Understanding, designing, constructing and using radio antennas

# 16 Jul 07 Modelling a 40m Dipole at Various Heights

Last time we looked at the performance of a 40m dipole at 7.5 m or 3/16 wavelengths high. We discovered that its DX potential was limited but the high angle radiation made it useful for NVIS service. Now we will look at the change of performance as we increase the height of the antenna up to a maximum of 1 wavelength. At 40m obtaining a support structure of greater height than this is unlikely to be practical for most amateurs.

First we will look at the effect on impedance as the height incleases.

At 7.5m (3/16 wavelength) height band centre impedance is 62.8 ohms.

SWR of 40m dipole 7.5m high

At 10m (1/4 wavelength) height band centre impedance is 81.2 ohms.

SWR of 40m dipole 10m high

At 15m (3/8 wavelength) height band centre impedance is 92.1 ohms.

SWR of 40m dipole 15m high

At 20m (1/2 wavelength) height band centre impedance is 71.8 ohms.

SWR of 40m dipole 20m high

At 30m (3/4 wavelength) height band centre impedance is 70.0 ohms.

SWR of 40m dipole 30m high

At 40m  (1 wavelength) height band centre impedance is 75.2 ohms.

SWR of 40m dipole 40m high

Clearly, for a dipole it would be worth considering using a 75 ohm coax cable feeder and a transformer or tuner at the shack end, but a match to 50 ohms would be within the range of most transmitter output circuits, even at the band edges.

Now we’ll look at the radiation patterns at these heights.

First the elevation patterns:

Elevation plots for a 40m dipole

As we saw earlier, at 7.5m height the signal is largely radiated vertically, but as the antenna height increases the angle decreases towards the horizon. It is easy to see why for good DX performance a height of at least 1/2 wavelength is recommended.  The gain figures are as follows:

Now for the azimuth plots:

Azimuth plots for a 40m dipole

The following table gives the figures for takeoff angle, gain, fron to side ratio and beamwidth for each height. For the lower heights the figures for 45 degrees are also given to allow a comparison of performance to be more easily made.

Takeoff angle, gain.front to side ratio and beamwidth for a 40m dipole at various heights

Again, a height of 1/2 wavelength is seen to be a reasonable compromise between performance and the size of mast or tower needed.

# 25 Jun 07 Modeling a 40m Dipole at Low Height

Varying the height of an antenna above ground affects many of its characteristics, among which are its feed impedance, its gain, the angle above the horizon at which maximum gain occurs, and the length of the elements for resonance. We will model a 40m dipole and investigate its performance at 7.5m, 10m, 15m. 20m, 30m and 40m above ground. These heights embrace the practical limitations placed on most amateur radio antenna installations. They represent a range of from 3/16 to 1 wavelength height.The elements for our dipole are as follows:

Wires for a 40m dipole at 7.5m height

For resonance at 7.15MHz we need a 2mm diameter wire to be 20.11m long. We will feed it at the centre. Now we find the SWR of the antenna:

SWR for a 40m dipole at 7.5m height

The impedance at 7.15MHz is 62.8 ohms, rising to 57.7 – j34.5 ohms at 7.0MHz and 68.3 + 35.1 ohms at 7.3MHz. This results in an SWR below 1.94:1 across the band for a 50 ohm feed with 1.26:1 at band centre. The match to 75 ohms is much better, being below 1.78:1 across the band and 1.19:1 at the centre.

The radiation pattern is interesting. In elevation we have the following:

Elevation plot for a 40m dipole at 7.5m height

The outer circle indicates a gain of 7.24 dBi. At 45 degree elevation the gain is down to 5.56dBi, and it reaches 3.0 dBi at 29 degrees. Clearly, this antenna is not going to be very efficient at sending a signal to the horizon. However, it would make a good cloud warmer or Near Vertical Incidence System (NVIS) antenna.

The azimuthal pattern at 29 degrees elevation is:

Azimuth plot for 40m dipole 7.5m high at 29 degrees

Here is the plot in 3D:

3D radiation pattern of a 40m dipole at 7.5m height

The pattern is not as omnidirectional as is commonly assumed, with 3.02 dBi gain perpendicular to the dipole and -4.34 dBi off the ends. That’s a difference of 7.36 dB front/side ratio! The beamwidth is 90.0 degrees.

At 45 degrees elevation the pattern is similar, but the gains range from 5.56 dBi to 1.48 dBi, a front/side ratio of 4.08 dBi.

So, an antenna at this low height of 3/16 wavelength is quite directional and will need careful alignment. It also aims most of its signal high in the air, rather than towards the horizon where it would do a DXer most good. It will probably work well enough for short range contacts in directions perpendicular to the wire.

Next we will investigate the performance of this antenna at greater heights.

# 31 Mar 07 New features in EZNEC Plus v5.0 Upgrade

I’ve just upgraded from EZNEC+ v4.0 to EZNEC+ v5.0. The new version offers a number of new features. To quote from W7EL Roy Lewallen’s email:

“For the first time, you can get a program which analyzes a whole antenna system, from the antenna clear back to the transmitter! EZNEC v. 5.0 allows you to directly model L networks, transformers, and transmission line loss, so you can include tuners, matching networks, phasing networks, and realistic transmission lines in your model. L networks can be cascaded to make pi, tee, and other network types, and can include frequency-dependent loss resistance to mimic real coils. But that’s not all. Have you ever tried to put a load in parallel with a source or transmission line? Until now it was tricky at best, but EZNEC v. 5.0 has new parallel connected loads. Tired of making little wires to interconnect sources and transmission lines? EZNEC v. 5.0 has virtual segments to relieve you of that task.”

Hear is a list of the new features:

• New modeling objects – L networks, transformers, parallel connected loads.
• “Virtual segments” – Makes it easy to interconnect objects. It’s no longer necessary to create small wires.
• Transmission line loss – Changes realistically with frequency.
• Geometry scaling – Easily scale any number of wires, optionally including diameter and insulation thickness.
• New 2D plot grid style – Larger plots when a ground is present, and easier to interpret display.
• Smith chart display – Shows SWR sweep output on a Smith chart.
• Additional impedance displays – Shows SWR sweep output as return loss or reflection coefficient magnitude.
• Writes IONCAP/VOACAP files – Writes type 13 input files for these popular propagation programs.
• Advanced wire features – To make translated and/or rotated copies or reflections of a group of wires, or make a cylindrical structure.
• 10,000 frequency sweep steps – For detailed analysis over a wide bandwidth.

I’m looking forward to trying out the new version. One thing I noticed immediately is that the antenna radiation pattern plots are much clearer.

If you want to check it out try the free demo at http://eznec.com/demoinfo.htm, or get the full version from http://eznec.com/ordering.htm.

# 22 Mar 07 Modelling an Antenna with EZNEC

EZNEC is a powerful, low-cost, antenna modelling program for Windows developed Roy Lewallyn – W7EL. It is based on the powerful NEC-2 (which stands for Numerical Electromagnetics Code) software for modelling antennas and other electromagnetic structures using the Method of Moments. NEC was developed at Lawrence Livermore Laboratories.

EZNEC is easy to use, and is particularly effective for investigating the characteristics and performance of a design before construction, and for especially for trying out new ideas. The version I am using here is EZNEC+ version 4.0. The models will also work fine in EZNEC version 3.0 which I have used for many years.

The operations centre for EZNEC is the following window:

Main EZNEC Window

From here you can carry out everything needed for a simulation. The model used here as an example is for a simple 20m 3 element yagi.

The first step is to set the frequency you want the similation to run at by clicking on ‘Frequency’. It has been set here for 14.175 MHz, the centre of the 20m band.

The next step is to insert descriptions of the elements. This is done in the Wires window, accessed by clicking on ‘Wires’ on the main window. Here is the data for the yagi:

Wires for a 20m 3 element yagi

Click on the above image for a larger, more readable version in another window. We will look at the significance of some of the numbers in this table later.

Once you have the antenna data entered you can view the antenna by clicking ‘View Ant’. You will see something like this:

View of the 20m 3 element yagi

Here you can see the ‘wire’ elements (tubes in reality, but modelled as thick wires here), and the current in each wire with its phase. Depending on the ‘View’ settings you might see other information, such as the wire numbers:

Antenna view with wire numbers

The antenna will need feeding with power from a feedline, and this can be simulated by placing a source in the centre of the driven element, using the ‘Sources’ window:

Feed source for the antenna

Once you are satisfied that the data has been entered correctly and the antenna looks something like it should, you can begin checking its performance. For example, one of the things of great interest will be its match to the impedance of the feedline across the frequency range to be used. For this you can click on SWR. A window will come up allowing you to enter the lower and upper frequency of the band of interest, and the step size across the band. Then when you run the simulation you will obtain something like:

SWR of the antenna

You can see here that this antenna presents a pretty fair match to 50 ohms right across the 20m band.

The other features of interest are the directivity charactersitics of the antenna, such as gain, front-to-back ratio, and beam width. These can be seen by running an ‘FF Plot” (Far Field Plot).  With the ‘Ground Type’ set to ‘Real/High Accuracy’ and the ‘Plot Type’ set to ‘Elevation’ the following plot is obtained:

Elevation plot of the antenna

You can see here that maximum gain is obtained with an elevation angle of 27 degrees. So, with  the Plot Type’ set to ‘Azimuth’ and the elevation angle set to ‘27 Degrees’ we obtain:

Azimuth plot of the antenna

The gain turns out to be 11.05 dBi at an azimuth angle of 27 degrees. Front/Back Ratio is 19.79 dB and Beam Width is 72 degrees. This is quite a respectible performance for such an antenna.

It is possible to get a 3D plot to help us visualise the antenna’s performance:

3D plot for the antenna

Once we have tested the antenna we can begin to experiment. For example, we can easily change its height and check the effect on gain and radiation angle. Or we can vary the element lengths and spacings to adjust gain, front/back ratio, srw and swr bandwidth. We can put in alternative source impedances to see if this gives us a better match. We could even add another element to see what effect this has, and so on.

And all of this is possible, to a surprising degree of reliability, without once ruining a length of aluminium tube or risking the tower falling onto the neighbour’s house.

In future posts we will explore further aspects of simulating antennas, investigate some of the standard antenna designs, and then look at producing some new ideas of our own.