Today’s Whimsy
There was a young donkey named Sam
Who ate everything he could scam
He ingested a 16 foot wire,
Climbed a small hill to get higher,
And found he was resonant on the 10 meter band.
A Bit About Vertical Antennas
Vertical antennas are both praised and damned, with some users complaining that they radiate equally poorly in all directions, and others swearing that they really haul in the DX. Either case can be true, depending on the design. Here’s your key to figuring out what will work for you.
What I’m discussing here is vertical antennas working against a real or artificial ground, rather than dipoles oriented vertically.
To an electron, a sheet of conductor is a mirror. Move the electron, and its mirror image moves accordingly. This effect creates a virtual mirror image of moving electrons in a vertical antenna. A ¼ wave vertical in the midst of a large perfectly conducting sheet then becomes a ½ wave dipole, with a base feed point impedance of 36.5 ohms, half that of an ideal dipole. This is also the antenna’s radiation resistance, the effective resistance that converts current into RF radiation, just as an ordinary resistor converts current into heat.
As an antenna is shortened below ¼ wavelength, the radiation resistance, which is also the resistance component of impedance at the base feedpoint, drops.
The total resistance that will be seen at the base of a ≤¼ wave vertical will be the radiation resistance of the antenna (useful), plus the effective resistance of the ground (not so much).
Just for grins, I pushed a 9” rain gutter nail into my lawn and used it as ground for a ¼ wave vertical. Call that the worst case real ground. The total resistance was 135 ohms. The maximum efficiency of an antenna is its radiation resistance divided by the total resistance, so that antenna could not be more than 36.5/135 = 27% efficient. That’s close to a 6 dB loss, or about 1 S unit. Can you make contacts with such a contraption? Of course. Your 100 watt transmitter will sound like 27 watts, and 27 watts can take you a long way.
Substitute a ⅛ wave vertical, and the radiation resistance is about 8 ohms. Now the maximum efficiency is 8/135 = 6% and you’re really running QRP without knowing it. Multi-band trap verticals are shorter than ¼ wave on most of the bands they cover, and hence have low radiation resistance and high losses against poor grounds
Can we beat the rap? I’m so glad you asked.
One approach is to start burying ground radials, which simulates that large conducting sheet. Four quarter wave radials work better than that 9” nail, reducing ¼ wave vertical loss to very roughly 3 dB. But to get the ultimate low loss ground, you have to lay out 132 quarter wave radials. That shields the lossy Earth from your radio signal. Installing that much wire in the ground is a big job, and there are easier approaches.
Another solution is to make an artificial ground, and move the feed point up away from Earth. A ¼ wave vertical with 4 ¼ wave radials, with the base elevated ¼ wave is nearly 100% efficient.
Just a few feet of elevation makes a significant difference. For POTA, I use an 18.4’ vertical mounted on top of my Highlander, using the body of the vehicle as ground. 18.4’ is .14 wave on 40 meters, and the vehicle is far less than ¼ wave tall or wide. Still, my measurements indicate that it is better than 50% efficient there, and closer to 100% on the higher bands.
Yet another solution is to make your vertical antenna longer than ¼ wave. At ½ wave plus a trifle to make impedance matching easier, the currents in the ground are very small and so also are your losses. I’ve run a 20 meter halfwave vertical with 4 radials from Antelope Island, with very satisfactory results. Lengths between ¼ and ⅝ wave are well behaved except when the antenna length is exactly ½ wave.
Whatever configuration you choose, I suggest that you install at least 5 type 31 ferrite beads on your coax. This prevents your coax from trying to be a radial, and rendering you and your rig hot with RF. Well, maybe your SO thinks you’re hot anyway. I don’t know.
Vertical Antenna Virtues
- Verticals with an effective ground provide a low angle of radiation. This favors DX. Poor grounds suppress the lowest angle radiation in addition to being inefficient.
- They don’t require a huge amount of real estate and are relatively unobtrusive.
- You can hear and transmit in all directions.
Vertical Antenna Vices
- Verticals disfavor local contacts via Near Vertical Incidence Skywave (NVIS) paths.
- On the lower bands, they are quite noisy. On 40 and 80 meters, you might want to consider a low noise receiving antenna. On 30 meters and above, it’s not a problem.
- Short vertical antennas with poor grounds will challenge your patience and operating skills.