The G5RV is simply--as an antenna--a 102' length of wire center fed. This
length has been used since the late 20s or early 30s with parallel feeders
to balanced link-coupled antenna tuners.
The G5RV part is the system that Gil Varney devised to provide a low SWR
to coax with a length of paralel line from the antenna to the coax. This
worked for many folks (and was off the mark for other installations) for
the harmonically related bands, but the advent of the WARC bands made the
system outmoded--since parallel line to a balanced tuner works for all
bands. If the line presents on one or another band a level of reactance
beyond the range of the tuner, an inserted length of line will generally
change the reactance presented to a matchable value.
No mention is here made of any kind of balun. I experimented with them in
the 70s and found them unnecessary except in one situation. If you have a
network tuner, then you will be using a balun inside the tuner. The
lossiness of these 4:1 baluns is quite variable from one make of tuner to
the next under conditions of high reactance. An external unit, like a
line isolator, which is 1:1 will likely have less loss, but certainly not
no loss. If you look at the value of resistance and reactance along
lengths of line under a wide range of initial load conditions, you will
discover that as often as not the impedance presented to the tuner is
already low in the resistive component--and a further 4:1 reduction does
not speak to good efficiency. Hence, a 1:1 system is better. In some
installations, indoor conditions may promote unbalancing the parallel
line, and in that situation, it may be best to place the 1:1 unit at the
shack entry point and use sizable coax (less than 20') to the network
tuner. Under virutally all (but not absolutely all) conditions, the
losses will at the higher end of the HF spectrum never exceed 1 dB, and
the advantages of not having RF in the shack to cotnend with may be worth
the loss.
These latter thoughts, based on experience, all depend on the variables
of the equipment, shack, and building situation. Hence, they represent
alternatives to consider, but not recommendations, since--at best--they
are compromise systems. For the long term, it is best to revise the shack
situation to place a balanced tuner like a matchbox near the entry/exit
point for the parallel feedline for maximum efficiency with a 90-110 foot
wire antenna (length is not critical with a parallel line feed system).
At my site are some notes and pattern plots modeled with this length of
wire so that one can get some expectation of the lobe directions, etc. for
the antenna on the bands from 80-10 meters. They are in the collection
called Antennas from the Ground Up done for Low Down. There are similar
compendia for other wire antennas in the same series. Knowing where your
lobes go is crucial to successful use (and expectations) of wire doublets.
Hope these notes are useful.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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