Hi guys,
Once before we had the white coax desolder itself and produce a powerful RF arc .
Put it back again and recently we had it happen again.
These are my findings.
The best way to tune this dual LDMOS amp is to isolate and tune each side separately ,THEN combine them .
So we isolate the power splitter feed on the inputs and the outputs , disconnect the 50v to the side we are not working on.
On the input there is that tiny green capacitor , this just snapped off first time I tried to adjust it , replaced it with a better ceramic version . Using a small hand held analyser I adjusted the input SWR to perfect match , then after disconnecting the input to the output combiner ,but directly to a dummy load.
Power up and with a small amount of drive checked the power , double checked and adjusted the input Capacitor for max power at this low level , once peaked I increased the drive and checked the power, we obtained around 500watt with the amp meter around 20amps , next , isolated that side and connected in the other side the same way exactly , this time I could only get around 350 watts max no matter how much drive I gave it but limited it to 10 watts .
While doing these test I used an infra red temperature probe to monitor the temp of that coax, the LDMOS , we were at 35 deg on the LDMOS and 35c on the coax. ( With fans running ), This was with a new cooling setup of 5 fans, 3 fans on the heatsink. One large fan over the main LDMOS pallet and another blowing across the board , all fans active on PTT except the large fan above the pallet which is always running.
This new cooling method kept the coax capacitors cool and the LDMOS cool not going above 45c.
So now the problem of the different power between each side as there was an imbalance . This imbalance generates heat in the white coax (TC12 ) and also dumps more power into the load resistor on the output combiner , this will burn it up as It did before. So beware of that.
So after asking around ( Roger of the DXShopUk was of great help ) found that the best way is to tune each LDMOS in turn is by adjusting the white coax capacitor (TC12 ) by a
few mm on the output line , it ended up moving the coax capacitor by just 2mm resulted in that lower power side recovering back up to the same levels as the other side. So both sides were now around the same , we can now reconnect the combiners and the LPF ( another fan ) in a wind chamber to help keep it cool , so up to 6 fans now !.
so we now connected both LDMOS back in line and applied power , .... wonderful getting proper power levels and the full KW at 40 amps , but requires more drive around 18 watts. Then I run the amp at full cycle checking the temperate very carefully, all well nothing overheating now.
So the main reason for the white capacitor re-flowing is due to heat and the side not tuned properly , just 2mm make a big difference in the position along the output line.
So this will answer that white coax problem on these amps.
This tuning method is not mentioned in the setup notes for this amp.
All the best , stay safe guys.
Now I can warm up some sheep’s behinds off the moon Hi.
William
GW4LWD |
