hi, i have construct this linear amplifier 500w with 12 mosfet irfp360 for 160m band: tzitzikas.webs.com/linear500w.jpg (schematic) The manufacturer and other amateurs who have construct the follwing set, claims that the linear provide in output at 110vdc 450-500w carrier (13A current), at 55vdc 100w and at 75vdc 200w carrier and the requirements for r.f driving power are only 2w (Also he claims that is broadband at frequencies between 1000-2000khz)
I took these measures: at 1450khz at 65V(voltage to drains of irfp360): 103W output power current 4.8Α and driving power 2w at 1700 at 65V 60W output power current 3.3Α (i don't change the driving stage settings (like voltage e.t.c) at 1200 at 65V 183W output power current 7.8Α (at 1850khz! the output power was only 50w!) swr=0 (all frequencies) After, i connect only the linear with 2 irf640 (driver) to the dummy load and i took these measures at 20v (voltage to drains
of irf640): 1431khz 2w 1700khz 1.9w 1200khz 4w So, you can see that the driving power was increasing when the frequency was decreasing.
After these measures i gave to linear 500w, 3.6w of driving r.f power at 1450khz and it gave only 130w at 65v (current was 5,5Α and the modulation in this test wasn't very good- AM modulation).
So, it is clear that at lower frequences linear 500w, and the other circuits give the maximum r.f power. The designer claims that this linear amplifier is broadband for 160m and mw band (1000-2000khz) I would like my linear to give 185w at 65vdc at high frequencies between 1400-1900khz. What changes do you propose to do at linear amplifier 500w, linear 2xirf640, modulator or oscillator to give at output 185w at 65v? Maybe some changes in transformers or capacitors?? Moreover, the one heatsink was colder than the other one.
My quick guess is that it's a much lower stage in the RF chain causing a variance in RF power. It may not be your power linear amplifier. I would suspect a much lower stage of the transmitter. Check the oscillator power output first while changing frequency, then the input power to the IRF640s and then their output and so on until you find where it's varying in RF output power. I would suspect it may be the oscillator itself since your driver and power amplifiers are linear and broadband. If it does happen to be one of the power amplifiers their is no simple fix all. You'd probably have to remove one of the windings on the RF transformers to raise their operating frequency range. Just pray it's not that If it is the oscillator I can think of a few ways to make its output solid across the band.
By the way, cool schematics. That looks like a fun project.
My quick guess is that it's a much lower stage in the RF chain causing a variance in RF power. It may not be your power linear amplifier. I would suspect a much lower stage of the transmitter. Check the oscillator power output first while changing frequency, then the input power to the IRF640s and then their output and so on until you find where it's varying in RF output power. I would suspect it may be the oscillator itself since your driver and power amplifiers are linear and broadband. If it does happen to be one of the power amplifiers their is no simple fix all. You'd probably have to remove one of the windings on the RF transformers to raise their operating frequency range. Just pray it's not that If it is the oscillator I can think of a few ways to make its output solid across the band.
By the way, cool schematics. That looks like a fun project.
hi. i don't have oscilloscope to check oscillator power. oscillator power is very low, approximately 200mv. what do you mean to remove "one of the windings " .for which transformer you talking about and what do you mean? thank you
hi. i don't have oscilloscope to check oscillator power. oscillator power is very low, approximately 200mv. what do you mean to remove "one of the windings " .for which transformer you talking about and what do you mean? thank you
You could simply use an old AC volt meter, one that reads down to a mV setting. Most multimeters can do that and will work fine for <3MHz. Just probe the RF output of the oscillator and take AC mV readings. Or you can use a diode and bypass capacitor if you only have DC measurements on your meter that go that low for voltage.
If a decent multimeter or VOM meter isn't available even a simple field strength meter with its sniffing wire wrapped around the oscillators output can give you a basic idea of what's happening.
As far as the transformers I am talking about the input and output coils. The shorter electrically a coil is the higher the operational frequency. If it comes down to the coils as being the culprit you would need to remove a single winding from those toroidal RF transformers/coils. This would be the last step you would want to take though.
One last word of advice, get a scope! Check around on ebay or thrift stores. I picked up an old Eico 5mhz scope for 20$. Works great for lower frequencies. Heck on Ebay you could probably find even cheaper ones. It helps a lot to see what your waveform looks like along with the ability to take quick and dirty power measurements. Dirty waveforms = harmonics and other issues. Also when working AM it's a must to know your modulation % which scopes can show you quickly. Not to be pushy, just some advice.
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