Posted by
Bob McGraw - K4TAX on
Jun 24, 2017; 2:47am
URL: http://elecraft.85.s1.nabble.com/ATU-and-Bandpass-Filter-tp7632019p7632047.html
If one chooses to use band pass filters then one should anticipate using
an external antenna tuner as well. This of course depends largely on
the antenna and the impedance of the antenna at a given frequency. If
one is using resonant antennas with a reasonably close 50 ohm impedance
then all is well. If one is using a multiband antenna the impedance is
likely to vary widely and be far from 50 ohms. As to a beam, if it is
correctly adjusted, it should provide a reasonably close impedance to 50
ohms on all band for which it is designed.
I find many hams are grossly over concerned with SWR values. For
example; 100 ft of RG-8X operating with a 2:1 SWR at 14 MHz will have a
total loss of 1.5 dB. Where as the same length of coax with a 1:1 SWR
will have a loss of 1.2 dB. A difference of only 0.3 dB. With the same
configuration at 28 MHz, the loss with a 2:1 SWR will be 2.1 dB and with
a 1:1 SWR the loss will be 1.8 dB a difference of 0.3 dB.
My point is..........hams are grossly over concerned about having a 1:1
match. Factually, it isn't that dang important. As you can see, there
is little difference in total loss. Thus of 100 watts power fed into
the line at 14 MHz and a 2:1 SWR the power at the antenna will be 70
watts. Where as 100 watts fed into the line at 14 MHz with a 1:1 SWR
the power at the antenna is 75.8 watts. These are real numbers, thus a
difference of only ~5 watts. The only thing affecting loss is the loss
in the transmission line which is present to some degree regardless of
the SWR match.
73
Bob, K4TAX
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