[Elecraft] Baluns and 450 ohm line

[Stuart Rohre]

John,
50 ohm based measuring instrumentation should have enough available current
drive, at its low impedance, to accurately drive, and allow measurement, of
the higher impedance cases.  The principle here is that a higher impedance
will not load down the instrumentation, given 50 ohm devices with sufficient
current sources.

However, remember that unless you have exactly a half wave length line, if
the antenna is some impedance other than 100 ohms, it may not show its true
Z thru a random length of 100 ohm line.  In the exact half wave length at
one frequency case, the 100 ohm line becomes transparent as to impedance,
and does not affect the total impedance, allowing you to measure the antenna
impedance back at the shack end of the half wave line.

You would look for the minimum, or dip, in the measured SWR on the MFJ.  The
actual value of impedance reported will be perhaps something different than
antenna Z, for random length feedline, but the dip will be the resonant
point of the antenna.  A resonant antenna at optimum height will measure
closer to the 72 ohms of free space, not 50 ohms anyway.  If you had 50 ohms
before, it was a fortuitous happening of the height of the antenna, and its
influence with nearby conductors or other near field couplings.  Don't worry
about the exact antenna impedance if you are using a transmatch anyway to
move over the band.  On 80m, no single wire antenna, that is high
efficiency, will hold a 1:1 SWR over the band without a tuner.  If your
antenna is so low Q as to have low SWR over most of the band, look for
losses, or other problems.

You should have to use a transmatch to move around the 80m band with most
ham wire antennas.  They are too high Q to load well at all 80m band
frequencies.  But this is OK, you can accept some SWR up to the limits of
your rig, or use your transmatch.  The efficiency of the antenna will not be
lessened by the SWR, all the reflected power is eventually radiated.

The losses come about in low impedance feedlines having higher loss
inherently, and the power having to transverse the line multiple times until
eventually radiated.  Although there are losses in the higher impedance
parallel lines, it is very much less than coax loss, when open wire or even
insulated ladder lines are used.

I do not think there is any advantage to using twin coax to feed an antenna,
when you consider the downsides of extra weight on the center insulator as
compared to most parallel lines and the extra dielectric in the coax..  The
use of twin coax is often thought to lessen RF from the line, but certain
lengths of feeder, or failure to have balanced currents at the feed point
may introduce RF onto the outside of the shields and you are right back to
having a radiating unbalanced condition.  Beware of the line, if choosing a
particular line broadened your SWR curve flat portion!  Yes, the rig will
load it, but the RF is not going where it is needed.

The wonderful thing about ham radio, is under the right propagation
conditions, you can have a poor antenna, balanced in construction, but
unbalanced by feeding by single coax and no balun, and it will work out just
fine.  I doubt you could measure the difference on an S meter between the
ideal feed and whatever you have at hand. It is the other effects of
unbalance that might cause problems, (line radiation, etc.)   But, an
antenna is a system that consists not only of the feeder, but also the
antenna and its materials, and the local RF ground character, elevation, and
anything close enough to couple to the antenna.  You must have knowledge of
all factors affecting your installation to get it operating to top
efficiency.
-Stuart K5KVH