[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: [bod] Fwd: 70 cm band plan???



Hi, Gene!

Robin Haighton, VE3FRH, asked me to take a shot at answering your call for 
help.  Let me see what I can do.

As I understand it, the heart of the problem is summed up in these two 
paragraphs.

> >The second group is proposing a second ATV repeater for 70 cm. They have
> >selected a frequency pair which includes a wide-band  TV (fast-scan, full
> >color) input frequency at 434.000 MHz. The local ATV activity with a 6 MHz
> >bandwidth would/could prevent weak signal reception from satellites. I am in
> >a turmoil here!!! I've asked the second group to change the input frequency
> >with no avail.
> >
> >I have heard that "ARRL or the FCC" has indicated a band-plan for 70 cm, but
> >no one here seems to be paying attention to my thoughts. The second ATV
> >group is preparing to request "frequency coordination" from "T-MARC" ( the
> >local repeater coordinator). I have attended the annual T-MARC meeting today
> >and they indicated little or no objection to the requested 434.000 MHz input
> >frequency. I need some help here to convince all that "a bad choice is in
> >the making".

Some thoughts for you.

ARRL does have a suggested band plan which is published in several places 
including the repeater directory, last I checked.  Within this band plan 
are some local options which are considered by the frequency coordinating 
committee.  And, there is the band from 435-438 MHz for the 
amateur-satellite service.

FCC respects local frequency coordinators for repeaters and auxiliary stations.

However, every amateur radio operator signs away the right to any specific 
frequency as a condition of application.  Sharing is the order of the day.

Now, there are some technical considerations.  First, is the ATV power 
spectrum.  While the total bandwidth of an ATV signal may be on the order 
of 6 MHz, the power spectrum in not uniform.  Most of the energy in common 
vestigial sideband NTSC color tranmissions is concentrated at three 
points.  One is at about 1.25 MHz from the bottom of the 6 MHz "channel" 
for the visual carrier and it's most significant sidebands.  The vast 
majority of the sideband energy is most likely to occur within about +/- 1 
MHz of the visual carrier.  The other two are the color subcarrier at about 
3.58 MHz above the visual carrier and the aural (sound) carrier at 4.5 MHz 
above the visual carrier.  What's important here is that, while the power 
from an ATV station in the band 435-438 MHz is not zero, it may be low 
enough to be tolerable.

How can you tell what will be tolerable?  This is a normal exercise in 
checking compatibility.  You'll have to look at your own station with its 
directional antenna, where the antenna is likely to be pointing, and the 
signal you'll be receiving and transmitting.  (Yes, transmitting!  You may 
be a source of interference to the ATV repeater.)  Then, repeat the 
exercise for nearby ATV stations.  (You may have to go a ways to find one 
as they're not all that common even in our neighborhood.)  Keeping in mind 
the power within the bandwidth of your receiver, figure out how the power 
received from the unwanted station will compare to the signal you want to 
receive.

Clearly, if you have an ATV station next door, you're much more likely to 
have a problem.  But, as mentioned, that isn't too likely.

Just run the numbers and see how it comes out.  The ATV repeater folks 
should be doing the same thing as they will have to deal with you as a 
source of interference as well.

Then, there's the time factor.  Neither you nor nearby ATV stations will or 
should be on the air 100% of the time.  So, there may be more sharing 
opportunities here.

Would it be wiser for the ATV'ers to look to wider bands like 902-928 MHz 
or 1240-1300 MHz or other bands?  Sure.  These bands could stand some 
heavier use to help justify their existence in the minds of those who look 
at such things.  Probably so.  But, do the ATV'ers have to look beyond the 
420-450 MHz band?  No.  Not at all.

The resolution of this problem is most likely to be by means of mutual 
enlightened self interest.  Both you and the ATV group need to find a way 
to get along.  Technically, that may well be possible.  You'll both have to 
run the numbers taking into account frequency, power density, receive 
bandwidth, time, space, and other relevant factors.  T-MARC may be able to 
lend a hand.  (I'm sending a copy of this to a friend who is a member.)

This may not be the answer you were seeking.  But, I believe it's 
realistic.  I hope it helps.

73, art.....
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home