A kit to produce 30W at 1296MHz has been engineered by members of the San
Bernardino Microwave Society http://www.ham-radio.com/sbms/ and the San Diego
Microwave Group http://ourworld.compuserve.com/homepages/edmunn/ :
- power gain 10 - 13dB (so budget for 3W i/p) - uses a PTF 10021 FET
- 28v dc supply; current drain about 2A (or use 24v & lose <1dB power o/p)
- cost $45 + $5 s&h (shipping USA; kit does not include heatsink,
connectors or T/R relay)
For full details, assembly & test info, and to order a kit, email
1296amp@cox.net
Note: this kit contains surface mount components; familiarity with handling
these is helpful (BUT: it just ain't that hard - see comments below)
For pic of completed amp, see
http://kahuna.sdsu.edu/~mechtron/hamshack/1296Amp/1296Photo1a.jpg (note: you do
not have to erode the heatsink as much as our brave beta-tester did <g>)
Personal comments from a builder:
- I have successfully completed one kit, with a second on the way;
splitters/combiners also under construction
- I used a Model 99 heatsink from Communications Concepts ($22 + s&h)
http://www.communication-concepts.com/heat_sink.htm
- to accomodate the FET height vs PCB thickness, I chose to use a sheet of
0.032 in thick brass beneath the PCB instead of washers or bending the FET
tabs; I used item #253 from K&S Engineering www.ksmetals.com. Many hobby stores
have a display area with K&S products.
- I decided to use a 4in x 6in slab of copper between the FET and the
aluminum heatsink to better distribute heat. I bought mine from Industrial
Metal Supply Co., who sell metal offcuts by the lb http://www.imsmetals.com/
locations Irvine, Riverside, San Diego & Sun Valley in CA
- thermal grease: to use or not to use? There are many opinions. Advice I
have had is that use of grease prevents getting a good rf ground - leads to
possible oscillation & thermal runaway.
- I found the bias adjustment pot somewhat fragile, so, being ham-handed
(sorry!), I am using a 10-turn encapsulated pot in my Mark II version
- the clearance holes in the PCB are large enough to permit the use of #6
machine screws (NOTE: must use #4 for mounting FET); having broken a #4 tap
recently, which stuck in the project... need I say more
- power relays; for brick amps,I have standardised on an NTE SPDT, 12v
coil, 40A contacts, encapsulated relay from Mouser http://www.mouser.com ;
$7.25 ea @5-off; part # 526-R51-5D40-12F. Spade terminals and easy mounting
with a #10 screw.
- rf relays; I use Transco SPDT, SMA relays from DEM
http://www.downeastmicrowave.com/ $45ea guaranteed working; I have been told
they will tolerate 80W COLD-switched at 1296. They are nom 28v, but DEM sells
neat kits for operating from 12v (which I use)
- connectors; I used right-angle SMA, PCB-style; I found I needed to file
the pins slightly to achieve a sliding fit; pyrojoe claims SMA is good to 400W,
so have at it...
- surface mount parts; I use demagnetized tweezers & a fine Weller
soldering iron bit; if, like me, you suffer from the "slippery little suckers"
syndrome, a la "Pretty Woman," spread a sheet on the floor & work directly on
it. Sounds daft. Wait 'til you're searching for an SMD cap on your workshop
floor...
Now make some noise on the 1296 nets:
- WSWSS http://www.wswss.org/ Tues & Fri evenings
- PNWVHFS http://www.pnwvhfs.org/ Sun am
Any info on other reflectors you think this email should be listed on, please
advise the writer off-list
Rod Adkins WI6M
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