Having the correct antenna height is essential for antenna performance.
Usually, you may want to mount your antenna as high as possible, but this can also have counterproductive effects as you may experience radiation losses beyond a certain height.
You should also keep in mind that there are regulations that put restrictions as to how high your antenna can be.
So how high should your antenna be?
As a general rule, most antennas should be at about half the wavelength of the transmitting frequency depending on the type and resonance frequency of the antenna. But this may not be feasible at the lower spectrum of the ham bands where the antenna may be too tall using this rule. Most of the time, antennas are usually between 15 feet and 70 feet off the ground depending on several factors.
The factors that can have an impact on the height of the antenna you are using are:
- The ham frequency type
- Antenna gain and radiation patterns
- Fresnel zone and interference
- Feedline and ground losses
- Local and state regulations
These are the most important factors to keep in mind when choosing the right height for your amateur radio station.
So, let us look at each of these factors in more detail.
The Ham Frequency Type
Just like we mentioned in our definitive guide to the amateur radio hobby, there are three main ham radio bands which are:
- High-Frequency bands or HF
- Very High-Frequency bands or VHF
- Ultra-high Frequency bands or UHF
Each of these bands has a range of frequencies that one can operate on with each frequency requiring the optimal antenna height to achieve the highest possible DX (radio distance).
As a general rule of thumb, the antenna needs to be at least half the wavelength of the frequency you are operating on.
However, this is not always possible for the lower spectrum HF bands with wavelengths of up to 262 feet, which would mean having an antenna that is about 131 feet which may not be feasible even from a legal standpoint.
For such cases, the antenna should be made about a quarter of the ham frequency you are broadcasting on. In the case of the above example, that would be about 70 feet in the lowest frequencies amateur radio operators can broadcast on.
You can calculate the wavelength by using the following formula:
λ = c/f
Where is λ the wavelength, C is the speed of radiation (300,000,000 m/s) and is a constant, while f is the frequency of operation.
From the above formula, you deduce that increasing the transmission frequency will reduce the wavelength since they are inversely proportional. This will, in turn, reduce how high you need your antenna to be.
However, it is important to keep in mind that the antenna height will mostly affect the wavelength of a vertical antenna and not so much so for horizontal antennas.
Antenna Gain and Radiation Angle
Antenna gain is a measure of how efficiently an antenna can radiate ham radio waves in a certain direction in reference to an isotropic antenna. An isotropic antenna is a theoretical antenna that can radiate radio waves uniformly in all directions in free space.
Gain is, therefore, denoted as dBi where “dB” is the pressure level while the “i” represents the isotropic antenna.
Meanwhile, the radiation angle is the angle of the ham radio field in relation to the ground plane.
By tinkering around with your antenna’s height, you affect both the gain and the radiation angle from the ground.
With all factors being equal, increasing your antenna’s height by a factor of two is said to increase the gain by 6 dB. An increase of 3 dB doubles the power output and, therefore, a gain increase of 6 dB will increase the power by a factor of 4.
On the other hand, increasing the antenna’s height will lower the radiation angle of the lowest radiating lobe on a perfectly conducting ground plane. Depending on your frequency of operation, you may need a lower or higher radiation angle for proper propagation.
HF bands will need the highest radiation angle since they mostly rely on Ionospheric refraction for propagation. It is important to find the optimum angle where the HF waves will hit the ionosphere to be refracted back to earth where they will be reflected towards the ionosphere for refraction and so forth.
Very steep radiation angles will cause the radio waves to be lost into space while very low angles will mean that the waves may not hit the ionosphere at the right for refraction to occur.
To find the radiation angle in relation to antenna height for the lowest lobe, you can use the following formula:
θ = sin 1 (0.25/h)
Where θ is the angle of radiation and h is the height of the antenna above a ground plane.
Higher radiation angles are essential for long-distance transmissions such as HF transmissions while lower angles will be needed for short-distance communications but it important to keep in mind that not all ground planes are equal, and they will have different conductivity levels with hilly areas having the worst conduction and saltwater surfaces having the best conduction.
It is also important to strike a balance between height, radiation, and gain. You can also use software to determine the perfect height to maintain this balance depending on the terrain.
Fresnel Zone and Interference
Fresnel zone is more like the line of sight between a transmitting antenna and a receiving antenna where they are at most 40% obstruction or interference that can lead to loss of the radiating signal.
This zone can be represented using an ellipse.
You need your antenna to be above the Fresnel zone when broadcasting on VHF and UHF bands to reduce the amount of signal loss or interference that may occur in between due to absorption or reflection of the radio by different obstacles.
This will mean having the antenna above obstacles such as trees, buildings, and so on to increase the range of transmission and coverage.
Feedline and Ground Loss
Having an antenna that is too high from the ground will mean one thing and that is that you will need to have a long feedline usually a coaxial cable that can carry signals from your transmitter to your antenna.
However, as the signals are transmitted through the cable, they experience resistance due to cable impedance and in the process, some signal strength is lost in long lines.
You can get around this by using a cable with a lower gauge (thicker cables) that minimize that amount of resistance, but this also creates problems with flexibility and, therefore, you must balance between the gauge and the flexibility of the cable to keep feedline loss low for longer runs.
Losses can also be experienced if your antenna is too close to the ground which is a phenomenon known as ground loss.
Because of this, some of the signal power is lost to the electrically conductive ground and is converted into heat. Some of the signals may also be reflected by the earth causing losses if the waves are out of phase but the power of the signals may also be increased if waves are in phase.
The closer the antenna gets to the ground the more power is lost, reducing the efficiency of propagation.
Therefore, when choosing the right height for your antenna, ensure that there is enough clearance from the ground to the top of the antenna but do not make the antenna too high as this can have diminishing effects on transmission.
Local and State Regulation
Different regulations stipulate the maximum height your ham antenna can have which can differ from state to state and depending on local HOAs.
Most states may not allow you to have ham antennas that are over 45 feet tall from a building or above 70 feet from the ground. 30 feet is considered an average height for amateur radio antennas in most places while others only permit heights of up to 15 feet.
It is important to check your state laws to ensure that your antenna will be within the maximum permitted height.
HOAs on the other hand, may not even allow you to have an antenna that is visible to the public.
Most residential areas see amateur antennas as eyesores or are just afraid the antenna may cause interference to their TVs, radios, and other RF signals.
This may force you to place your antenna in the attic or chimney where you will be limited by how high your attic or chimney is. You may experience losses depending on the building material used but in other cases such as when the roof is made of metallic materials putting your antenna in the attic may not be a viable option.
It is important to have the right antenna height to not only have better coverage but better reception too.
This does not necessarily mean having the longest antenna possible as this can also have diminishing effects.
For the most part, increasing your antenna height may improve your coverage but this is not always the case.
Finding the right antenna height will also mean taking other factors such as terrain, ground conductivity, and type of antenna you intend to use for your ham ops into account.
I would also suggest installing a ham height software to calculate how high your antenna should be with all factors such as terrain considered.