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DF3VI > WLAN 17.05.05 18:33l 45 Lines 2236 Bytes #999 (30) @ WW
BID : H5FDB0II_02N
Read: GUEST DK7JAN OE7HNT DK5RAS OE7FTJ DL7ALE DG0LGJ DO6NP DK7NR DD1OV
Subj: Re^2: Packet vs WiFi
Path: DB0FHN<DB0THA<DB0ERF<DB0ROF<DB0ACH<DB0PKE<DB0II
Sent: 050517/1717z @:DB0II.#NRW.DEU.EU [BCM M'Gladbach] obcm1.06b51 LT:030
From: DF3VI @ DB0II.#NRW.DEU.EU (Patrick)
To: WLAN @ WW
Reply-To: DF3VI @ DB0OVN.#NRW.DEU.EU
X-Info: Sent with login password
ZL4JAS wrote:
> was under the impression that 9k6 takes alot more signal than 1k2.
> That's what all my things say anyway and it makes sense. The higher the
> baudrate, the wider the bandwidth, the greater the noise, the stronger
> signal strength needed to overcome it.
This is only true if you use the same kind of modulation.
But 1k2 uses AFSK (normal FM modulated with tones), while 9k6 uses FSK
(the carrier frequency is modulated). You surely are a ham enough to
understand the fundamental difference?
The result is, 9k6 FSK does not need more bandwidth than 1k2 AFSK at all.
An overmodulated 1k2-signal is actually far wider than 9k6!
What is correct though that 9k6 needs a better S/N ratio, because of the
higher data speed it is more sensitive to noise.
But the difference is not so big that it makes really a difference :-)
> > 23cm links *can* work where 2m links don't.
>
> Not often! Using the same power and antenna gain at each end, 23cm will
> be a weaker
Not true because of two reasons. You have to compare the same antenna size
(a 4 element on 2m has the size of a 13 element on 23cm), and then the gain
on 23cm is much bigger. Also 23cm is less affected by fading.
On the other side, 23cm needs a more optical line of sight (no free sight
means no good link in general, but I work my node on 23cm without seeing it).
A second advantage of 23cm is that there is much more space, so you can run
many individual links, instead of crowded places on 2m. That increases the
throughput dramatically again! We run here 23cm fullduplex with 19k2 on many
links, and duplex filters are so small on 23cm...
> That's why they use 300 baud on HF. The weaker and noisier a signal, the
> lower the baudrate that can be used.
Even 300 baud packet is inefficient on HF. Because of the low data rate
frames take very long time and the possibility of packet loss is very high.
Remember that a single bit lost in a frame means the frame is lost.
But there is no forward error correction which is absolute needed on noisy
channels. Other modes like Pactor are much more efficient here.
73, Patrick
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