Posted by
Geoffrey Mackenzie-Kennedy-2 on
Oct 02, 2006; 12:21pm
URL: http://elecraft.85.s1.nabble.com/135ft-flat-top-with-K2-KAT2-BL2-and-450-ohm-feedline-tp394239p394248.html
Ron, AC7AC wrote:
But we then need a decent
tuner to feed the antenna & open-wire. Here I drop back to the old
tank-circuit tuners which make things "sing!" I don't like to use a
balun here - it's just another device to burn up the precious RF when
you hit a severe mis-match node.".
<snip>
Even among open wire lines, losses are not always insignificant. It depends
upon the feed point impedance (that's at the antenna or load end of the
feeder). For example, a short doublet or some transmitting loop antennas can
have an impedance of 10 ohms or less. With 600 ohm line that's an SWR or
60:1. The losses in the open wire line may be as great as those in coax
under those conditions.
One way to reduce those losses is to reduce the ohmic losses. That's where
virtually all the loss in any kind of transmission line occur: RF converted
to heat at the high-current points. Ladder line and "twinlead" that use
small conductors is very susceptible to such losses. RF flows in the skin of
the conductors; resistance is directly proportional to surface area. My open
wire lines are seldom built using smaller than #12 wires.
------------------------------------------------------------------------
To put a balun at the shack end of a balanced feeder is, I agree, one way of
burning up the precious RF unless the feeder presents a suitable
non-reactive load compatible with the balun's characteristics. On the other
hand a current choke at the 50 ohm input of the old type of tuner, the TX
connection, could be seen as being necessay but in my experience not
essential - assuming that the tuner presents a load close to 50 ohms and
that the amount of common mode current is insignificant i.e. the outside of
the coax to the TX is 'cold'.
Another way to reduce ohmic and radiation loss in open wire lines is to use
the 4 wire cross-connected type. The characteristic impedance of such a line
is very approximately half that of a two wire line using the same size
conductors and spacing, which makes a 4 wire line useful when feeding a low
impedance load such as a short doublet. The downside is that 4 wire lines
are more difficult to build than two wire lines and need to be kept under a
small amount of tension, unless a fair number of spacers are used which can
increase loss. They also need to be 'cleaned' occasionally, especially in
the Fall after the trees have shed their leaves. The upside is that in
addition to reduced ohmic loss, a 4 wire line loses less power due to
radiation than a two wire line, and nearby objects such as masts have less
effect on the line. It could be argued that the effort to build 4 wire lines
for HF use is not worthwhile, however for use at 50 MHz it is very
worthwhile where the radiation loss in a long run of 2 wire line using a
practical spacing between wires can be significant.
73,
Geoff
GM4ESD
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