Recently I was asked about the shortened dipole. In a nutshell: the question was pretty simple - is this possible to create a small yet effective antenna for 80 meters band. Below is a result of my thoughts on this topic as well as some number of diagrams, which, as everybody knows, worse one thousand words.
Let's start from basics: the antenna is a wire conducting the RF current. As far as we are talking not about the conducting of the DC, the basic understanding of the RF transmission is a must, although this is out of the scope of this article.
In general, the frequency has its own wavelength. The higher frequency is, the shorter antenna matches it. Therefore on VHF (and above), you don't need a lot of space to build an effective antenna. But the situation gets worse when the frequency goes down. When you reach the 14 MHz, the size of the regular half-wave dipole become so significant, so that the simple task of raising it on the reasonable height might become unmanageable.
These circumstances make the shortening of antenna very reasonable. This sounds very optimistic to have the antenna as short as possible. But be sure, you will pay for this. Diagrams below display you what happened to the antenna when its electrical length becomes much bigger than the physical one.
We all know: an antenna has the impedance. It consists of two parts: active and reactive. Active part can be positive only, whereas the reactive impedance (or reactance) can be either positive or negative. When on some frequency the reactance is equal to zero, the antenna is resonant on this frequency. Increasing the antenna size increases the reactance and vice versa.
In case of the half-wave dipole having no additions, the physical dimensions are the same as electrical. Using formula λ = с/F (where c is the speed of light, F is the frequency) we can calculate the length of the wave for some frequency. For 7 MHz the wave is 42.86 meters. The half-wave dipole should be 21.44 meters long to be resonant on 7 MHz. These 21.44 meters is the physical and electrical length of this antenna. The question is: how to change the physical dimensions without changing the electrical length? The answer is very simple: the coil. Adding coil into the middle of the antenna makes it longer electrically whereas physical dimensions remain intact. In order to get the electrical length back to its previous value (to make it resonant again), we have to shorten the physical dimensions accordingly.
Let's look how the simple dipole looks on 7 MHz. The simple model tells us that placed on the 10 meters tall mast, the 20 meters long dipole is resonant on 7.088 MHz. The impedance on this frequency is about 77 Ohms. The antenna is quite wideband - 612 KHz by SWR 2:1.
But what will happen to the antenna if we add some coil? I added two 35 microhenry coils near to the end of each shoulder. The resonance swept to 15.6 MHz. Then I started decreasing the antenna to have it resonant again on 7.05 or so. After 10 minutes the new dimensions were found. The overall length of the antenna become as low as only 8 meters. The antenna was resonant again (on 7.088 MHz). Good work! But do you recall what I told you in the beginning? You have to pay for the shortening. Let's take a look at SWR plot.
Although the SWR is low (using transformer 2.5:1), the antenna becomes very narrow - only 22 kHz by SWR 2:1. The 7 MHz band is much wider than this 22 kHz strip so that the achieved result is unacceptable. In order to have the antenna covering at least the CW part of the 7 MHz band, I had to decrease coils to 15 uH and increase the length of the antenna to 11.2 meters. Now it is as wide as 89 kHz which makes contacts using it much more comfortable and confident.
Below are two images displaying how RF currents spread along the antenna. In the case of the full-sized antenna, we have the smooth distribution, whereas shortened antenna has very big current between coils and significant decreasing by the ends.
To sum up. A shortened antenna has advantages and disadvantages. The main positive aspect is a significant decreasing of the antenna dimensions. At the same time, the shortened antennas are much narrower than the full-sized. Also, any small change in the shortened antenna's neighborhood has a bigger effect. So that when you want to shrink your antenna a little, you have to decide yourself how wide should it be and how much the dimensions should change.
Let's start from basics: the antenna is a wire conducting the RF current. As far as we are talking not about the conducting of the DC, the basic understanding of the RF transmission is a must, although this is out of the scope of this article.
In general, the frequency has its own wavelength. The higher frequency is, the shorter antenna matches it. Therefore on VHF (and above), you don't need a lot of space to build an effective antenna. But the situation gets worse when the frequency goes down. When you reach the 14 MHz, the size of the regular half-wave dipole become so significant, so that the simple task of raising it on the reasonable height might become unmanageable.
These circumstances make the shortening of antenna very reasonable. This sounds very optimistic to have the antenna as short as possible. But be sure, you will pay for this. Diagrams below display you what happened to the antenna when its electrical length becomes much bigger than the physical one.
We all know: an antenna has the impedance. It consists of two parts: active and reactive. Active part can be positive only, whereas the reactive impedance (or reactance) can be either positive or negative. When on some frequency the reactance is equal to zero, the antenna is resonant on this frequency. Increasing the antenna size increases the reactance and vice versa.
In case of the half-wave dipole having no additions, the physical dimensions are the same as electrical. Using formula λ = с/F (where c is the speed of light, F is the frequency) we can calculate the length of the wave for some frequency. For 7 MHz the wave is 42.86 meters. The half-wave dipole should be 21.44 meters long to be resonant on 7 MHz. These 21.44 meters is the physical and electrical length of this antenna. The question is: how to change the physical dimensions without changing the electrical length? The answer is very simple: the coil. Adding coil into the middle of the antenna makes it longer electrically whereas physical dimensions remain intact. In order to get the electrical length back to its previous value (to make it resonant again), we have to shorten the physical dimensions accordingly.
Let's look how the simple dipole looks on 7 MHz. The simple model tells us that placed on the 10 meters tall mast, the 20 meters long dipole is resonant on 7.088 MHz. The impedance on this frequency is about 77 Ohms. The antenna is quite wideband - 612 KHz by SWR 2:1.
But what will happen to the antenna if we add some coil? I added two 35 microhenry coils near to the end of each shoulder. The resonance swept to 15.6 MHz. Then I started decreasing the antenna to have it resonant again on 7.05 or so. After 10 minutes the new dimensions were found. The overall length of the antenna become as low as only 8 meters. The antenna was resonant again (on 7.088 MHz). Good work! But do you recall what I told you in the beginning? You have to pay for the shortening. Let's take a look at SWR plot.
Below are two images displaying how RF currents spread along the antenna. In the case of the full-sized antenna, we have the smooth distribution, whereas shortened antenna has very big current between coils and significant decreasing by the ends.
To sum up. A shortened antenna has advantages and disadvantages. The main positive aspect is a significant decreasing of the antenna dimensions. At the same time, the shortened antennas are much narrower than the full-sized. Also, any small change in the shortened antenna's neighborhood has a bigger effect. So that when you want to shrink your antenna a little, you have to decide yourself how wide should it be and how much the dimensions should change.
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