[Elecraft] ATU and Bandpass Filter

[Bob McGraw K4TAX]

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