The previous article caused some amount of questions in respect to the VHF/UHF antennas and how the height of the mast affects the radiation pattern. This is to answer these questions and bring light on this very interesting topic.
We all know that VHF antennas are vulnerable. There is a lot of things which might destroy the antenna - wires, trees, birds, even its own mast. Therefore we try to place the antenna as higher as possible to avoid the impact of the mentioned above objects. For the birds, we have the scarecrow. The practice tells us: the higher antenna is, the farthest QSO you have. This is the true and the false simultaneously. The devil is in details. Two antennas for the 433 MHz GP located on the 20 meters mast and on the top of 20 meters building are very different antennas, even if they are twins. This is because the overall height of the antenna is calculated relative to the nearest more or less flat conductive plane: the ground or the roof. The radiation pattern of the roof-mounted VHF GP is the same as of ground-mounted one.
Below is the set of pictures demonstrating the radiation pattern of the VHF antenna (quarter wave GP with sloped radials) having the same dimensions but placed on different heights - 1, 2, 3, 4, 5, 10 and 20 meters. I have chosen the 2 meters band to match the wavelength to whole integers. Thus, the mast height is 1/2, 1, 1.5, 2, 2.5, 4 and 10 lambdas respectively. In all positions, the antenna has the same SWR (1.4-1.5).
Figure 1. The mast is 1/2 lambda. The pattern is nice, but not ideal. The elevation is 13 degrees. No gain.
Figure 2. The mast is equal to lambda. The pattern changed more. The radiation pattern has two identical maximums: at 9 and 29 degrees. Gain is about 3 dBi.
Figure 3. The mast is 1.5 lambda. The radiation pattern is changed more and more. The elevation is 8 degrees. The gain increased a little.
Figure 4. Two lambdas. The pattern is awful. The elevation is 6 degrees. The gain raises.
Figure 5. The mast is 2.5 lambdas tall. The elevation continues to lower. Now it is 5 degrees. The gain is still raising.
Figure 6. The mast is 4 lambdas. The elevation angle is as low as 2.7 degrees. But the pattern is rather the hedgehog.
Figure 7. The 20 meters tall (10 lambdas) super mast. The pattern is awful, but the elevation is only 1.4 degrees. The gain is the best (7 dBi).
To sum up. Increasing the height of the mast increases the gain and lowers the elevation angle. Meanwhile, the radiation pattern becomes fragmented which can lead to even worse performance because of the lowered density of the radiation. After examining the pictures I could conclude: 2-2.5 lambda mast is the most effective for the VHF/UHF operations: the gain is good, the angle is pretty good as well as the density of radiation.
We all know that VHF antennas are vulnerable. There is a lot of things which might destroy the antenna - wires, trees, birds, even its own mast. Therefore we try to place the antenna as higher as possible to avoid the impact of the mentioned above objects. For the birds, we have the scarecrow. The practice tells us: the higher antenna is, the farthest QSO you have. This is the true and the false simultaneously. The devil is in details. Two antennas for the 433 MHz GP located on the 20 meters mast and on the top of 20 meters building are very different antennas, even if they are twins. This is because the overall height of the antenna is calculated relative to the nearest more or less flat conductive plane: the ground or the roof. The radiation pattern of the roof-mounted VHF GP is the same as of ground-mounted one.
Below is the set of pictures demonstrating the radiation pattern of the VHF antenna (quarter wave GP with sloped radials) having the same dimensions but placed on different heights - 1, 2, 3, 4, 5, 10 and 20 meters. I have chosen the 2 meters band to match the wavelength to whole integers. Thus, the mast height is 1/2, 1, 1.5, 2, 2.5, 4 and 10 lambdas respectively. In all positions, the antenna has the same SWR (1.4-1.5).
Figure 1. The mast is 1/2 lambda. The pattern is nice, but not ideal. The elevation is 13 degrees. No gain.
Figure 3. The mast is 1.5 lambda. The radiation pattern is changed more and more. The elevation is 8 degrees. The gain increased a little.
Figure 4. Two lambdas. The pattern is awful. The elevation is 6 degrees. The gain raises.
Figure 5. The mast is 2.5 lambdas tall. The elevation continues to lower. Now it is 5 degrees. The gain is still raising.
Figure 6. The mast is 4 lambdas. The elevation angle is as low as 2.7 degrees. But the pattern is rather the hedgehog.
Figure 7. The 20 meters tall (10 lambdas) super mast. The pattern is awful, but the elevation is only 1.4 degrees. The gain is the best (7 dBi).
Hi there!
ОтветитьУдалитьI can add, if I may, that 2 to 2.5λ height is excellent in Height Gain vs Coax Losses ratio!
The higher you install your antenna the more lossy coax you use!
Very good article OM! Congrats de sv3auw/m0lpt
Hello! Yes, the coax loss is the thing being ignored by a lot of people. But using some very good (and expensive) coax you can eliminate the loss at all.
УдалитьJUST TO SAY NICE ARTICLE AND T/U KF9F RICH
ОтветитьУдалитьYou have given us very useful information about VHF Radio and marine vhf radio reviews in general.
ОтветитьУдалить