Build an amplifier chain with 0dBm input and 50dBm output. Any technology can be used but the amplifier chain must cover 1.8 MHz to 30 MHz and be easily constructed using standard parts.
If you are buying I'm building. First I want to see how clean that 0dbm source is.
I've used both bipolar and LDMOS FETs for power amps and driver chains and it's all about
heat and power. You have to burn a bit of power and get rid of the heat. Most all the
homebrews I have sound good for a reason, that reason. that also doesn't mean being
stupid about it. the other is design for 3 to 6DB more output than will be used. Half the
signals (even some that sound good) I hear are wide because they are running right
up to the 1db compression point and that can dirty an amp up regardless of the device
used. It also means the power source has to be able to support the load!!!
FYI: find an old AIL wideband amp, 10mhz to 400mhz, flat gain of 50DB, 50W
(100w saturated) class A, only 80 pounds and oh about 20+ years old. Did I
mention solid state and bullet proof?
Seriously it can be done and is being done. You have to be willing to pay for it and accept that the
unit will be large. Look at the AR313 or AR305 design as the staring point, we are talking 300W.
Those Grandberg/MOtorola designs are very clean and with care can put serious gain and clean power.
They are also antiques or more than 20 years old. So the arguement is not how or with what
Every stage leading to that has to be clean and often that's where people screw up.
NOTE: while the K3 has a poor IMD we do NOT know if that due to the power stages or earlier
in the system.
Some observations. All the better and cleaner solid state linear amplifiers run on 28, 36 and 50V.
There a reason for that, it's I^R and also heat. as you go up the impedences become more
manageable even for LDMOS parts. Amplifiers used for analog TV and DIgital YV are very linear
and none I know of work at less than 28V even for the 25 or 50W drivers.
Right now I'm completing two amps, both take less than 4W. One is a pair of BLF177s push pull
and the output match is optimized for 20 through 6M (my span of interest for it ). The base
Phillips/NXP design was for 1.6 to 28 at 300W, 22db gain (300w watts with less that 2W drive).
Worst case IMD (3rd is -33 below carrier at 30W and better at 300, 5th is -36 at 30W) I think
a bit more bias current should clean the IMD hump up some as it is not uniformly that level
and at some frequencies it's closer to -45! This is a simple amp. My 2M is a variation on the
granberg/PK0V/NXP designs and runs a BLF278 for the same power I expect similar numbers.
My change allows operation though the 6M band with the BLF177s as the ferrite used in the apnote.
is very lossy at VHF and lower loss ferrites limit the lower HF usability and efficiency. It only takes
10 or 15 of those 300W to make the ferrite get very (80C) hot if its wrong.
So the NXP/Phillips amp plus a driver that is very clean for the needed 4W (2w + 6db headroom)
is a trivial task and there are many good sub 12W LDMOS devices that run at 28V with 20+ db gain
over that range and the apnotes for them. Add a driver for that and run them with heavy feedback
for a total of say 30db of clean gain.
Theres your amp. An easy 47db(or more) of gain and with 0Dbm in that should yield more than
100W of very clean power (its capable of 300).
Of course TR switching, power supply, supervisory (Over current, high SWR, and high input power
protections), cute metering and low pass filters that can stand power (and deliver suitable 2 and 3rd harmonic suppression) and their switching.
Are there tricks. well yes and no. This is not a slam it together on perfboard thing. It requires a
well laid out board and more than trivial mechanical work to lay it out and build it. there are odd
material used like UT141-25 (25 ohm copper jacket coax) and parts like ATC 100B caps, ELENCO
metal cad mica caps, oddball and large ferrites of the correct types. The mechanical magic is a
slab of .25" thick copper 4"x6" to move the heat to the aluminum heatsink and who knows how
many hours of polishing the copper and the matching surface of the heatsink for flatness. Add
a few days of taping holes for screws. You don't do this and run full power and it goes pop,
don't cry to me.
So you issue a challenge to engineers that need skills of a solid sate power designer, a healthy dose
of mechanical skills, and knowledge of mechanical fabrication techniques. And the heart of a lion
as your spending $100 for the BLF177s and maybe $100 more for a set of spares in case an Oops
happens. Sure and you want this cheap in kit form that's foolproof.
There's little rocket science in doing that just hard work.
The the user turns up the compression and clipping, replaces the SSB filter with a wider
essb filter, adds base boost treble boost, and runs power till the average reading meter says
"yep the amps doing what you paid for". With all that the LO phase noise and the noise
from 50DB(or more) of amplifiers doing their work products are at the level that people
for 10 miles around can hear them across the entire band before you start to call CQ.
We blame the amp but the signal was already trash by time it got to the maybe -10dbm
post filter amp never mind the power stages.
Remember GIGO, garbage in, garbage out.