It's true that the effect a given SWR has on an amplifier output depends
on the actual impedance presented. That 2:1 SWR can be anywhere from 25
to 100 Ohms, plus various reactive impedances in between. On
simple-minded way to look at it is to imagine an amplifier as a
low-impedance source. (A crude approximation, but a useful one.) With
the same drive level, it might deliver 200 W to a 25 Ohm load, or 50 W
to a 100 Ohm load. If you reduced the drive to the 25 Ohm load for 100
W output, the amplifier would be very inefficient, as current would be
high. On the other hand, if you increase drive with the 100 Ohm load, it
is likely the amplifier will saturate before reaching full power, due to
limited voltage output capability. The only way to make an amplifier
tolerate such big load differences is to make it capable of much higher
output, which increases cost. A good ATU is probably more cost effective.
73,
Scott K9MA
On 11/29/2018 19:22, Bob McGraw K4TAX wrote:
In checking my K3S into an antenna that presents a 2:1 SWR as
indicated on my external Power/SWR meter, my KPA500 in STBY, and my
KAT500 in bypass mode, it does deliver 100 watts. Although, I prefer
not to operate in that configuration if not necessary.
I think in general we fret too much over values of SWR which are 1.5:1
or less at the station end i.e. between the tuner and the amp or
transceiver. This is without regard to the actual SWR on the
feedline between the tuner and the antenna load. That is where the
real power loss occurs. Also, in many cases, using a tuner in a
feedline where the SWR is 1.5:1 or less, most likely the tuner
introduces more loss than if the tuner was not in the path. There
is a nice XLS spreadsheet
http://www.dj0ip.de/antenna-matchboxes/matchbox-shoot-out/ showing the
actual loss of many different tuners, different bands, and different
matched and unmatched losses. Rather eye opening.
Regarding the SWR fault at 1.4:1, is that at 35 ohms or at 70 ohms?
Significant difference in current demand of the PA between the two
values while SWR appears to remain the same. This is largely the
reason I often state that SWR only tells 1/2 of the story of antenna
system performance. A PA correctly designed will monitor the current
in addition to SWR and not just SWR as most seem to do.
73
Bob, K4TAX
On 11/29/2018 6:16 PM, K9MA wrote:
That certainly is not the case with the KPA1500. An SWR of 1.4:1 can
cause it to fault well below 1500 W due to either excess current or
excessive drive power, or to greatly exceed 1500 W output.
I don't think the K3 will deliver anywhere near 100 W at 2:1, either.
73,
Scott K9MA
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