FT4 via satellite

Interesting tests with FT8 and FT4 via linear transponders. I hope we will see more of this in the future, for hams not interested in SSB or/and CW.

Digital Mode Experiments Conducted on Linear Satellites
From ANS-152

[EDITOR’S NOTE: Be aware that the experiments described below utilize
very narrow AFSK modulation with fairly advanced computer control for
Doppler correction and frequency stabilization. They do NOT involve
use of narrowband FM signals such as those used for terrestrial APRS or
dedicated APRS satellites. So please, NEVER transmit FM on the uplink
to a linear satellite. ALSO, in should be emphasized that power levels
must be kept very low, as all the WSJT modes are 100% duty cycle.]

Recently a group of regular satellite-using amateurs conducted experi-
ments with FT-8 and FT-4 on a variety of linear satellites. Alan
(WA6DNR), Carlos (W7QL), Dave (W0DHB) and Ron (W5RKN) were involved and
made many satisfactory QSOs. The primary results and observations from
these tests are:
• Digital modes can successfully be employed on the linear birds
while not interfering with concurrent users by operating close to
the bottom end of the passband, using the lowest power practicable
and using very narrow signals.

• We avoided the satellites known to be power-sensitive, FO-29 and
AO-7. Testing was conducted with CAS-4A, CAS-4B, RS-44, XW-2A, XW-2B,
XW-2C and XW-2F.

• FT-4 was the most robust signal format, compared to FT-8. Other
of Joe Taylor’s digital signal modes should be investigated.

• The rate of change of Doppler just before and after TCA is high on
the lower-orbit satellites and must have compensation. 200 Milli-
second Doppler updates allowed 100% copy of FT-4 transmissions
throughout several passes of different linear satellites. This can be
accomplished in SatPC32 by setting the “SSB/CW Interval” in the CAT
menu to zero and checking the 5X box. Note that this setting is not
retained when SatPC32 is shut down.

• Very little power is needed for QSOs throughout the duration of the
pass, from AOS to LOS. W7QL set IC-9700 power to “zero” (roughly 500
milli-Watts, with over 3 dB cable/connector loss to a Leo-Pack anten-
na pair) and copied every packet on several satellites.

• FT-4 is very tolerant of voice signals which might drop on top of
an ongoing FT-4 QSO. However, an FT-4 signal dropping in the middle
of an SSB QSO would be quite annoying to the SSB operators.

• According to Joe Taylor, the occupied bandwidth of an FT-4 signal
is 90 Hz. So theoretically over 200 such signals could be present on
a 20 KHz channel. Obviously that will not, and should not happen. But
a dozen closely spaced FT-4 QSOs at the bottom of the band, each run-
ning very low power should hardly be noticeable to current voice and
CW traffic.

We invite other Satellite operators to join us in this expanded utili-
zation of the linear satellite resources available to us. We recommend
using FT-4 AT VERY LOW POWER, in the bottom few KHz of the downlink
frequency range, with appropriate Doppler compensation, as described
above. We hope to have a digital QSO with you sometime soon on the
linear birds.

[ANS thanks Carlos Cardon, W7QL, for the above information]


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