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

[James Duffer]

Sounds reasonable to me Aaron Hsu.

I operated and maintained an ARSR-1D Long Range Radar (L-Band) that was 
equipped with an Raytheon Amplitron  which boosted the output power to 
around 4 megawatts peak.  I can assure you that VSWR was constantly 
monitored and if it became sufficient it would shut itself down.  Since the 
transmission line was waveguide and the antenna a horn and it operated only 
in the L band VSWR was usually not a problem, but the amount tolerated was 
very low, I am not sure but it may have been less than 2:1.  The monitor 
system was checked periodically by inserting a pin through the wave guide 
and slowly increase power to determine what the amount of power would shut 
it down, the monitor circuit which enable the shut down was adjusted 
accordingly.

So if the FAA specified a maximum VSWR in their standard and tolerances for 
this system, I am sure they had sufficient reason.

There are some books which may give the impression that high VSWR is just 
fine, but this is usually on the line and a matching device is used so that 
the transmitter itself is working into a reasonable matching impedance to 
obtain maximum transfer of power (i.e low VSWR).

I am inclined to agree with you on this matter.

73, de wd4air (Jim)

>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
snip

>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.
73s John W5TD
>----
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