[Lowfer] 1 watt in class 'AB' into 32 Ohm load, BW - 600 KHz, and it's an audio amp

[Clint Turner]

Along similar lines, I've been running a PSK31/QRSS MedFER beacon for 
years that uses "non-conventional" modulation to achieve a linear 
output, described here:

http://www.ka7oei.com/psk_transmitter.html

What this does is to modulate the PA's output power with the amplitude 
portion of the PSK31 waveform and do the phase modulating portion 
elsewhere and this way I can use a non-linear PA for best efficiency.

The web page is a bit outdated (so is the circuit - I could greatly 
enhance and simplify things if I were to use a more-modern PIC) but the 
transmitter is still up there and running since 1999 (it's in QRSS3 mode 
at the moment and the antenna current may be down slightly due to recent 
high winds that tend to vibrate the variometer off-peak).

Then again, as it was pointed out, you could just bang a hard-keyed PSK 
signal (from an XOR gate) onto a LowFER transmitter and no-one would 
care - nor would it affect the receive signal quality in the slightest:  
My prototype did this and other than making clicks up and down the 
entire band (which would be filtered to legal levels by a half-decent 
loading coil on the tuned to the LoweFER watering hole) it worked just fine.

73,

Clint
KA7OEI/CT


jrusgrove at comcast.net wrote:
> Andy
>
> PSK, BPSK, QPSK, WOLF generally require a linear amplifier although at least several lowfer stations
> have run some of those modes through non linear amplifiers with no problems. There's little chance
> that a PW 1-watt lowfer is going to cause intereference - especially if run outside of the watering
> hole window as is customary when transmitting a 'digital' mode.
>
> RTTY, Pactor, Jason, Amtor, MFSK, Olivia, Domino, Throb, WSJT, WSPR do not require a linear
> amplifier and can be run through your existing lowfer 'Class D' final with no concerns.
>
> Jay
>
>
> ----- Original Message -----
> From: "Andy - KU4XR"<ku4xr at yahoo.com>
> To: " European&  UK) and MedFer bandsDiscussion of the Lowfer (US"<lowfer at mailman.qth.net>
> Sent: Thursday, November 25, 2010 5:20 PM
> Subject: Re: [Lowfer] 1 watt in class&quot; AB&quot;into 32 Ohm load, BW - 600 KHz, and it's an
> audio amp
>
>
>    
>> --- On Thu, 11/25/10, JD<listread at oswegoblade.com>  wrote:
>>
>>      
>>> We also need to bear in mind that Andy was asking about
>>> Part 15 operation, which is limited to 1 watt DC INPUT.
>>> The efficiency of Classes A and AB are pretty poor,
>>> and B is barely adequate.
>>> _________________________________________________________________
>>>        
>>
>> Hi JD:
>>
>> I suppose that this fact is probably one of the biggest reasons
>> that there are not many lowFERs sending linear modes on 1750 meters.
>> 70% efficiency would be excellent performance in a Class " AB " amp.
>> Unless I'm mistaken, I think the distortion in a Class " B " amp
>> might be a little too high, but on the other hand; at LF would it
>> even be noticable ?? Possibly so if it distorted the transmitted
>> waveform of a digital mode enough that decoding it would be difficult,
>> or impossible. I don't have enough experience to know.
>> I did build an HF to LF Transverter using a SA612, and a TDA7052B.
>> The IF was at 28 MHz, and I was trying to use an old Radio Shack HTX-100
>> 10 meter mobile as the tunable IF. It works, but the drift in the old
>> radio is excessive, so not very useful. Antenna current readings with
>> the QRSS Class " D " setup on the old antenna were 180 Milliamps, and
>> on the Linear Transverter set for 1 watt DC Input on the TDA7052B audio
>> amp, the Antenna current was 110 Milliamps. So there is certainly a
>> drop in output efficiency with any Linear system.
>> I do still have the desire to get a Linear PA going at some time, and
>> run something in addition to the QRSS signals on the band. Probably
>> just need to change the crystal in the transverter, and use another
>> radio as the tunable IF. I guess that's the most logical route to take.
>>
>> 73:
>>
>>
>> Andy - KU4XR - EM75xr - Friendsville, TN. USA
>>
>>
>>
>>
>>
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