Hi All, as Ron and others said, the theory is solid: swr is determined by
the ratio between the transmission line impedance and load impedance.
What is often overlooked is that the load impedance of a cathode driven
amplifier is determined primarily by the cathode current. This impedance is
different for moderate drive levels compared to high drive levels.
One can see the amount of drive delivered to the amplifier will be less if
the swr seen by the transmitter is elevated. When the drive is less, the
cathode current is less and the input impedance of the amplifier is higher.
With a 2:1 swr, the load impedance can fall between 25+j0 and 100+j0 on the
chart. a mid range line length that is 45 degrees removed from 25+j0 would
be caused by a load impedance of 40+j30. With low loss coax, the impedance
seen by the transmitter would be within the same 2:1 circle on the chart.
So, by altering the length of the coax between the transmitter and
amplifier, a different load impedance can be presented to the transmitter. As
the
line length is adjusted, some of the lengths will allow a better match
between the transmitter and the coax line so that more drive power to be
delivered to the amplifier cathode. As the drive power increases, the load
impedance will decrease and cause the swr seen by the transmitter to decrease.
The point of all this is to explain how swr can change as the line length
is changed. An antenna tuner can fix the problem as can a properly tuned
input matching circuit (basically the same thing). The adjustments are correct
for only one drive power setting though. Tuning things for maximum goo
sort of makes for the optimum setting. At lower drive levels one is not overly
concerned about amplifier input swr.
73,
Gerald K5GW
In a message dated 1/11/2015 9:36:32 A.M. Central Standard Time,
ka4inm@gmail.com writes:
On 01/10/2015 11:53 PM, KB8NTY wrote:
> Sounds like a neat program, thanks for link... however with all due
> respect...I'm thinking the "feedline" is not being referenced. It is the
> "transmission feedline" the coax that connects the amp to the
> transceiver/transmitter, not the coax that runs to the antenna.
> Theory is solid, yet real time first hand experiences speak loudly. When
> users experience controlled SWR by means of "transmission feedline"
> lengths, it's is hard to convince otherwise.
> I would think though as has been mentioned there is a dynamic that
> exists, by means of use of various transceiver/amplifier's. So what one
> finds acceptable, may not work the same for others. /* snip */
The theory NEVER fails! The language is frequently wrong.
The standing wave ratio is the standing wave ratio and it refers only
to the match between any transmission line impedance and it's load's
impedance. (never anything else)
What happens when you change the length of the interconnecting feed
line between two amplifiers (100 - 1,000 W) is, you are changing the
impedance transformation caused by transmission line feeding a reactive
load. Only if a transmission line feeds a load that matches it's surge
impedance and is resistive at all frequencies involved does the input
impedance of the transmission line equal the impedance of the load.
Otherwise an impedance transformation occurs.
If exactly half wave length (and even multiples) at the one frequency
in use it will repeat the load impedance and reactance, which is very
handy for testing antennas with the test equipment out of the near
field of the antenna.
If exactly quarter wave length (and odd multiples) at the one
frequency in use it will invert the load impedance and reactance,
which is very handy for feeding extremely high impedance loads such as
the end of a half wave long dipole antenna.
Any transmission line length between these will transform in ways
between these two lengths.
If the input impedance of an RF amplifier isn't resistive and many
are, just not a lot of HAM amplifiers and the output impedance of it's
feeding amplifier isn't adjustable the impedance transformation of the
interconnecting cable (unless less than a foot) will effect the load
impedance that the driving amplifier will be.
A Heath DX-100B could feed a huge range of load impedances and
reactances, a common cathode amplifier with a swamped input can have a
fixed resistive input at all used frequencies, with either the length of
transmission line will not be sensitive and you are using
neither, you either need a tuner between the two amplifiers set to mimic
the effects of that golden length or that (compromise if
multi-band) length of line.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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