Most people think that the famous G5RV antenna was designed as a
multi-band antenna, but in fact, it was actually designed as a three -
half waves in phase antenna for 14 Mhz by Louis Varney, G5RV. Mr. Varney's version used open wire (600 ohm) feed line all the way to the
transmatch instead of the short section of 450 ohm window line and 70
feet of 75 ohm coax that most of us know today.
Mr. Varney also found out that his antenna would work other bands as well with a fair amount of efficiency with the use of the transmatch. As designed, the G5RV exhibits a low SWR on 14 Mhz of around 1.8:1 into a 75 ohm load, but the SWR varies significantly on the other bands, so the transmatch must be used when using it on bands that it was not designed for.
To figure the length of the G5RV antenna as we know it today using the short section of
twin lead and coax combo, we simply use 2 simple mathematical formulas.
This type of antenna can be designed for any frequency, so the
conventional dimensions will be different for different bands.
To figure the "flat top" (dipole) length, we use the formula: 1428 / F Mhz = Length in feet, and the formula of the matching is: 468 / F Mhz X velocity of the twin lead = Length in feet.
To demonstrate the difference in size for different bands, we will
now figure one for 15 meters as an example. So, 1428 devided by 21 Mhz =
68 feet, this is the flat top, so we devide it by 2 to find the center
and this makes each side 34 feet long. For the matching section, we
devide 468 by 21 Mhz = 22.29 feet multiplied by 98% = 21.84 feet of 450
ohm window line, we now attach 70 feet of 75 ohm coax and have a fair
match on 15 meters.
Notice that the 15 meter version is almost the same size as a conventional coax fed dipole which is about 66 feet long, so this 15 meter antenna will work very well from 40 meters on up when using a transmatch. Of couse, you can do as Mr. Varney did and run the 450 ohm window line all the way to the balanced output of your tuner eliminating even more loss and giving your efficiency a big boost!