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Sunday, 28 April 2013

QRP OP but been sent a PA for DX trips and a plea to use.

This is a complex issue but one that I will investigate.

I'd rather do all trips qrp but out of the blue someone send me a PA with a type written note no explanation could I please consider using this next time I did a DX trip (for some of it at least) as they could not hear me. They went on further to suggest it might need some modification before use and suggested BIAS and an LPF.

So I thought time to investigate it.

The PA is one of those built by RM the KL-500. Yuk, nasty horrid CB amp I hear everyone say but I pulled up the circuit diagram and the core it is not that different from most PA's I had seen so I thought I would put any preconceptions to one side and actually test it to see if it might be of any use.

Driven with one of my KX3 that I can wind right down to 0.1W and up to 10W so it is perfect to use as it also contains a two tone test.

So I connected the PA output to a G6ALU VSWR meter and a 1KW dummy load. The input to my KX3. I added between the KX3 and PA and PA and VSWR two samplers so that I could display on a 4 channel oscilloscope the signal in and out of the PA and also sample the frequency spectrum. I meant to take photos but just did this quickly.

The results were interesting.

I do not own a lot of test kit but I have run two main tests one is a simple repeating cw key down sending AAAA and the other is the usual SSB two tone (from my KX3) into the PA. As I mentioned the output from the PA is fed into a (I RF shielded it inside a big tin) dummy load and then sample the output on a spectrum analyser and oscilloscope input and output and see what it outputs. Obviously before going on the air I would start looking at LPF etc.

I tested the amplifier on the bench briefly after chatting to someone else with a pass through (you will find a circuit for one on this blog) to a big dummy load and a sampling and used a cheap Vellemann spectrum analyser to evaluate the out of band signals up to 25MHz and an oscilloscope (tektronics 4665) to monitor the input and output. I was pleasantly surprised to find the results seemed fine from 3.5 to 14MHz and harmonics (that I could see) were more than -45dB down at least as long as input drive power was 8 Watts or less, I'd normally be closer to half that however if more was used (KX3 will go to 12W) the amplifier started to go non linear and produces lots of harmonics. No surprises (for me anyway).
I measured the RF voltage directly at output and used a G6ALU VSWR meter for some of it and the power output, with 5 Watts drive manages over 80-90W and with 10 Watts drive >240W output on some bands it is not quite linear over the whole range but close (might be down to my quickie measurements), there are 6 power settings and the output appears to be much better with lower drive. Current draw at maximum was close to 34 amps ouch! and there is a lot of heat after a while but I suspect would be reduced at lower output. Adding a 12V 4" fan cooled the PA well. The final power transistors according to data sheets are capable of 80 Watts each and therefore the max power out as there are 4 should be 320 Watts if in parallel.

It is not that heavy 1.5KG and was free, albeit I might have to put in some effort and some components to get it to work so maybe an option

I know about the fact that on AM these things fail (google search) and as the PA have no VSWR protection you need an ATU or resonant antennas. So might be time to add in the VSWR protection circuit I put together in 2009 as the transistors are, like most in PA, not free of wierd effects if the VSWR goes above 3:1

I would still have to figure out what could I use to power it as it is not like you can plug into a mains outlet when you are on a DX beach and I suspect a car battery but I'd be prepared to stick a car battery charger in the luggage and source a car battery locally.

I'll run through tests again after I have added a fan, an HF LPF and a switchable BPF and the VSWR protection circuit and post some images.

It was a version that has the BIAS modification already performed but I will do the triple fuse to single fuse swap out.

I did some on air tests this w/e and one evening during the week (with an LPF in place) CW only and boy does a 200W signal go a long long way! Thanks to those stations that I contacted and allowed me to wind the power up and down from 5w to 150W and were able to clarify my thoughts that 5W was not always audible, but as qrp fans we already know this, 20W was usually heard if not always understood, between 50-80w 85% copy all the time even with QSB, 100W+ reliable copy even if deep QSB on band.

I guess I need to find a real DX station to test this with whilst having an antenna on the beach and do some field strength measurements too.

I did not notice any 88-108MHz FM radio interference or TV (mast head pre-amp) interference once LPF used (-70dB cut off at 35MHz). Which is what locals would complain about most I suspect.

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