Posted by
Don Wilhelm-3 on
Jan 30, 2005; 5:52am
URL: http://elecraft.85.s1.nabble.com/Elecraft-RFI-Problems-tp374033p374036.html
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" <
[hidden email]>
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.
_______________________________________________
Elecraft mailing list
Post to:
[hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help:
http://mailman.qth.net/subscribers.htmElecraft web page:
http://www.elecraft.com