Re: Re: [Repeater-Builder] Repeater input getting interference FROM a TV?

[Robert Koblish]

On Tue, Mar 11, 2008 at 10:59 PM,  <[EMAIL PROTECTED]> wrote:
> At 3/11/2008 12:34, you wrote:
>
>  >Ron,
>  >
>  >I think that Bob was trying to say that the N-th harmonic of the
>  >15.734 KHz Hsync was 1.5 KHz away from the receiver's center
>  >frequency. But Bob, I calculate that N=9402.8857. If you take N=9403,
>  >that ends up 1,8KHz away from 147.945 MHz.
>  >
>  >These calculations are bogus, though, since the Hsync frequency has a
>  >few more decimal places than I've used. At such a high harmonic the
>  >decimal places matter.
>  >
>  >Bob, was the TV old enough to use hollow-state devices? Could have
>  >been Barkhausen oscillations in that case.
>
>  It wasn't a TV - it was an analog security camera.  Still had lots of
>  15.7xx kHz horizontal sync in it, enough to generate the mix.  We simply
>  disconnected it to solve the problem (with the site owner's permission, of
>  course).
>
>  Bob NO6B
>
>
>
>
>
>
>
>
>  Yahoo! Groups Links
>
>
>
>

Ah, so!
It was 15750 Hz in the original monochrome (RS-170?) standard.
With NTSC they adjusted it to 15734 Hz and the vertical sync changed
from 60 Hz to 59.94 Hz. That's why you see a slowly scrolling hum bar
in the pic when the receiver's power supply starts to go; the 59.94 Hz
vertical refresh beats against the 120 Hz hum.

Everything in NTSC is based on dividing down the 3.579545 MHz clock.
Brilliant work, especially considering that it took a 6SN7 and a bunch
of discrete parts just to make a single flipflop at the time it was
designed and first implemented.

Older monochrome CCTV cameras derived their timebase from the 60 Hz
line, so the one that gave you grief might have used 15750 Hz
horizontal sweep. I dunno about recent ones.

Bob N3HAT

Re: [Repeater-Builder] Indoor antenna question

[Robert Koblish]

Chris,

Assuming that you've got a suspended ceiling with steel grid, just get
a mag mount antenna and stick it upside-down to the grid. Attach the
feedline to the grid for a couple of feet so it's perpendicular to the
radiator for 1/4 wavelength or so. And turn down the TX power. That
close to the repeater, you could even do without the PA, I suspect. So
if your radio shop has a mobile with a blown PA you might be able to
give it a home.

I used to operate indoors at home by sticking a mag mount to the top
of the fridge; a metal file cabinet would serve equally well as a
groundplane. But you'd have to be careful not to transmit when anyone
was near the file cabinet.

You could also make a ground plane with an SO239 female connector and
five 1/4 wavelength scraps of AWG14 solid copper house wire (one for
the radiator and four as radials). Put a loop in the top of the
radiator and suspend the antenna from the ceiling grid (or a hook)
with monofilament line.

Take a piece of coaxial cable, strip the jacket for something over 1/4
wavelength, spread  the shield braid and fold it back over itself, and
over the jacket. You've got a coaxial dipole. You need to strip more
than 1/4 wavelength because the braid shortens as you spread it. Trim
the braid and the exposed center conductor to 1/4 wavelength each,
apply tape or shrink tubing, and hang as above. The end of any antenna
will be a high impedance (hence high voltage) point, so use several
inches of monofilament to suspend it.

If your office has a window, you may have enough room to make a J-pole
out of burglar-alarm or copper foil attached to the glass.

Just some ideas.

Bob N3HAT

Re: Re: [Repeater-Builder] Repeater input getting interference FROM a TV?

[Robert Koblish]

On Tue, Mar 11, 2008 at 1:51 PM, Ron Wright <[EMAIL PROTECTED]> wrote:
> Bob,
>
>  I think you ment 15 kHz, not 1.5.  The horizonal freq in a TV is 15,734 Hz.
>
>  73, ron, n9ee/r
>
>
>
>  >From: [EMAIL PROTECTED]
>  >Date: 2008/03/11 Tue AM 09:59:16 CDT
>  >To: Repeater-Builder@yahoogroups.com
>  >Subject: Re: [Repeater-Builder] Repeater input getting interference  FROM a 
> TV?
>
>
>  >
>  >At 3/11/2008 02:20, you wrote:
>  >
>  >>Hi Folks,
>  >>
>  >>Here's a new one for me. In the same building as one of
>  >>our repeaters there's a Sony TV hooked up to Dish/Echostar.
>  >>The repeater is on 147.345/945. When anyone turns on the
>  >>Sony TV hash is produced on the repeater input and locks
>  >>up (keys) the repeater. It does not matter if the sat rcvr
>  >>is on, off, powered off, or disconnected. The TV itself
>  >>is producing the RFI.
>  >
>  >Is the interfering signal ~1.5 kHz above or below your input?  If so, it's
>  >the 38th harmonic of the horizontal sync. of the TV mixing inside the TV
>  >with your output & landing on your input.  Had this happen with a security
>  >camera once.
>  >
>  >Bob NO6B
>  >
>  >
>
>
>  Ron Wright, N9EE

Ron,

I think that Bob was trying to say that the N-th harmonic of the
15.734 KHz Hsync was 1.5 KHz away from the receiver's center
frequency. But Bob, I calculate that N=9402.8857. If you take N=9403,
that ends up 1,8KHz away from 147.945 MHz.

These calculations are bogus, though, since the Hsync frequency has a
few more decimal places than I've used. At such a high harmonic the
decimal places matter.

Bob, was the TV old enough to use hollow-state devices? Could have
been Barkhausen oscillations in that case.

73
Bob N3HAT

Re: [Repeater-Builder] Re: Gmrs Repeater

[Robert Koblish]

Note also that a 0.005% frequency tolerance is 3 times as big at 450 MHz as
it is at 150, hence the wider channel spacing that has been used on UHF in
the past.

-Bob N3HAT

On Jan 24, 2008 5:16 PM, <[EMAIL PROTECTED]> wrote:

> Paul,
> the underlying and unexpected contridiction here was not between 15Khz vs.
> 20Khz channel spacing but instead between 20Khz and 25Khz channels more
> commonly found on UHF which as you know is where GMRS resides. Historically
> there's more than enough guard between channels to compensate for slight
> overdeviation or slight drift off channel but now everybody wants to tighten
> things up so new gear is coming stock with 4Khz max dev associated with
> 20Khz channels vs. 5Khz dev which we've all enjoyed for years on 25/30Khz
> channels. Seems a bit odd of them to expect no interference problems from
> +/-5Khz peak dev on a 20Khz max channel but it is better for us users to
> have the option I suppose.
> Gary
>  Paul Plack <[EMAIL PROTECTED]> wrote:
> > Gary, 5 kHz deviation with a 3 kHz audio cutoff creates a signal at
> least 13
> > kHz wide, counting only the first set of sidebands. That's why many
> states
> > went from 15 kHz to a 20 kHz channel bandplan on 2m a few years ago.
> There's
> > nothing inconsistent in the FCC imposing both limits.
> >
> > 73,
> > Paul, AE4KR
> >
> >   _
> >
> > From: Repeater-Builder@yahoogroups.com
> > [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED]
> > Sent: Thursday, January 24, 2008 1:33 PM
> > To: Repeater-Builder@yahoogroups.com
> > Subject: Re: [Repeater-Builder] Re: Gmrs Repeater
> >
> >
> >
> > I wrote NARROWER GMRS SPECS. I did not write narrowband. After more
> study I
> > see some possible contridictions in the rules as currently written for
> > example, F3E/G3E GMRS emissions (probably the most common) are limited
> to
> > 20Khz bandwidth but at the same time those emission types are also
> limited
> > to +/-5Khz peak dev. I suspect the FCC doesn't really give a damn these
> days
> > especially with rebanding, auctions, and now a congressional
> investigation
> > on their doorstep.
> > Gary
> >
> >  wd8chl <[EMAIL PROTECTED]  com> wrote:
> > > [EMAIL PROTECTED]   wrote:
> > > > Good idea Richard especially since Mitreks are wideband radios so
> making
> > them meet the narrower GMRS specs is probably a monumental task. I
> wonder if
> > a Mitrek can even meet the required frequency tolerance.
> > > > Gary
> > >
> > > You're not talking about the Part 90 narrowbanding? Part 95 (GMRS)
> does
> > > not have to narrowband. The 12.5 tertiaries are NB, but the normal
> > > channels can stay as they are.
> > >
> > >
> > >
> > >
> > >
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> >
> >
> >
> >
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> Yahoo! Groups Links
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