Posted by
Jim Brown-10 on
Nov 30, 2018; 2:20am
URL: http://elecraft.85.s1.nabble.com/KPA-1500-issues-tp7646571p7646656.html
Lots of important stuff here, most of which I agree with. See comments
interspersed.
On 11/29/2018 5:22 PM, Bob McGraw K4TAX wrote:
> 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.
I paid a lot less attention to the SWR that my Titan 425 amps saw
because their manually tuned Pi-Li output stage could make the tubes
happy working into much higher SWR than the auto-tuned Pi-L in the 87As
that replaced the Titans. And they both use the same tubes. The Alpha
will fault as the SWR approaches 2:1. And, has been noted, both KPA
amps introduce loss in the drive circuit to reduce power if the SWR gets
too high because it makes the output devices too warm.
> 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.
Yes and no -- most hams believe that excess loss due to SWR is much
higher than it is. I got into this many years ago when specifying low
loss 75 ohm CATV coax for remotely located wireless mic antennas for
sound systems in large spaces. Engineers for the major wireless system
mfrs told me that 1) they didn't know what the input Z of their
receivers were and 2) that was probably somewhere between 50 and 100
ohms. My logic was (and still is) pretty simple -- thanks to the very
high volume of coax for CATV systems, low loss 75 ohm coax is FAR
cheaper than low loss 50 ohm coax, and even assuming a 50 ohm RX input,
worst case excess loss for a 1.5:1 mismatch is 0.18 dB no matter how
long the line is.
That said, because the bandwidth of 80M is a rather percentage of its
operating frequency, SWR can get fairly high at the band edges of a
dipole cut to the center of the band. My dipoles are up about 120 ft at
the end of about 170 ft of coax with feedpoint Z are resonance about 88
ohms (measured), so they're fed with the lowest loss RG11 I can find
(currently Davis RF RG11, which measures like Belden 8213).
> 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.
Nice chart, which I've seen before. Over the years, ARRL has published
this sort of testing for a lot of tuners. I'd far rather see this in dB
-- 10% lost power is 0.46 dB, 20% is 0.97 dB.
73, Jim K9YC
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