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Drake - Which IF frequency? (was: help needed...)



Hi Jordan,
It is really kind of early to tell, but the only sat with 2.4 GHz transponder
is the as yet unlaunched Phase 3D.  A couple others have beacons on mode S,
but I am not aware of any current ones using it for actual data transmission.
The two most commonly mentioned are UO-11 and DO-17.  UO-11 has power level
problems and is rather weak, but receivable on a good system.  DO-17, Dove,
(currently inoperative) has a strong beacon, but the data is corrupt in some
fashion.  Still it makes a good signal source for testing when it is on the
air.  No forecast when/if it will return.

But those satellites aren't really affected by your question, since you have
no need (that I know of) to uplink to them while receiveng the mode S signal.
(I know this doesn't apply to control ops, but I'm addressing the average
user's needs).  

Most of the Mode S activity is being done in preparation for the upcoming
launch of the Phase 3D satellite.  When launched, it will have a versitile
array of band combinations using many combinations of uplinks and downlinks.
It is beyond the scope of this note to describe them in detail, but more info
can be found in past issues of QST or on the AMSAT web site at: 
http://www.amsat.org

I believe that P3D will have a receiver for 2M uplink, but the uplink
anticipated to be most often used with 2.4GHz downlink is 435 MHz.  A close
second will likely be 1.2 GHz uplink, as more people become equipped to use
that frequency.  The antenna gain/size benefits realized by going to the
higher frequencies will likely make them the more often used bands.  

Based on this, it is desirable to keep the downlink IF frequency away from 435
in order to avoid unwanted reception of signal leakage from the uplink rig.
While either combination could be made to work, the simpler choice seems to be
to use 2M for the downconverter's output frequency.  This also allows the dual
band capability of most satellite active hams to be used instead of two
redundant 435 rigs.  Signal loss between the downconverter and the station rig
would also be less using 2M than with 435MHz.

On the other hand, there are some benefits to using the 440 band for the IF.
It is closer to the original passband of the Drake downconverter, and would
probably not require the IF mod to be performed (coil and cap removal).  It
may be the case that by using an IF frequency closer to the original that the
circuitry of the Drake would be more effiecient there, but I have seen no data
to back up this supposition.  There is also an inexpensive microprocessor
crystal that can perform this conversion, as previously mentioned on this
list.  

For those that missed it previoulsy, K9EK wrote, "An even better DigiKey
crystal for converting the Drake 2880 to 70cm IF is the P/N X163-ND. This is
also a 7.68 MHz, 20pf load cap crystal, but it is in an "HC49/US" package
which is miniature and fits much better in the (short) converter housing than
the HC49. And it's cheap ($1.38)."

Personally, I find the benefits of using a 2M IF outweigh the benefits of
using 70 cm for me.  If you have both 2M and 70 CM capability, this is likely
your best option.  Either will likely get the job done, but your choice really
depends on what equipment you intend to use with it.  

Since it's easier to change the Drake from 70cm to 2M than the reverse
(assuming that you would replace the IF filter  - may be unnecessary), one
approach would be to convert it to 70 cm by swapping the crystal and using it
that way until P3D is launched and then converting to 2M if your operating
experience shows that this would work better for your situation.

Keep in mind that much of this conclusion is based upon the anticipated
operating schedules of an unlaunched satellite.  Changes in the way P3D used
will affect the conclusions you reach as well!

73,
John Oppen, KJ6HZ



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