Sometimes you don't have an opportunity to choose an antenna. You can be either limited in finances or space, but the only antenna you may build is something small. Just imagine you live in a country house and have two 10-meters-tall trees with a distance of 15 meters between them, leaving you only 11-12 meters of free space for your antenna. You want to be present on 40 meters band, where the propagation does live the whole day. Since you don't want to build a ground plane, you have chosen to build the dipole. Good choice: the dipole is a pretty good and easy to build antenna requiring not so many materials. But you have realized you have no space for this antenna: diagrams tell you that the antenna must be 20 meters long. So sad...
But, don't panic! A shortened dipole is your choice. Let's try to design the shortened dipole that could fit the space between your trees!
As you know for the moment, actually you may shorten the antenna as much as you want. But don't forget: the loose of power is a penalty for doing this. The shorter the antenna, the more power you lose. But, again, don't panic! Let's start!
Shortening with coils is a good and quite simple way to get a tough antenna for lower bands. The trick is to find the place where these coils should be placed. But making assumptions is a pretty bad practice: you can lose too much effort and get no result. Therefore, we have to use the modeling software to find out these points.
I use the MMana-gal, the modeling software by Igor Goncharenko. You can read about this software on his website http://dl2kq.de/
In order to start, let's use the model I have created while writing my previous article. The model is provided below, just copy the entire text and put it into the MMana.
*
7.05
***Wires***
3
-3.1, 0.0, 0.0, 3.2, 0.0, 0.0, 0.004, -1
3.2, 0.0, 0.0, 5.6, 0.0, 0.0, 0.004, -1
-3.1, 0.0, 0.0, -5.6, 0.0, 0.0, 0.004, -1
***Source***
1, 0
w1c, 0.0, 1.0
***Load***
2, 1
w1b, 0, 16.0, 0.0, 0.0
w1e, 0, 16.0, 0.0, 0.0
***Segmentation***
800, 80, 2.0, 2
***G/H/M/R/AzEl/X***
2, 10.0, 4, 32.0, 120, 60, 0.0
As you can see, the overall length of the antenna is about 11 meters. To be precise: 11.2 meters. Meanwhile, the full-sized half-wave dipole is 20 meters long. Let's see, what have you got and what have you lose with the antenna as twice as shorten?
As stated in the previous article, the antenna is quite narrow. 11.2 meters long antenna is as wide as 90 kHz. Although this is not enough to cover the entire 40 meters band, you still can hope to be pretty confident on either CW or SSB pieces. If the main purpose is to use for FT8, you don't have to worry at all, just tune it to be resonant on 7074 kHz and you'll be satisfied for many years.
Now let's examine the design. The antenna consists of two shoulders divided by coils. The main view is shown below. The image shows the antenna itself and its currents. The maximal current is being spread alongside the piece of wire between coils.
Below is a radiation pattern of the antenna. Nothing special for the low-mounted antenna for 40 meters.
Coils are very simple: the single wire is wound around a piece of a plastic pipe. The antenna being examined here uses two 16 uH coils. Using the coil32 application we can easily know all the parameters of our future coils. Just imagine you have a plastic tube having 50 millimeters in diameter and some amount of the 1 mm thin wire. Short series of calculations showed me that 16 turns of this wire on 50 mm tube should give us 16.5 uH - pretty close to the inductance we are looking for. By the way, 28 turns of the same wire being wounded on the 30 mm tube will give you 15.9 uH. Thus, you have to check what you have and what should be better in your case. Also, don't forget about the insulating layer around the wire.
Now, everything is ready to be implemented. Cut a desired piece of pipe, wind a required number of turns, secure ends using holes in the pipe. Then use bolts, washers, and nuts to make two joints on each side of a coil. Then use any sort of protective material (hot-shrinking tube is a good choice) to protect the coil against the open air full of moisture. It's up to you, how to make the coil. But in case you have no idea about this, you can always drop a glance at pictures, you can find using your favorite search engine.
As soon as your coils are ready, you can start mounting the antenna between your trees. Please, keep in mind, that every change you can make against physical dimensions of the antenna or its placement will change its electrical characteristics. Below are some simple rules.
But, don't panic! A shortened dipole is your choice. Let's try to design the shortened dipole that could fit the space between your trees!
As you know for the moment, actually you may shorten the antenna as much as you want. But don't forget: the loose of power is a penalty for doing this. The shorter the antenna, the more power you lose. But, again, don't panic! Let's start!
Shortening with coils is a good and quite simple way to get a tough antenna for lower bands. The trick is to find the place where these coils should be placed. But making assumptions is a pretty bad practice: you can lose too much effort and get no result. Therefore, we have to use the modeling software to find out these points.
I use the MMana-gal, the modeling software by Igor Goncharenko. You can read about this software on his website http://dl2kq.de/
In order to start, let's use the model I have created while writing my previous article. The model is provided below, just copy the entire text and put it into the MMana.
*
7.05
***Wires***
3
-3.1, 0.0, 0.0, 3.2, 0.0, 0.0, 0.004, -1
3.2, 0.0, 0.0, 5.6, 0.0, 0.0, 0.004, -1
-3.1, 0.0, 0.0, -5.6, 0.0, 0.0, 0.004, -1
***Source***
1, 0
w1c, 0.0, 1.0
***Load***
2, 1
w1b, 0, 16.0, 0.0, 0.0
w1e, 0, 16.0, 0.0, 0.0
***Segmentation***
800, 80, 2.0, 2
***G/H/M/R/AzEl/X***
2, 10.0, 4, 32.0, 120, 60, 0.0
As you can see, the overall length of the antenna is about 11 meters. To be precise: 11.2 meters. Meanwhile, the full-sized half-wave dipole is 20 meters long. Let's see, what have you got and what have you lose with the antenna as twice as shorten?
As stated in the previous article, the antenna is quite narrow. 11.2 meters long antenna is as wide as 90 kHz. Although this is not enough to cover the entire 40 meters band, you still can hope to be pretty confident on either CW or SSB pieces. If the main purpose is to use for FT8, you don't have to worry at all, just tune it to be resonant on 7074 kHz and you'll be satisfied for many years.
Now let's examine the design. The antenna consists of two shoulders divided by coils. The main view is shown below. The image shows the antenna itself and its currents. The maximal current is being spread alongside the piece of wire between coils.
Below is a radiation pattern of the antenna. Nothing special for the low-mounted antenna for 40 meters.
Coils are very simple: the single wire is wound around a piece of a plastic pipe. The antenna being examined here uses two 16 uH coils. Using the coil32 application we can easily know all the parameters of our future coils. Just imagine you have a plastic tube having 50 millimeters in diameter and some amount of the 1 mm thin wire. Short series of calculations showed me that 16 turns of this wire on 50 mm tube should give us 16.5 uH - pretty close to the inductance we are looking for. By the way, 28 turns of the same wire being wounded on the 30 mm tube will give you 15.9 uH. Thus, you have to check what you have and what should be better in your case. Also, don't forget about the insulating layer around the wire.
Now, everything is ready to be implemented. Cut a desired piece of pipe, wind a required number of turns, secure ends using holes in the pipe. Then use bolts, washers, and nuts to make two joints on each side of a coil. Then use any sort of protective material (hot-shrinking tube is a good choice) to protect the coil against the open air full of moisture. It's up to you, how to make the coil. But in case you have no idea about this, you can always drop a glance at pictures, you can find using your favorite search engine.
As soon as your coils are ready, you can start mounting the antenna between your trees. Please, keep in mind, that every change you can make against physical dimensions of the antenna or its placement will change its electrical characteristics. Below are some simple rules.
- decreasing the coil inductance by 1 uH moves the resonance of the antenna by 150-170 kHz up.
- increasing the coil inductance by the same value moves the resonance down by the same 150-170 kHz.
- every 10 centimeters added between coils moves the resonance down by 10 kHz.
- cutting 10 centimeters from this part of the antenna causes the resonance to be moved up by 10 kHz.
- every 10 centimeters added to outer wires (beyond any coil) decreases the resonance by 60-70 kHz.
- cutting 10 centimeters from outer wire will lead the resonance to be increased by the same 60-70 kHz.
- increasing the height of the antenna above the ground increases the impedance; the active part is being affected faster than the reactive.
Also, it's good to use some kind of balun in the feed point as well as the RF choke to prevent the RF currents from being walked along the coaxial cable back to your shack. But this is out of the scope of this article, though...
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