[Uram-rejections] What repeaters can you access?

Wed Mar 16 22:47:52 GMT 2016

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> From webstump at chiark.greenend.org.uk Wed Mar 16 22:26:44 2016
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> From: Brian Reay <no.sp at m.com>
> Newsgroups: uk.radio.amateur.moderated
> Subject: Re: What repeaters can you access?
> Date: Wed, 16 Mar 2016 22:23:26 -0000 (UTC)
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> Jerry Stuckle <jstucklex at attglobal.net> wrote:
> > On 3/16/2016 12:14 PM, Brian Reay wrote:
> >> On 16/03/16 15:36, Jerry Stuckle wrote:
> >>> On 3/16/2016 10:25 AM, Brian Reay wrote:
> >>>> On 16/03/16 12:52, Jeff wrote:
> >>>>> 
> >>>>>>> You are right, nowadays stability is not an issue, even the amateur
> >>>>>>> repeaters use GPS synchronized oscillators.
> >>>>>>> 
> >>>>>>> At VHF doppler (one TX behind you, one TX in front of you), is
> >>>>>>> limited
> >>>>>>> to 200 Hz (900 km/h), but at UHF it can be around 600 Hz for 900
> >>>>>>> km/h.
> >>>>>>> For aircraft flying above Mach 1 the situation is different (worse).
> >>>>>> 
> >>>>>> Can amateurs transmit from aircraft in P land? Our licence doesn't
> >>>>>> allow
> >>>>>> it?
> >>>>>> 
> >>>>>> If you can't, we only need to consider the 200Hz figure, as air band
> >>>>>> is VHF
> >>>>>> (other than mil. air band).
> >>>>>> 
> >>>>> 
> >>>>> If you are not considering /AM then the figure is a lot less than
> >>>>> 200Hz,
> >>>>> I can't think of a car that goes at 900 km/h!!!!!
> >>>>> 
> >>>>> Jeff
> >>>> 
> >>>> Indeed Jeff, which is why I asked about /AM in P land. Plus, of course,
> >>>> even light aircraft (the type most PPLs fly) aren't that rapid. I forget
> >>>> the recommended cruising speed of the Robin DR400 but I think is was
> >>>> about 65kts.Even allowing for a tail wind, getting to 900km/h would be
> >>>> 'adventurous' ;-) but I'm sure Wimpie wasn't suggesting that could
> >>>> happen. However, mil. jets do use the VHF aviation band so Wimpie's
> >>>> point is valid.
> >>>> 
> >>>> Perhaps it would be better to come at from the other direction (no pun
> >>>> intended), and ask what difference/error in frequency due to Doppler
> >>>> shift is the the performance degraded to a point defined as being out of
> >>>> spec. Then, use that to calculate the speed (or speeds, they will vary
> >>>> slighlty across the band) the vehicle can be travelling.
> >>>> 
> >>>> I suspect the tolerable error is small as trouble has been taken to lock
> >>>> the units frequency together but there may be another reason for that
> >>>> and I confess I'm not with the details of the system.
> >>>> 
> >>> 
> >>> Most single engine light planes (not light sport category) cruise in the
> >>> 100-140 kt range, with a few above or below that.  Turboprops may get up
> >>> to 200 kts.  Most can't afford multi-engine planes as a hobby (not that
> >>> single engines are cheap).
> >> 
> >> Oh, quicker than I thought. Perhaps I was thinking of the optimal climb
> >> speed, it is some decades since I let my qualification lapse. I have
> >> thought of renewing it but I can't see my having the time to make it
> >> worthwhile. Plus my stroke may be an issue.
> >> 
> > 
> > Which optimal climb speed?  There's best angle of climb (Vx), where you
> > get the maximum altitude for the minimum horizontal distance.  Then
> > there's best rate of climb (Vy), where you get the maximum altitude for
> > a time period.  Vx is somewhat slower than Vy, but in small planes
> > somewhere around 65-75 kts would be about right.
> > 
> 
> All I recall is 65kts. I may have my notes in the loft but, after  about 30
> years, finding them
> would be a challenge.
> 
> 
> 
> >>> 
> >>> And at those speeds, Doppler shift would be less than error margins
> >>> unless you had some expensive, lab grade equipment.
> >>> 
> >>> GPS detects location by the difference between time signals received
> >>> from the satellites, and velocity by the differences between locations.
> >>> AFAIK, it doesn't use Doppler shift at all.
> >> 
> >> Trigulation. But we weren't discussing Doppler on the GPS signal, Wimpie
> >> was referring to Doppler shift on the voice channel- hence the
> >> difference at UHF.
> >> 
> > 
> > Yes, I was just pointing out that even GPS doesn't use Doppler shift.
> > And Doppler shift on a VHF channel (118-136 MHz) is virtually
> > unmeasurable unless you have a very stable reference such as a GPS
> > signal and are using lab quality equipment.
> > 
> 
> Perhaps it would help if I recapped the path of discussion:
> 
> Wimpie mentioned the voice Channel frequencies were all GPS locked and
> mentioned Doppler. I commented I thought the Doppler would be low, Wimpie
> calculated 200Hz at VHF  for civil aircraft (he didn't mention speed). I
> suggested perhaps it would be better to approach it the other way, what
> shift was a problem, then work out the corresponding speed. 
> 
> I think you joined in the latter part and perhaps miss-understood the
> aspects we were discussing, rather than how things like GPS etc work. 
> 
> 
> 
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