[Elecraft] Elecraft: RFI Problems

[W3FPR - Don Wilhelm]

Ron, Bob and all,

It is quite true that losses increase as the SWR increases - the ohmic 
losses are the contributing factor with increased current, but the 
dielectric losses will be important as the voltage increases - so both are a 
contributing factor.  How much contribution is due to the ohmic loss or the 
dielectric loss is dependent on the feedline construction and frequency. 
With smaller feedlines and lower frequencies, the ohmic losses are likely to 
predominate, but as the physical size of the feedline increases, the 
dielectric losses may become the greater factor.  The crossover point 
depends on frequency - and at HF, the ohmic loss usually predominates.

Perhaps I am being overly technical here - and it really may not be 
important - if you have losses, you simply generate heat instead of RF - 
these subtle points are only appropriate if you want to persure the proper 
path to reduce the loss.  If you have already made the compromise choices 
that are right for your station, there is not much that can be done in the 
way of improvement - just understand your choices (decisions).

If you want to adopt a 'rule of thumb', a feedline with a higher 
characteristic impedance will have lower loss when comparing feedlines of 
similar size. Even though the center conductor of 75 ohm (RG-11) has a 
smaller diameter that 50 ohm (RG-8) coax - compare Belden 8237 with #13 
center conductor with Belden 8213 with a smaller (#14) center conductor and 
you will see that the 75 ohm Belden 8213 has lower loss at higher 
frequencies even though the ohmic loss through the #13 center conductor will 
be smaller - the higher impedance cable wins in the loss game.  Several 
other comparisons will reveal similar results.

73,
Don W3FPR

----- Original Message ----- 
From: "Ron D'Eau Claire" <rondec at easystreet.com>

Quite true about the relative losses, Bob. We've kicked this subject around
here in the past. It's been pointed out that, while it is true that  there
are dielectric losses in feedlines, feedlines operating at high SWR's are
more prone to resistive losses in the conductors than dielectric losses.