The really attractive thing about open line, and even window line, is
that its loss is so low you can use it at a very high SWR, and take care
of the matching at the station end. This is especially useful for
multi-band antennas. The famous G5RV, for example, will work just
exactly the same fed with window line and a tuner as it will with the
usual matching section.
73,
Scott K9MA
On 1/6/2019 16:09, W2xj wrote:
And those broadcast stations generally work within a 2:1 VSWR range.
Sent from my iPad
On Jan 6, 2019, at 4:51 PM, Bob McGraw K4TAX <rmcg...@blomand.net> wrote:
Correct on all points.
That is the reason I gave my feedline type, and length and antenna length alone with
specific balun type and common mode choke type. Any old ragged combination of chunks of
wire and feed line is not assured to work and is largely the reason many hams shy away
from balanced feed systems. They tried it once, it didn't work well thus they deemed
all balanced feed systems as being "bad". While the facts are, a balanced
system correctly installed and with correct lengths, it is the lowest loss feed of any
system reasonably available. Which is one of the reasons most short wave broadcast
stations used balanced feed systems.
73
Bob, K4TAX
On 1/6/2019 3:40 PM, Don Wilhelm wrote:
All,
I think a bit of extended study on transmission lines and their impedance
transformation characteristics is in order - refer to any ARRL antenna book,
even those from the 1950 an 1960 era, not much new has been added, other than
baluns. The feedpoint impedance to the transmission line can vary from quite
low to quite high dependent on the frequency, the length of line, and the load
at the far end. It does NOT depend on the characteristic impedance of the
transmission line. The ARRL TLW program can do a lot of computation for you.
You may be able to adjust the length of the transmission line so there is a
moderate shack end feedpoint impedance on all bands of interest and be able to
use a good current balun which has either a 1:1 or 4:1 transformation ratio.
You have to measure the transmission line at the point where you intend to
install the balun to be sure which to use.
Yes, just hanging any balun on any length of parallel feedline connected to any
length of antenna and expecting it to work without problems is just not going
to happen in all but exceptional cases.
How many cases where you see "this antenna works great" do you see any mention
of how long the parallel feedline may be. Most hams simply ignore that, but it is likely
the most important part of how the antenna works and what type of balun of you need to
use.
Your experience may be very different with the "same" multiband antenna than
your local ham or that in the artic
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