When we say vertical antennas we normally mean a quarterwave monopole stick, or ground plane antenna. While the ground plane antenna has many virtues it relies heavily on a good ground for its performance.
A good ground system often means laying out a number of radials below the antenna proper. As many references note and advocate, a prohibitively large number of radials is required for the best performance. Four radials is perhaps mostly used but the number should be increased to several dozens, or even hundreds, to make an electrically acceptable ground system. That kind of ground system cannot reasonably be made anything but stationary. If at all, considering the real estate needed.
An attractive alternative is the vertical halfwave dipole. This is a seldom used antenna for shortwave but it proves to be a good performer. The antenna is symmetrical and does not rely on a ground system like the quarter wave vertical to achieve a low take-off angle. With a proper ground, the angle gets even lower, but for average ground conditions an angle lower than 20 degress can be expected!
There are some drawbacks but they can generally be overcome:
- The size. Being a halfwave dipole it is twice as long as the quarterwave groundplane. This is mainly a hindrance at wavelengths longer than 20 or 30 meters. In practice there are two ways of keeping the antenna upright. One is to use guy wires, the other to attach the lower leg of the antenna to a pole (wood, glassfibre etc) in the ground.
- Feeding point. The dipole must be fed at its center which is halfway up the antenna element. Its feedpoint impedance is some 70 ohms meaning that a match to ordinary coax is not absolutely necessary if one accepts a SWR in the 1.4 range.
- Feeding point again. To acheive the lowest take-off angle a balanced feeding system is a must. That can be a balanced ladder-type feeder or a balun at the feed point. See below.
Current on the feeder. Fed with a coax the cable normally slants down to the transmitter. If the cable is closer than about λ/4 to λ/2 to the antenna then the preferred orientation is at a right angle to the antenna. As this can be difficult to maintain an alternative is to use current chokes at strategic points along the feeding coax to reduce the unbalance currents.
Among the benefits I clearly see:
- The very low take-off angle. Apart from the quarterwave antenna there is no way to get such a low take-off angle so easily. A quad or beam will give low angles too but only if these antennas are erected at considerable height. Which entails towers or large structures of some kind.
- Portability. Built with collapsable fishing rods this antenna can be use on a backpacking tour even.
- Multiband use. Generally the VHD:s radiation pattern gets better at higher frequencies. Unfortunately running it outside of its design frequency gives very large standing wave ratio which in connexion with coax feeding, results in very high losses. But, if fed with ladder-type feeder, which has inherently very low loss even at great SWR:s, the VHD can be used at frequencies both below and above its design frequency.
My VHD is cut for the CW-portion of the 21 MHz band i.e. 21050 KHz. The total length, according to the formula L=(300/21.05)*0.95/2, is then 6.77 metres.
The vertical dipole in place. The anchoring lines are overdimensioned and this setup has survived both severe autumn winds and heavy snowfall. Coming from the right is the ladder feeder. This feeding arrangement allows me to use the vertical from 10 MHz and up with only a few dB’s loss even though the vertical is dimensioned for 21 MHz.
As the antenna is to be placed close to ground there will be larger capacitive coupling between the lower of the antenna legs to the ground, than between the upper leg and the ground. This will disprupt the balance of the dipole as well as cause the resonant (resistive) frequency to shift somewhat.
Apparently some in-place tuning will be necessary, so I made the ends of the dipole adjustable. This turned out to be necessary to achieve a sound SWR on 21 MHz. And I also noted that the antennas height greatly changes the SWR.
The antenna is strapped to a 2″x2″ piece of wood about 40 cm above the closest ground. To get rid of the antenna I installed it on top of an old cellar in the garden. This way it is almost 3 metres above the surrounding area, and does not have to confront the lawn mower.
For feeding it I made 50 metres of ladder-type feeder from insulated electrical cable. Small pieces of PVC tubing was used as spacers 7 cm long, one every half metre. The cable is attached at each end of the tubing and strapped into place with the smallest plastic straps I could find. The resulting impedance has not been measured but will be in the 400 to 600 ohm range. The exact value is not critical here.
The dipole is fed with almost 50 metres of ladder line hanging on top of some bushes in the garden.If you build this and feed it with coax it turns into a one bander. You will then need a choke at the feeding point to reduce unbalance currents on the outside of the coax. This choke is most easily made as a loop of several turns, say 6 to 10 turns with a diameter of 20-30 cm or so, of the coax itself. It is also a good idea to make another choke λ/4 further down the coax. (NB. This is electrical λ/4 which is normally 0.96 times shorter, due to the plastic insulation of the coax, than expected with the formula 300/f/4.) The reason for this placing is that we have a current maximum at the feed point and also a current maximum at λ/4 down the wire. If the first balun is of the suggested coax type then measure how much coax that went into the balun and include this length in the λ/4. I have also seen examples where the coax shield has been tied to a ground pole at this λ/4 point. That would effectively quench any remaining unbalanced currents!
As I am not using coax for it, the radiator length was no longer critical so I made it as long as I could before installing it. Simulations i MMANA show that the radiation pattern gets better at higher frequencies i.e. the VHD gets better as the dipole is longer that λ/2.
As the seasons go the feeder has been covered with a fair amount of snow and has been pulled taught so that it now is a bit longer than when I erected it.