On hf, any balanced center fed antenna, i.e. zep, dipole or loop etc., operated
with a high vswr should be fed with open wire line, where feasible. This is
because open wire line has extremely low loss and very high vswr
characteristics. Open wire line can be connected to coaxial cable quite
easily, and remain balanced. This is accomplished by using two parallel runs
of coax of equal length. Simply connect the shields together at each end. One
then has two shielded center parallel conductors that can be connected to the
open wire line. The coax can then be pulled through walls easier than the open
wire type. (Of course for low power, TV type of ribbon line can be used with
good success and can be easily pulled through walls.) If one uses RG-11 the
nominal impedance of the two parallel lines will be 150 ohms instead of 75.
The larger the coax the better because the larger cable usually has lower
losses and higher voltage ratings. Also, this makes for a fairly good match to
300, 450 or 600 ohm open wire line and the small mismatch is insigificant. By
using two parallel runs of coax in this manner, the peak voltage rating is
doubled. I have written about using this technique before, as well as several
articles have appeared in QST recommending this technique to feed open wire
line to a balanced antenna. Also, open wire line will not radiate unless one of
two following conditions exists: 1. The spacing of the conductors exceeds one
percent of the wavelength, and 2. There is an inherent imbalance somewhere
in the antenna system, i.e. one side longer than the other that will cause the
currents on the transmission line to be out of phase or if the open wire line
is fed with unbalanced current. As long as one is using a highly efficient
transmission line and a very low loss antenna tuner (i.e. E.F. Johnson Matchbox
or equivalent) then the presence of high vswr on the transmission line is not
important as no sigificant losses will occur. This has the very optimum
advantage of allowing one to use one's
results. This technique has been clearly explained by Mr. Walter Maxwell
(W2DU) in his series of about seven articles entitled " Another Look at
Reflections" that appeared in QST from about 1973 to 1976. He later used the
articles as a basis to publish a book by the same name. The book was published
by the ARRL for a brief period and is well worth reading. Unfortunaly, it is no
longer available from ARRL. Also, older issues of the ARRL Antenna book had
chapters clearly explaining how to build a simple all hf band system using open
wire line and link coupled antenna tuners. Also, I quote from Mr. Maxwell as
stated in QST: " A LOW VSWR WILL KILL YOU !" - 73's- Corn -k4own
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