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

[Hsu, Aaron (NBC Universal)]

I can think of a couple reasons...

Protection of the device - most RF power transistors can take a good
beating, but, if you were a radio manufacturer, would you want to
constantly replace the finals due to problems induced by high SWR (such
as heat)?  Even more so considering some might be replaced under
warranty?  Imagine the customer feedback after a year or so if the rig
was spec'd to 3:1 - "My YaeComWood TC-9000 needed new finals right after
the warranty expired!  And I was only running RTTY driving a light bulb
at the specified <3:1 SWR!".  Consider the number of Toshiba 2M/440
finals modules needing replacement due to heat problems - high SWR was
proably a catalyst in these replacements.

Also, considering the state of electronics, the marketing life of a
power transistor is relatively limited.  After a few years, the part may
not be available anymore.  Consider the somewhat popular TenTec 444 amp
- Motorola only sold the RF transistor for a few couple before
discontinuing it.  Due to numerous blown finals on the 444 and the
non-availability of replacement power transistors, TenTec got hit quite
hard and probably lost many customers (they have a reputation of
servicing stuff "forever" and this is the one exception).  TenTec hasn't
manufactured a solid-state "low-power" amp since. 

Cost - to "guarantee" a higher SWR rating, manufacturers would need to
spend more on a device that's capable of operating at such high SWR for
long durations.  Generally, you'll select a device that has 3 to 4 times
the power rating vs the amount of power you want to actually put out.
For example, you'll use a 400W rated device for a radio being designed
for 100W.  This might allow you to operate at 3:1 SWR key-down, but at a
higher cost.  400W RF transistors are not that common and are probably
quite expensive!  Plus, don't forget the costs of all the beefed-up
support circuitry.  We HAMs are generally CHEAP!

Operating specs - can the device work into high SWR and still have a
"clean" signal?  I believe tubes were more robust in this sense since
they tend to only generate even harmonics.  Solid-state devices generate
both even and odd harmonics possibly causing a "dirtier" out-of-spec
signal.  I'm not an EE, so please correct me if I'm wrong.

Efficiency - Higher SWR causes higher current draw for the same amount
of forward power.  So, if you wanted the rig to put out 100W into a 3:1
SWR, expect to see a 30+ amp current requirement for the power supply.
Unfortunately, this can be used as a marketing tool - our radios only
need 20 amps to put out 100W - they won't mention that it's with a 1:1.5
SWR, but the customer probably won't ask either.

And, we're getting way off topic here.  Please reply direct if need be.
Apoligies to the rest of the list.


  - Aaron Hsu, NN6O (ex-KD6DAE)
    {nn6o}@arrl.net
    {aaron.hsu}@nbcuni.com
    No-QRO Int'l #1,000,006
    . -..- - .-. .-   ".... . .- ...- -.--"
 

-----Original Message-----
Sent: Sunday, June 18, 2006 3:16 PM
Subject: [ICOM] Why are we so SWR-phobic

Am I the only one who things that the SWR protection circuits in
currents are ridiculous?  I mean, most of them start cutting power back
with a 1.5:1 SWR?  Are today's rigs so poorly designed that you could
damage the finals with a 2:1 SWR?  Older solid state rigs used to handle
that just fine.  The ad for the Yaesu FT107M advertised that it would
put out 75% power into a 3:1 SWR!!  Most Ten Tecs will put out almost
full power into a 3:1 SWR.  But most other moderns rigs are putting out
50% power or so into a 2:1 SWR.

The worst is the Yaesu FT857/897 series which cuts up the audio if the
SWR gets above 1.5:1 or so, and people think that this is a good thing?

So what is going on with the rig designers?  Do people really want this
aggressive of an SWR circuit?  I find it kind of annoying to have to
return the tuner every 25 kcs or so on some radios.  Are the finals
today really that fragile?

Or is it just me?  Anyone else with ideas?

73s John W5TD
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