25 Mar
2019
25 Mar
'19
3:36 p.m.
Hello Brian, Hello Michael,
Brian is right about multipath.
But with an unobstructed omnidirectional antenna for the NPR Master (the
central repeater), and diretional antennas (Yagi) at clients-user side
(even partially obstructed), you avoid almost totally the multipath
problem, and you can increase the symbol rate. That is exactly what we
do here in the suburb of Paris with my protocol NPR (New Packet Radio)
at 500kS/s.
This is also well explained here by the Slovenian team.
http://lea.hamradio.si/~s53mv/nbp/link.html
Furtheremore, here in France, the radioamateur rules request us to use
horizontal polarisation for “high bandwidth” protocols, on the 70cm band.
If you want to make data links with only omnidirectional antennas, then
you should
* either use lower symbol rates (<200 kS/s)
* or use more complex modulations, like COFDM
Last, but not least, in order to achieve good distance links, you need
much better radio links with such wide band protocol, than with a narrow
band protocol. Therefore, you need gain antenna, and Yagi is an easy
solution.
73,
Guillaume F4HDK
Le 25/03/2019 à 08:30, Michael Fox - N6MEF a écrit :
Although my
understanding of such things far from complete, I have
heard that one of the biggest problems with high data rate radio
is "smearing" of the symbols by multipath, and that if you can
achieve little or no multipath (akin to analog TV "ghosting"), you
can use much less robust modulation schemes to achieve higher symbol
rates.
No doubt Guillaume can correct me on this.
- Brian
Brian,
Yes, I understand that. But he shows an omni at the central site. And a
directional antenna doesn't eliminate multi-path (although it can help
reduce it, depending on the source of the multi-path). So, I didn't want to
assume what he meant by " In order to achieve links with omnidirectional
antennas, you have to lower the datarate drastically".
Michael