I have always seen mentioned many times when someone writes about the
G5RV antenna that you need to use a minimum of 68 feet of coax connected
to the 30ft of twinlead or ladderline. I'm pretty sure that you can
blame me for that urban legend. Sometime around 1965-1967 several of us
in Maine were fooling around with the G5RV configuration on 75 meters.
Some with good results and some with not so good results! A ham by the
name of Charlie Murch decided to manufacture the antenna, which he
called the 51-30-51 for obvious reasons. He sent many of us a prototype
to test. In that era the rigs were tube finals with a pi network output
which could handle a modest range of input impedances. Antenna tuners
were not in vogue and Lou McCoy's design of the "Ultimate Transmatch"
was just coming upon the scene, the granddaddy of most of the tuners
used today.
So with my trusty HQ129X rcvr and DX100 xmiter, a Jones MicroMatch swr
bridge plus a brand new Omega T noise bridge I set out to solve the
mystery of the G5RV. The first antenna was erected about 200 feet from
the shack and 35 feet in the air with 200 feet of coax hooked to it.
Worked good and showed a less than a 2.1 swr on 3.940MHz. The next
location for it was closer to the shack and required only (you guessed
it) 68 feet of coax. The 2.1 swr went to over 3.1, mystery solved.....
.........anything less than 68 feet would result in a too high swr on 75
meters for most pi networks to handle it. This finding was duly reported
to Charlie, with a recommendation that at least 100 feet of coax should
be used for use on 75 meters for best results. When it went into
production his ad in QST and the instructions packaged with the antenna
said "should be used with at least 68 feet of coax".
Of course I have since learned why the longer coax showed a lower swr.
I wonder if the legend would be different if I only had needed 34 feet
to reach the shack. Of all my G5RV antennas that I have built, and I
have built many, My conclusion is that it is a good tri-band antenna.
80,40 and 20 mtrs. Not an "all band" antenna. 300 ohm ladder line is a
better choice than 450 ohm. The reason is that on 40 mtrs the 1/4 wave
length better transforms the high center impedance nearer to 50 ohms
than 450 ohm line. On 75/80 mtrs it is a 1/8 wave length feeding a very
low impedance which results in adding inductive reactance to the system.
And again 300 ohm line is better than 450 ohm which adds a bit too much
inductive reactance. It would make no difference on 20 mtrs since it is
a 1/2 wave length there and WYSIWYG on both ends of the line. Your
mileage may vary. And like all wire antennas, the biggest factor for
their performance are three things........location...location and location.

John K1BXI