[1000mp] Re: Increasing MK V Transmit Power Beyond 200W

[Steve Fraasch]

Don't do it, for so many reasons:

Before I go on, in a previous employment lifetime, I was a RF designer and developed Cable telephony/data modems for a telecom Co here in Minneapolis.  The system was based on Orthogonal Frequency Division Multiplexing (OFDM) which also happens to be the same system used in your WIFI system at home or the coffee shop (802.11G).

It was that working experience where I had truly learned to appreciate the need to reduce and manage  transmitter intermodulation distortion (IMD).

I will leave this non-analytic, but here is the problem with IMD:  First off, since most hams now are running 2.8K and even more of transmit BW, the 3rd order can fall out to 9 KHz from carrier, and 5th order out at 15 KHz.  Since transceiver and amplifier combinations should maintain IMD at below -30 dBc, that implies roughly 1.5W output power at some of these offset frequencies.  On a calibrated S Meter, that means if the signal is S9 + 30 dB over on 15 meters during the Sweepstakes phone contest, these products will be heard at a S9 level, and that's for a nominally acceptable system !  In my book, -30 dBc IMD is still WAY TOO much: I would like to see levels down -40 dB or more.

Before you consider modifying your Mark V for more power, you should understand IMD summation.  First, overall IMD cannot be better than the first stage IMD, it can only maintain or degrade in a linear sytem.  Therefore, if the tx is generating -30 dBc, the composite running your amp can be no better, and will probably be worse than -30 dBc.  For a signal such as AM, sideband, QPSK, QAM, the transmitter designer usually constructs an IMD spreadsheet to balance gains and choose appropriate devices with high enough 3rd order intercept points to maintain low IMD.  A transmitter "tweak" would be tantamount to overdriving a stage, thus causing it to generate large IMD and ruin the budget.

The other reason to not do it is reliability: if any of the changes result in more emitter or drain current, chances are the RF power device junctions will run much warmer than the recommended 125C.  Reliability will be reduced and you will start popping components.  Of course, power supply and tuner components are also vulnerable.  Last, greater loads on the power supply will also cause non-linear modulation products, because the supply will not be "stiff" enough to handle the aditional load, hence greater distortion.

So, what does this mean ?  If you "tweak" biases, device power, remove attenuation circuits in your Mark V (or anything else) you will inevitably ruin the IMD performance.  Yaesu included the Class A mode for that reason:  if you have a reasonably low drive amp (like a QRO HF2500, Alpha 87/91/99, ACOM), you can run the Mark V transmitter in Class with -50 dBc IMD and obtain better IMD from the rig and amplifier combo.  The non-class A generates more IMD, where the extra barefoot power is useful, and IMD is not as much a concern since it is not amplified by a linear.

If you have questions, don't hesitate to email back.  Here is a link to an article you might want to read:

www.highfrequencyelectronics.com/ Archives/May03/HFE0503_Tutorial.pdf

73,

Steve Fraasch, K0SF

-----Original Message-----
From: Bob Martin <bob.martin at charter.net>
Sent: Oct 7, 2005 10:57 AM
To: 1000mp at mailman.qth.net
Subject: [1000mp] (no subject)

I was told that the Mark 5 was capable of more output than 200 watts without
causing damage to the finals.  Is this true, and if so, what internal
changes are necessary?

 

Every Day Hamday

 

 


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