The Trail Friendly 5/8 Wave Antenna
Special to The ARS Sojourner
Short verticals are nice for putting up, but they have a low radiation resistance which translates into poor efficiency unless all the other elements in the antenna system are exceptionally low resistance as well. This means that a short (i.e. 1/4 wave or less) vertical if matched with components that have very low internal resistances ( large diameter inductors with heavy gauge conductors) and driven over a very good ground, can be efficient. But if there are any sizeable losses in the ground system or matching system then the overall efficiency is poor.
The 5/8 wave vertical has a radiation resistance in the same range as a dipole making it easier to efficiently transfer more of those precious milliwatts into the ether instead of uselessly heating up a coil or the ground. The other thing about this antenna is that I normally hang it with the feedpoint at roughly 5 feet above the ground against four 1/4 wave radials which slope downward to within a couple of decimeters ( Americans read 1 foot) above the ground and are each held by a short length of mason twine and a tent peg. This elevated groundplane reduces capacitance between the radials and the lossy earth further increasing system efficiency the matching network is part of the base of the radiator so that a short length of 50 ohm coax can be used as a feedline. This allows whatever tent or fly is being used for shelter to be pitched directly below the feedpoint and a very short length of RG-174 can then be used as feedline saving weight and bulk without a major penalty in loss on the 20 meter band. At higher frequencies it may be better to use real coax.
Construction is simple. I use 22 gauge insulated wire. The insulation is thin and since the ratio of wavelength to wire radius is somewhere around 20,000 or more, the wire length would be only slightly less than the freespace value. I also do not tie knots or use egg insulators at the ends of my wires which act like capacitances and effect the electrical length of resonant wires. My insulators consist of strips of thin lexan or polyethylene plastic with a series of three holes drilled in, which the wire is fed through serpentine fashion creating a slip proof attachment point for a tie line. I use the formula 300/f (MHz)to calculate the wavelength in meters ( 984/f (MHz)for feet) and multiply this number by 5/8 for the radiator or 1/4 for the radials.
Matching can be done in different ways. According to tables compiled by Hallen which I obtained from Electromagnetic Waves and Radiating Systems 2nd Ed Prentice Hall, the impedance of a 5/8 wave vertical over a perfect ground with length to diameter ratio in this range should be roughly 75 ohms in series with 425 ohms capacitive reactance. With an elevated wire above a groundplane these numbers will be somewhat different but I didn’t measure the actual feedpoint impedance, and the matching networks that I used to suit these theoretical values worked just fine. These values can be fed into a piece of software called TLA by R. Dean Straw, N6BV which will calculate the component values of an L network which can match this impedance, or alternatively a coil can be wound which has the same reactance as the reactive component of the antenna feedpoint impedance, in this case 425 ohms, and a tapped feed can be used.
I have tried both and have had good success with each. The L network has the advantage of easy adjustability but the variable capacitance tends to be very small which tends to be a bit dubious and will be sensitive to the physical arrangement of the feedpoint. I have used a clear plastic film canister made of HDPE plastic which is an excellent low loss dielectric as the coil form and put a small air variable capacitor inside with the shaft mounted through the lid and had excellent results. Once the system is tuned to resonance it has plenty of bandwidth for any of my QRP rigs and seems quite insensitive to variations in mounting. Indeed when it has been windy and the sway of the branches has caused the whole antenna and groundplane to bob up and down I have noticed very little effect on SWR. This indicates that once tuned, the air variable could be taken out and measured and replaced with a fixed value and the variable returned to the junk box for more advantageous use in a VXO (one day).
The tapped inductor approach is not as convenient to adjust but once adjusted is quite permanent. I have used a T130-2 Amidon core to get the required inductance. A plastic coil form could also be used but soldering the tap in place presents a problem in this case. If a toroid is used, some form of protection should be provided for the coil due to its fragility ( I have yet to do this).In either case be sure to use good heavy copper wire, 16 gauge or better, to keep losses to a minimum.
How well does it work? I built one for 20 meters for the Flight of the Bumblebees. The week prior to the contest, after the band had recovered somewhat from a geomagnetic storm, I took it out to a local park and tried it out. Within 1 hour of tuning it I had worked UA9FAR East Russia, and OM0SX Slovak Republic, and OH5HO in Finland with 5 watts. Signal reports were 459, 559 and 559 respectively. Generally if I can hear them I can work them. Close in stations are tough though, as evidenced by the fact that I haven’t yet worked a single other VE3 but the DX sure rolls in. A 5/8 wire is getting pretty long on 40 meters, but the antenna does still work fairly well with a small section at the top bent over inverted L style. This would probably result in more high angle radiation which would be an advantage for close in work. It seems like it was more work to write this than it was to build the actual antenna, so the bottom line is just get out there and build one and see for yourself!!
Joesph Street, VE3VXO, is an avid builder, QRPer and outdoorsman living in Waterloo, Ontario, Canada.