On 9/2/2013 4:32 PM, Don Wilhelm wrote:
One can feed the 43 foot vertical with ladder line, and do the tuning
in the shack (yes, you can feed a vertical with balanced line),
I strongly object to the word "balanced" with respect to transmission
line. A better description is 2-wire or parallel wire line. The balance
of a circuit is determined by the impedances with respect to the
reference plane (usually the earth).
but if feeding with coax, the best place for the matching network is
at the base of the antenna. Coax is good for an swr of 2.0 or less
(depending on the frequency). If the SWR is greater than that, some
kind of matching device at the base of the antenna is in order.
That depends entirely on the length of the coax, how lossy the coax is
at the frequency(ies) of interest, and the power level.
Ladder line feed OTOH is quite OK with an SWR of 20:1 if it is
properly routed - away from conducting surfaces by at least 3 times
its spacing and similarly away from the earth.
Again, yes, balanced feedline is an alternative for vertical antennas
- the antenna may be *unbalanced*, but the feedline does not care, You
still need balanced current and return current for the antenna to
functon correctly.
Calling a feedline "balanced" does not make the currents equal. If the
system is unbalanced, the currents will be UNequal, and the difference
will be a common mode current. There are several ways to make the
currents equal, or more nearly equal -- we can use an ordinary 2-winding
transformer at each end, or an impedance-transforming balun, or even an
ordinary common mode choke.
I don't disagree that higher impedance 2-wire line can be a reasonable
way to connect a badly mismatched antenna to an antenna tuner. But if
like many hams, the vertical is close enough to the shack that it can be
fed with 50-75 ft of coax, I'd seriously consider running a good quality
RG213 or hard line to a tuner in the shack. Yes, line loss increases
with high SWR, but one of the most misunderstood things about
transmission lines is how SMALL that increase is for short runs of big
coax. This is clearly shown by a family of curves that has been
published in every edition of the ARRL Handbook since I've been a ham.
It's Fig 20.4 in the 2010 Handbook. Plugging in loss data for LMR400
(0.3dB for 75 ft at 10 MHz), the loss would be only 1dB with 10:1, and
2.3 dB with 20:1 SWR. For 1/2-in hard line, divide those loss numbers by
2. Think that coax is expensive? Try pricing a decent weatherproof
enclosure and other costs associated with remoting the tuner.
Where you get in trouble with loss in coax is when you use long runs of
small stuff -- because you're only running 5 watts. :) Our Field Day
group holds the all time record for 1A battery, and we run RG213 to all
of our antennas.
73, Jim K9YC
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