[aprssig] Fw: [nwaprssig] UHF 9K6 Baud vs. VHF 1200 Baud
krm1012 at qwest.net
Thu Oct 12 13:38:15 CDT 2006
----- Original Message -----
From: "R.M. King" <krm1012 at qwest.net>
To: <nwaprssig at nwaprs.info>; <aprssig-request at lists.tapr.org>
Cc: <bruninga at usna.edu>
Sent: Thursday, October 12, 2006 10:45
Subject: [nwaprssig] UHF 9K6 Baud vs. VHF 1200 Baud
> For sometime now I have been puzzled by the response of several persons on
> both the NWAPRS and National APRS sigs to UHF 9k6 Baud operations.
> I have heard:
> a) There is a 9db penalty for using 440Mhz vs. 144MHz. Implying this
> is an unacceptable or significant loss in performance for APRS operations.
> b) There is a 2db or so penalty for using G3RUH FSK vs.AFSK. Implying
> the wider bandwidth required by 9k6 Baud vs. 1200 Baud causes an increase
> in noise thus causing a higher BER rate adding to performance degradation
> at 9k6 Baud.
> c) QSB and phase distortion are worse at the higher frequency and other
> d) It is more difficult to get 9k6 Baud operations to work properly.
> Of course, all of this is keeping some people from trying UHF 9k6 APRS
> None of this seemed to agree with my three years of practical on-the-air
> operation of a small 440.875MHz 9k6 Baud APRS network in the Puget Sound
> area of Washington State. My observations on both 144.390 and 440.875MHz
> seem to point to 440.875MHz 9k6 Baud as a better choice for APRS
> Let me address each of the complaints I have heard.
> The 9db path loss penalty - It is true there is a 9.7db path loss when
> using 440.875MHz compared to 144.390MHz. However, in every case for
> practical operations this is NOT a significant factor. It does NOT reduce
> performance for distances used on Earth for APRS mobile or fixed
> operations. The real limiter is the same for 144.390MHz as it is for
> 440.875MHz. And that limiter is LOS distance and curvature of the Earth.
> (slant range) The signal strength at the ANTENNA TERMINALS for a typical
> 440 mobile reciever is well within the lower limit for an acceptable BER
> for APRS. A practical working range may be 20 miles. This is true for
> 144 and 440MHz.
> Using two identically equipped base and mobile stations on 144 and 440MHz
> (25Watts at the base antenna, a 6db gain gain antenna at the base station,
> a 3db gain antenna at the mobile station, and a 20 mile path), the power
> at the mobile station antenna terminals is -55.8dbm for the 144MHz station
> and 9.7db less or -65.5dbm at the 440MHz station. BOTH of these signal
> strengths are sufficient to drive the typical FM receiver into limiting
> enough to produce an acceptable BER for APRS. It looks like this is true
> even at 100 miles. This is far beyond the slant range for most stations
> on Earth.
> The reverse path loss (mobile to base and mobile station using 25W) in
> this case is 3db more. This is still not enough to cause problems at
> The 2db penalty for using G3RUH FSK vs. AFSK - This breaks down to NOT a
> conversion penalty for G3RUH FSK but a concern about BANDWIDTH in the I.F.
> needed for the 9k6 Baud modulation vs. 1200 Baud modulation. It turns out
> if you use the same receiver, the bandwidth for both modulations is the
> same. About 15KHz. The noise in the I.F. section is the same whether
> 1200 Baud or 9k6 Baud is used.
> QSB, phase distortion, etc. - These are very hard to quantify and would
> take a lot of work to compare between 2 meter and 3/4 meter operation.
> But let me say this; The packet length at 9600 Baud is less than 1/3 the
> length in time at 1200 Baud when a 200ms. TXD overhead is added to both
> packets. This is significant for several reasons. It means there is 60%
> less time for all the man made things to kill an APRS packet. Although I
> can't put this into quantifiable units, practical experience indicates
> this is very significant in improving APRS performance. The shorter
> packets do improve throughput. There is a 3X improvement to be sure.
> It is more difficult to get 9k6 Baud operations working. If this means
> you must be sure your transmitter deviation is set properly with a
> deviation meter, then it's true. It is near impossible to set 9k6 Baud to
> 3-kHz without a deviation meter. It's only blind luck if one gets it
> right without a deviation meter. So PLEASE, don't even try without a
> I have measured the deviation of several D7's and D700's and have not
> found a bad one.
> At 1200 Baud you can get away with murder. Even an obscenely overdeviated
> transmitter may get decoded. But as Bob Bruninga indicated in an earlier
> post, just taking a 1200 Baud TNC out of the box and hooking it to a
> transmitter is VERY bad practice. And it quite likely will be illegal.
> You should use a good deviation meter even at 1200 Baud.
> So then. Why try UHF and 9k6 Baud? The big reason is throughput. Much
> more activity can be tolerated on a simplex channel at 9k6 Baud than 1200
> Baud. More net reliability is built in, NOT less as many people seem to
> think. It improves the state of the art. Better equipment is used,
> generally, at 9600 Baud.
> When 144.390 finally chokes itself to death, UHF and 9k6 will still be
> working. Who knows, maybe the equipment manufacturers will pay attention
> and develop gear for higher speed data operations. One thing is for sure.
> We can't grow if we stand still. We can't learn new things if we don't
> Anyone want to try 102.4K Baud APRS?
> Bob King
> krm1012 at qwest.net
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> nwaprssig at nwaprs.info
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