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Testing the DEM 144/1268 Tx Converter


Thursday night I had my first QSO on mode-LS with WL7BQM, and yesterday
Mike came over to the house for a visit and we ran the new converter
through some power tests using his and my Bird-43 meters.  Thought I'd
share what we found.

I bought a 10K element [10w, 1.1-1.8 GHz] a couple years ago for measuring
1296 equipment, but its first use was by Mike to measure his FT-736R/1.2G
module [10w].  Mike then bought a 25K element for his own use.  I had made
some power measurements of my new DEM unit using a 50E element [400-1000
MHz] but wondered about the accuracy.  Also the 10K seemed to disagree
greatly with the 50E {that was later attributed to "pilot error"}.

So we hooked up my FT-817 thru my Bird [5C-element] to the 144 input of the
DEM unit and connected the output to Mike's meter terminated with a 100w
Bird load.  Initial test were disappointing...only 8w out using the 25K and
10K elements.  I said this was not right as I had measured near 20w on the
roof.  Lots of head scratching and I removed the two-foot RG-8 coax between
the DEM and the Bird meter.  We reconnected the DEM to the Bird with a
double-male N-adapter...15w!  I don't recommend using RG-8 with PL-259
connectors at these frequencies!

We proceeded to make a calibration chart using all three elements: 10K,
50E, and 25K for varying drive levels.  The FT-817 steps power from 1/2, 1,
2.5, and 5w out; all tests were using cw-carrier.  We later added a 2w, 3
dB attenuator to get 1/4w drive.  Here's the results:
1/4w ---> 6.8w[10K], 7w[50E], 7.25w [25K]
1/2w ---> 10w [10K], 10.5w [50E], 10.5w [25K]
1w ------> 14w [50E], 13w [25K], ...10K element not used above 10w.
2.5w ---> 15.5w [50E], 15w [25K]
5w ------> 15.5w [50E], 15w [25K]

So the DEM with the internal power control at max saturates at 15w output
using ~ 2w drive [I have the 3-10w transceiver drive option installed].
DEM rates the unit at >15w at 11.0 - 16.5 vdc @ 6 amps [uses the M68719
power module]; drive requirements are 1 mw thru 10w (with high power
option).  My unit is model 1268-144TX, serial no. 102 and cost $375,
assembled and tested.  DEM expects to offer a complete kit
1268-144TCK...$300, and a pcb kit...$250.

I ran my tests at 13.5 vdc. and current was not measured but in the region
of 5-6 amps as indicated on the Astron power supply meter.  On-air
modulation sounds very clean even when run at saturation...IM products were
not measured.


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