is used.
> The A line people really only have 439.25 or must forgo normal sound
> subcarrier.
>
> We here are satisfied with the legal reasoning we have given to use either
of
> these two ATV video carrier frequencies given the spirit and purpose of
the
> FCC rule of preventing interference from terrestrial repeaters within
431-433
> and 435-438 MHz segment.  Each area will have to make their own minds up
but
> those who just want to eliminate ATV from the band, or have had a personal
> bad experience with an ATVer, or enjoy debating or strictly interpreting
the
> FCC Rules will just have to spend the time battling it out with local
ATVers
> - it seems that is more of a hobby for some than enjoying all the many
modes
> of Amateur Radio and finding ways to work it out.
>
> Let me restate the legal reasoning, because we did not arrive at it
lightly.
> Also we feel to put in a request for a rule change would stir up the pot
much
> like it has done on remailers and most of us did not feel we wanted to put
> the time in trying allay the interference fears by educating and
responding
> to so many people across the country.  Hopefully, as each area works out
> their local all mode band plans, the true technical characteristics and
proof
> would come out to everyone concerned, just as we had to do here with our
weak
> signal and satellite people.  There is no way we could travel around the
> country with a spectrum analyzer and gear to make the tests to so many,
and
> no amount of written technical proof seems to over come the fear of the
> possibility.
>
> We moved down to 434.0 in Southern California over 20 years ago to allow
more
> FM voice channels which were growing very rapidly in the 70's.  Our FM
voice
> repeaters go down all the way to 440.0 and have been full up for years -
over
> 600 coordinated voice repeaters, most of which are on high sites greater
than
> 4000 feet above the coastal plain.  At the same time we did not want to
> exchange interfering with one mode only to have it with another.  Given
the
> spectrum power density of an ATV signal being 95% +/- 1 MHz from the
carrier,
> the 434 frequency had little energy in the weak signal or satellite bands
and
> on the air tests confirmed it to the satisfaction of all 3 parties.  So we
> feel we have satisfied the prime directive of 97.101(a) of good amateur
> engineering and practice and 97.101(d) where the use of 434.0 does not
cause
> interference to the two modes who wanted protection from repeater users
with
> the exclusion.
>
> In addition, the ATV repeaters use horizontal sync detectors (15734 Hz) to
> key up the repeater.  Therefore it does not intentionally key up and
repeat
> in the presence of any other mode - just its intended AM video modulated
> carrier at 434.0 MHz.
>
> The received energy at the repeater is below the 26 dB in the 431.0-433.0
and
> 435.0 to 438.0 MHz segments per the definition of bandwidth per 97.3(a)(8)
> when using the 434.0 MHz video carrier frequency.  The sideband energy
from
> camera video is very low and random +/- 1 MHz from the video carrier.  If
you
> have access to a spectrum analyzer, watch a broadcast TV upper side band
and
> you will notice the levels are way down.  A photo can be found in the
> 1995-2000 ARRL Handbook page 12.48 figure 12.61 also.
>
> The sync is the only video component that is constant and according to the
> Television Engineering Handbook - 1992 - Benson - Fig. 5-11, the
horizontal
> sync harmonics sideband energy is more than 40 dBc +/- 1 MHz from the
video
> carrier.   So if you are transmitting 150 watts pep from a Teletec amp,
the
> sync energy in the weak signal and satellite segments is no worse than
> transmitting a 15 milliwatt pulse every 63.5 microseconds.
>
> In addition, a narrow band receiver is like a low pass filter to the video
> since at any given frequency, the amplitude of the video pulse depends on
the
> rise time as the camera horizontal sweep goes across and sees a white to
> black, or vice versa transition.  A full white to black vertical line in
the
> picture is going to be a rare occurance, more likely smaller amplitude
> changes in the shades of grey.  Since the video amplitude change is not a
> continuous sine wave, but occurs at the 15.7 kHz sync rates in time, this
> pulse is further attenuated by the narrow IF and audio filters.  Try
tuning
> across the band with a SSB or CW receiver as an ATVer is transmitting.  I
> doubt you will note anything outside of +/- 100 kHz of the video carrier,
in
> fact I can only remember one case of interference here in 20 years where
the
> ATVer was less than a half mile away from the 432 receiver and they worked
it
> out on two meters.  Some ATVers have multimode voice rigs and come on
during
> contests just to give the 432 gang some points now.
>
> In any given area, the local hams must decide to go to 434.0 or 439.25
Lvsb -
> you cannot run both 434 and 439.25 upper or lower VSB since the video will
> overlap.  Therefore the decision will have to be what two ATV frequencies
> will be part of the local band plan.  They need to be at least 8 MHz apart
to
> prevent interference in the weak on channel vs. strong adjacent channel
case.
>  This is why broadcast TV skips a channel on VHF and 2 channels on UHF.
> Cable TV does not have that problem because they go to great lengths to
> equalize and filter all the channels to the same amplitude.
>
> If more than one repeater is to be in the area on 70 cm, then sharing a
> common input can be done and coordinate on the two meter talk back
frequency.
>  Often, the two meter frequency is repeated back - at about 1/2 the sound
> subcarrier input level - on the ATV repeater sound subcarrier output.
Here
> in So. California, a person coming on in the southern part of Los Angeles
> County could possibly key up 5 ATV repeaters at one time if running an
omni
> on 434.0 MHz as well being linked to Las Vegas - we have to co-operate.
See
> the ARRL Repeater Directory ATV section.
>
> When using 434.0 and 439.25 lvsb, ATVers must accept the fact that local
hams
> up-linking to a satellite will occasionally wipe out the video depending
on
> beam headings.  But most satellites are only in view for up to 15 minutes
on
> a pass, and track in azimuth and elevation so be patient and wait.   We
even
> have some ATVers who put the satellite tracking map from their computers
on
> the air so we can see when the pass will be over.  As a consequence, many
> ATVers here enjoy working satellites (myself included) and vice versa.
>
> Those using 439.25 MHz ATV can use horizontal polarization to give up to
20
> dB of cross polarization loss to the vertically polarized FM repeaters.
> Conversely, since 432 MHz is horizontal, those using 434.0 MHz ATV should
be
> vertical.
>
> There is plenty of spectrum for all of us if those concerned are willing
to
> work out the local band plan on a technical basis with all mode users.
> Southern California has the highest communications density of hams and the
> unusual geography of a high mountain range full of repeater sites that
cover
> 100 miles up and down the coastal plain.  If we can work it out here,
other
> areas should be able to figure out engineering solutions also.
>
> 73, Tom O'Hara W6ORG
> ARRL Technical Advisor for Spectrum Management and ATV.
> SCRRBA Technical Committee member and 23 cm and ATV bands manager.
>


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