What I learned from building my own G5RV / ZS6BKW Antenna- Part 1
In the Beginning
When I first got my F call, not so very long ago in July 2008, it was not long before I wanted to improve my antenna. I had been using a Bushcom broadband antenna in an inverted V. I was limited as to maximum height, and I felt the performance of the Bushcom though good, could be improved upon especially with a bit more height which I could get using two trees in my back yard.
I could have simply moved the Bushcom, but I also thought it was time I revisited antenna building, as my last forays, including a 4 element quad, had been in the heady CB days of the 1970′s hihi. I settled on a G5RV as many beginners do, because it appeared to best suit my needs. My brand new F call allowed me to operate on 80m 40m 15m and 10m, as well as 2m and 70cm, so I wanted a “multiband” antenna that was easy and simple to build, and would fit in the space between two very tall trees in my backyard. The position of these trees meant a doublet such as the G5RV could be fed from a point very near or at my existing 10m mast. This would be convenient, as coax cables already exited the eaves at this point, and I didn’t really want to drill more holes in my house. The highest points in the trees would be somewhere around 18 metres if I could reach that high with my slingshot.
A Little Learning is a Dangerous Thing
...Drink deep or taste not that Perian spring…..etc
So knowing just enough about antennas to get myself into trouble, I set about building the G5RV. For the arms of the doublet I used 3.5 mm stainless steel, as I wanted some strength given the strain that would be put on the antenna in windy conditions. The tree branches tend to move quite a bit in the wind. For the open wire feeder I chose enameled copper wire of suitable thickness separated by beverage tubing “ladder rungs.” The spacing of the open wire feed-line I selected after reading various web articles on the G5RV was pretty wide at about the length of the average ballpoint pen. At the time I had no idea what effect this may/may not have.
The same web sites informed me that a balun was not required, and quoted comments by the antenna inventor and others testifying to that “fact.” I noted that many people ran 50 ohm coaxial cable from the rig/ATU to the open wire feeder. Since my rig had built-in tuner, I did the same. About 22 or so meters of RG58 coaxial cable runs from the rig through the ceiling to the bottom end of the open wire feeder. Initially I simply terminated the feeder with an SO239 socket and the coax with a PL259. I was concerned about going from unbalanced to balanced without a balun, but the web sites and much literature I could find suggested I didn’t need one. I was pretty soon testing the antenna, and though the SWR was high on all of the bands I used, the built in tuner of the 703 seemed to handle it quite well achieving a 1:1 match on all bands used.
I was however quite disappointed with the performance of the G5RV. It was very noisy on receive, and several stations over a period of time reported my signal as poor especially on 80 meters. The first thing I had noticed was the receive noise, so I spent a fair bit of time reading up on noise suppression and various ways to deal with noise, but that is another story. After determining that the noise was external to my house, I put up a 160 m horizontal loop which gave little noise on receive, while I worked on improving the G5RV. The first thing I did, was to coil the coax up at the feed point, onto a PVC former. I had done this years before on an inverted V that I made for an RFDS radio setup. This air-wound coax current balun (now I had a name for it) did little to improve things.
Truth or Fiction?
Before I go on with the story, let me share some of the things I had read about the G5RV. As it turned out, many of these things were untrue or misrepresented, and I had too little knowledge and experience to critique all of them. No doubt many beginners are in the same position.
1. SWR is low on all bands
2. SWR is high on all bands except 20m
3. The open wire feeder acts as part of the antenna on lower frequencies and this contributes to the antenna effectiveness on those bands.
4. The length of this open wire feeder is critical to the success of the antenna as it forms part of the antenna elements (radiates).
5. If using coax between rig and open wire feeder, the coax must be cut to the specific length that gives lowest SWR. (I’ve heard this one before with many antennas).
6. A balun is not required for the G5RV.
7. The G5RV was made for the pre- WARC bands and is no good on modern bands. The ZS6BKW modification is better on modern bands.
As I go through the process of explaining my experiences with this antenna, I hope I can dispel some myths. As I said, I am a beginner, so I may not understand all, but I hope my experiences are useful to other beginners.
So let’s return to where I was in the build process where we had already dispelled myth number one that the SWR is low on all bands.hihi. Though my 703 has a built-in SWR meter, I had a Foundation license at that time, and it did not allow me to transmit on 20m, so I could not measure SWR there.
Desperate to make this antenna work, I did lots of reading. Pretty soon I came across a modification of the G5RV known as a ZS6BKW, basically a shortened doublet with a longer feedline. Antenna theory by now was appearing to be some sort of black magic. Obviously this ZS6BKW was the “magic” I needed.
I set to work on the antenna. At the same time, I set about building a 1:1 current balun wound on a ferite torroid and housed in an aluminium box.
A Great Antenna At Last- How Did I Do It?
I finished the mods, fitted the balun and returned the antenna to its spot in the trees to find the reception had improved, and the transmit performance much better, even on 80 meters. So what had happened. Initially I didn’t care, as long as it was working. But eventually curiosity got the better of me, and I had to examine the G5RV/ZS6BKW a bit more closely……Part 2 here.
73 de VK4MDX