[ICOM] RE: Why are we so SWR-phobic

[Adam Farson]

Hi Aaron,

Thanks for an excellent post. You are correct about a correctly-matched load
as a requirement for a clean output signal, but linearity (IMD) is more of
an issue here than harmonics. Low-pass filters following a solid-state PA
are an acceptable trade-off for freedom from "valve pains", and will reduce
harmonics to levels that meet or exceed regulatory requirements.

To re-quote a key sentence from Sabin, "HF Radio Systems & Circuits": The
collector-to-collector (or drain) load impedance must be maintained as close
to resistive as possible. This in turn requires optimizing the output
wideband transformer(s), combiner (if used), and lowpass filter passband
VSWRs to as low values as possible. Another issue is that mis-termination of
the low-pass filters following the PA will compromise their
amplitude/frequency response.

In my view, these factors fully justify the use of a matching network or ATU
(manual or automatic) between a solid-state transmitter and the load, to
present the transmitter with the 50-ohm resistive termination it was
designed to see. There is absolutely no excuse for doing otherwise.

High reflected power will not necessarily increase DC input current to the
PA. The reflectometer in the output signal path of any properly-designed
solid-state PA will throttle back the drive via the ALC line, to prevent
damage to the power devices.

Whilst it is true that power transistors come and go (the 2SC2652, used in
the Icom IC-2KL and IC-4KL, and in the Yaesu FL-7000, is an example) proper
design and usage of the equipment will ensure very long device life. I had
an IC-2KL/AT-500 pair for 10 years, and never had a problem. The amplifier
always saw 50 ohms resistive, and ran quite cool (if not quietly). My
present amplifier, a Yaesu Quadra, is now 8 year sold and equally
trouble-free. The SWR scale on the screen stays in the 1.0 - 1.1 range.

"Going back" to the "guid auld valve days" is not an option - at least not
for me.

Cheers for now, 73,
Adam VA7OJ/AB4OJ
  



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