Past few years I have been building common mode chokes, 1:1 baluns aka 1:1 ununs on request. One of the questions asked by fellow hams is where to install it, which depends on the type of antenna. Then the ‘counterpoise’ enters the story and why certain antennas need one. So I will try to explain, starting with a light bulb.
A lamp with a classic light bulb needs electric current to glow The current is fed to the light bulb with a single wire. But when you connect one wire, nothing happens. We all know you need another single wire to transport the return current. Once connected and the circuit is completed, the light bulb glows. It works a bit the same for antennas.
Completing the RF circuit
With antennas it’s all about completing the antenna circuit: An antenna, whether it’s a dipole, vertical, or some other type, relies on a closed circuit for the flow of RF current. The counterpoise serves as the other half of this circuit, completing it. Without a counterpoise or ground plane, the antenna may not function optimally. RF energy will radiate inefficiently with a divergent radiation pattern.
Providing a return path
The counterpoise helps to improve the radiation efficiency of the antenna. When RF current is applied to the antenna, it creates an electromagnetic field around it. This field needs a return path to complete the circuit and radiate effectively. The counterpoise provides this return path, enabling the antenna to radiate efficiently.
Replacing ‘real’ ground
Especially HF antennas are often designed with reference to the Earth’s surface serving as a ground plane. But not all antennas use ‘real’ ground. The counterpoise ‘replaces’ the ground. Without it, the antenna may not be resonant.
Common mode currents
Antennas can induce common mode currents. Those are currents that flow on the outside of the coaxial cable or feedline. They are a source of RFI, interference on electrical devices like computers, monitors, audio and TV. The counterpoise can help reduce common mode currents by providing a balanced reference for the antenna system. A common mode choke or 1:1 balun isolates the feedline from the antenna, preventing the feedline of acting as a counterpoise.
A counterpoise should also help match the impedance of the antenna to that of the transceiver, in most cases 50 Ohm. An optimal match to impedance means an optimal SWR and minimal RF turned into heat in your transceiver.
Antenna’s that require a counterpoise
There are quite a lot antennas that do a lot better job when providing a counterpoise. Here is a listing of those antennas:
1/4 wave vertical
The 1/4 wave vertical is in fact a 1/2 wave antenna that use a ground plane as the other half of the antenna. When used with a ground rod, the Earth’s surface is the counterpoise. Sometimes a combination of a ground rod and several buried radials is used. When elevated, it needs several 1/4 wave long radials as counterpoise to function.
5/8 wave groundplane
The 5/8 wave groundpane (GP) is especially popular on 10 and 11 meter (CB) band. They use a counterpoise that exists of a few long radials or several short radials.
Mobile antennas use the metal body of a vehicle as a counterpoise. When using such an antenne on a balcony, you need to provide a decent counterpoise. A metal balcony fence, a long piece of wire or 1/4 wave radials will help accomplish that.
Maritime vertical antennas on a boat or ship, not only use the boat or ship’s hull as a counterpoise. They also use the water it’s floating in. When the boat or ship has a non conducting hull, like polyester, a large metal plate beneath the hull provides contact with the water.
Short antennas for handhelds
Short antennas for handhelds (HT) also need a counterpoise. Sometimes the circuit of the HT provides some counterpoise. Adding a counterpoise (like a piece of wire, metal can, etc.) could improve radiation efficiency.
Wire antennas such as the random length EndFed and halfwave EndFed (EFHW) will definitely benefit from a counterpoise. A single wire with a length of 0.05 x TX longest wavelength, can make a big difference. For example, if the longest wavelength your transmit on is 80 meters. Your counterpoise witre should be 0.05 x 80 = 4 meters long. You can also the Earth’;s surface, through a ground rod, as a counterpoise.
Counterpoises can be made from a variety of materials and objects. The one necessity is that the material is conductive, for example metals like copper, aluminium, steel and zinc.
- Single wires: copper electric wires, steel construction cable, clothes line, bob wire
- Fencing: balcony fence, chicken wire
- Construction materials: piping, masts, metal roof plating, metal wall plating, zinc rain gutters
In some areas the underground drinkwater piping is metal and can provide a good counterpoise. Always make sure that this piping does not turn into PVC further once it leaves your property.
Using central heating piping as counterpoise is not recommended. It’s notorious for picking up manmade noise. Putting RF on piping that runs through an entire apartment block could lead to serious RFI issues.
The ultimate counterpoise
Salt water is an extremely effective counterpoise. Sometimes island DXpeditions place 1/4 wave vertical antennas in shallow water to improve radiation efficiency in low take-off angles. Up to 6 dB low angle extra gain is achieved in comparison to a vertical on average soil.