Top Band (160m) external LPF

Bill NG1P provides a bit more detail on his Top Band (160m) mod success.

“I’m having a blast on 160m I did add larger heat sinks and a pair of small fans that I can turn on and off as needed to keep the finals cool if I’m long winded or using digital modes.

“Qrp labs makes a great low pass filter kit:
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Top Band (160m) with uBITx

Bill Richardson NG1P has reported on his modified uBITx on 160m operation using a doublet antenna.

“Doing some testing with my uBitx on 160m with a 160m LPF attached to the output. I’m seeing about 15 Watts out but the current draw is running very high at just over 3 amps. So not usable on 160m at the finals will blow I’m sure. The bias stays normal so it’s something else?

“Doing the same test on my Bitx40 on 160m I see about 10 Watts out and the current is about 1.5 amps and very usable.”

John AD5YE responded with:

“I think that what you are seeing is the “normal” variation in the output of the MOSFETs. Some are better than others at RF.  Gordon’s suggestion to turn back the drive is probably the right answer.

“Alternatively, you could turn down the gate voltage a bit, but that would probably introduce compression distortion. Or you could increase the size of the heat-sink and see how it goes.

“Probably the wisest thing to do is both turn back the drive a bit and increase the heat-sink. Some, but not all, IRF510’s are capable of 20W or more on 80m. you might happen to have some good ones. The key lies in the harmonic generation and the heat-sinking. Note that what you are seeing MAY be from UHF oscillation included in the signal. Be very aware of that. Check your output very carefully. “

Bill reduced his drive to overcome the high current, and reports:

“I turned the output down to about 8 Watts and I have made 4 contacts on 160 with great reports. I will add a larger heat sink to be safe. “

So the uBITx works fine on 160m with an outboard LPF to remove unwanted harmonics from the transmitted signal.