It has been said many times over that the G5RV or 102 ft dipole is not an all band antenna. That in certain cases is true such as when it is fed from the transceiver to the balanced line / ladderline feed point on the antenna with a long run of coax which looses power when it is connected to a mismatch.
What I am writing about here is where the balanced line (twin lead or ladder line) is attached to a good 1:1 current balun which is fed with a short as practical ( 8 to 10 ft) length of a good grade of coax such as LMR 400 to the coaxial output of a tuner in the shack at the operating position.
A 102 ft or G5RV dipole won’t work well on more than one band 20 meters without some sort of matching equipment or section that will allow the transmitter to load the antenna efficiently for most people that means an antenna tuner at some point in the feed line. An internal antenna tuner such as found on most modern HF transceivers may allow for operation on 20 & 40 meters depending on the doublet’s installation and the limitations of the built in tuner.
In my 4 decades or so of experimenting with HF wire antennas it is a rare exception to take an antenna from the work bench (or the box) pull it up into place on it’s vertical supports and have a good match right where one wants it. A G5RV is no exception to this, Owen Duffy wrote an excellent article “Optimising a typical G5RV” that describes in detail how to tune a G5RV for use on 20 meters that is well worth reading and doing.
Balanced feed line
The three basic types of feed line that can be used to feed the doublet are twin lead, ladder line and open wire. The first two are lighter and more compact if one is looking for a portable dipole to take into the field but are lossy and require retuning when there is fog, rain or ice. They both have more signal loss on high SWR mismatches than open wire. Open wire is the premium balanced feedline typical with wires spaced six inches apart by plastic spacers and an air dielectric offers the lowest losses on mismatches and is barely if at all affected by rain or fog, we don’t get ice here so I can’t attest to whether or not that is an issue. Open wire can be made at home with a few items from local home building centers namely irrigation tubing and stranded wire. Manufactured open wire can be bought at True Ladder , I am unaware of anyone else offering open wire.
For fixed QTH operation I recommend using a 1:1 current balun to help maintain feeder balance and lessen common mode currents, obviously a balun can be used for portable operations those sometime space and weight may make it difficult. A good balun can prevent the coax from radiating and becoming a part of the antenna instead of just being a transmission line. I tried a few coax chokes (sometimes called baluns) consisting of around 20 ft of solid dielectric RG-8 neatly wound on a length of 4 inch diameter PVC but was never really satisfied with the results when using it to interface balanced line. I have had excellent results using chokes at the feed point of coax fed dipoles such as fan dipoles. Several years back I bought a 1:1 current balun from Balun Designs a few months after that our QTH suffered a catastrophic flood leaving the balun under a few feet of water for over a week. After drying it out well that same balun is still in use today handling 1 KW SSB with no problems.
The secret to having success with a G5RV on the WARC bands – even80 – 10 is using as short of a length as possible of low loss quality coax to keep the power losses down. A good tuner is necessary and a good investment unless perhaps one is fortunate enough to have enough space for resonant antennas on each band of interest.