I have observed ascending multipath change to descending when switching antenna
polarity (H to V). The barber pole "strips" (notches) change direction. K3DCC
and I have experimented using 2 independent sync'd receivers with the same and
opposite polarity antennas achieving good to mixed results. We've noted the
sync'd receiver's combined output signal feeding FreeDV lowers the signal level
some but the noise floor is reduced even more increasing the SNR. In marginal
conditions this can help avoid dropouts and improve speech quality. Flex calls
this their "ESC" Enhanced Signal Clarity allowing the user to change the
phasing (magnitude and angle) with a simple control to find the sweet spot for
best reception. I did not notice if multipath was reduced but will make a
closer observation next QSO. I am sure some one on the list already knows the
answer. K3DCC and I are just experimenting. Maybe a cicular polarized
antenna would work better. In this article, describing one it reduced
multipath. http://www.bruhns.us/CP_on_HF/CP_on_HF.html
Mel
----- Original Message -----
From: Bruce Perens
To: [email protected]
Sent: Monday, March 18, 2013 7:50 PM
Subject: Re: [Freetel-codec2] FreeDV Robustness 3
Hi Oliver,
It's simply a matter of getting the most data in the least bandwidth. I think
there's room for experimentation with GMSK on HF, and FreeDV will be getting a
GMSK modem that is intended for VHF/UHF, so we can try it then.
Our main problem on HF is very clearly fading. I have a theory that circular
polarization of the receive antenna could help, but can't say when I would get
time to test it.
Thanks
Bruce
Oliver Johns <[email protected]> wrote:
Dear group,I have used the search tools in the list archives to try to find the
discussion about choice of modem. I wonder why FreeDV is using a modem with
quadrature signals rather than a pure FSK system similar to RTTY and MFSK or
Thor. (1) With a pure FSK system (one carrier on continuously, varying
frequency) I think one can use class C amplifiers. This would be a selling
point for DV. The need for amplifier linearity is a product of the SSB era,
but it need not be part of the DV era.(2) Ionospheric multi-path is notoriously
hard on PSK systems since the phase gets scrambled more severely than does the
amplitude or the frequency. This is why PSK31 communication often has a high
error rate in spite of seemingly good conditions.Could someone point me to the
white paper or email thread that treats these issues?Thanks,Oliver W6ODJ
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