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Re: upling frequency fo20/29

eduardo j valdez (du1ev / n1wlv) wrote:

> i have a problem with the ft-847 when i attempt to operate fo20 or
> 29.  i could not locate my downlink because i could hear myself
> coming down on the radio but not via satellite.  i do not know what
> you call this.  is there a way of not overloading the radio with my
> own signal?
> i would like to have the downlink on 435.850.  i would appreciate
> any help as to what frequency i should transmit on.


Reportedly, some production lots of the FT-847 have a "spur" on the 2M
output that falls very close to the 70cm downlink frequency of FO-20 /
FO-29 when transmitting on the proper uplink frequency.  Even though I
have one of the earliest production models, mine does not have a
significant problem with this.  There were times in the past when I was
just starting to work the Fuji's, when I had the receive gain and
output power maximized (described below), and was way off frequency in
a futile attempt to compensate for Doppler, when I could hear an echo
that was not coming from the bird, but was apparently the 3rd harmonic
of my transmit signal coming in on receive.  But if your radio does NOT
have the specific spur problem mentioned by others, the third harmonic
of 145.95 MHz is far enough away from 435.85 MHz that you should not
have a big problem with it.

Is your setup good enough to hear other QSO's in progress on the Fuji's
when you are not transmitting?  If so, does the QSO in progress get
covered up as soon as you push the PTT?  If not, perhaps your setup is
simply not good enough to get a reliable signal into the satellite and
back down to you.

For FO-20 and FO-29, I have a satellite memory programmed to start in
the middle of the passband as 145.950 LSB up and 435.850 USB down, with
tracking set to TCK-REV (due to the inverting transponder, of course).
Using STS-Plus for my tracking program, I can get a continuous display
of the appropriate doppler correction to dial in from there.  I find
that the numbers suggested by STS-Plus are close, but not quite right,
so I start there and dial around until I hear my downlink.  From that
point, the tracking of the two VFO's works very well to allow me to
move up and down the passband with the main tuning knob and keep the
uplink and downlink frequencies tracking together.  Now that I've had
some practice with it, I can fine-tune the Doppler correction during a
QSO most of the time, even while working the azimuth rotor manually.
But on high-elevation passes, I lose the bird during the middle of the
pass and have to re-establish my offset for the second half of the
pass again.

My FT-847 has a pair of RF Concepts "brick amplifiers" with switchable
preamps connected to it.  They are in the shack, not up near the
antennas which would be much better.  The antennas are fed with about
60 feet of LMR-400 coax, which is a little bit better than the Belden
9913 coax that may be more familiar to you.  Right now, the 2M antenna
is an old Cushcraft 11 element model, and the 70cm antenna is a recent
Cushcraft 719B (19 element) model.  Both are horizontally polarized
with fixed zero-degree elevation, and an azimuth-only rotor.  (This
will change dramatically in the Spring, with an elevation rotor added,
a stacked pair of better antennas on each of those bands, plus loop
yagis and transverter gear for modes L and S.  All the new goodies
except the transverters are already stacked up in the garage awaiting
the change of seasons!)

To get into the Fuji birds reliably with this setup, I use the internal
preamp in the FT-847 plus the external preamp in the RF Concepts model
RFC-4-310 amplifier.  Depending on range and elevation, I can sometimes
work these birds with most or all of the 50-watt output of the FT-847.
At other times, I need to kick in the 170-watt (maximum) external amp,
dialing back the output drive on the FT-847 to the minimum level that
gets me into the bird with a usable signal.  Due to lack of antenna
elevation, on real high elevation passes I can't get in during the
middle of the pass with all 170 watts, which is really no surprise.
But the point is that with a mediocre 2M antenna, a modest 70cm
antenna, more feedline than ideal (even though it is of good quality),
and either no external assistance or preamps/power amps in the shack
instead of near the antennas, the setup may be marginal without any
antenna elevation capability.  Just suggesting another possibility to
consider before assuming your radio is defective.

Good luck sorting it all out.

John Toscano, KBØZEV, EN34js
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