##### An HF Skeleton Slot Antenna

It is an array of two tightly coupled loops with a bi-directional pattern. Both loops are linked to share a single feed point. This antenna can operate in the 14 to 28 MHz bands with the appropriate impedance matching. It is self-resonant when the perimeter of each loop is around one wavelength. In this example, each loop is 3 x 4.5 m and the resonant frequency is 19.8 MHz.

##### Multiband J-Pole Antenna

This design is for 5 bands on a single pole. Each “J” corresponds to a band and therefore the feed point changes depending on the frequency. The resonance frequency is indicated in each case, which can be changed by moving the feed point. The height is about 5 m.

##### 145 MHz 5-Element Array of Square Loops

Here is a relatively compact array of 5 square loops for 145 MHz. It does not need a matching network since the input impedance is practically 50 Ohm. Gain 12 dBi. Beamwidth 50 deg. F/B 20 dB.

##### 17 m Band Spider Delta Loop

These are compact, lightweight antennas that can be used for DX applications. This 2-element array is the simplest we can build to get a directional antenna using delta loops. It is practically resonant with 50 Ohm of input impedance near the band center. This is an example where we need to enable the Exact Kernel option in AN-SOF since we have sharp angles between wires.

##### Moxon-Yagi Dual Band VHF/UHF Satellite Antenna

This model is self-resonant (50 Ohm) in the analyzed frequency range, so it does not need a matching network. Adjust the indicated gaps to minimize the VSWR.

##### Script for modeling 2-Element Quad Array

2-element quad antennas are very popular due to their compact size and gain similar to a more element Yagi. In addition, they can be designed to obtain an input impedance of 50 Ohm.

This design can operate at 27.5 MHz. We have added a script that allows us to plot the gain and front-to-back ratio as a function of element spacing. See this video >.

We can see that both cannot be maximized at the same time, but it is preferable to choose the maximum F/B since the gain changes relatively little.

To create the Scilab script, we started from a basic design in AN-SOF and then exported it as a *.sce file.

##### An 80 meters DX antenna: A spiral loop

A half-wave dipole would have a length of 40 meters in this band (3.75 MHz), difficult to install at home due to lack of space. Not to mention the complaints from our neighbors.

A spiral loop is attractive for its small size and relative ease of tuning because it is basically an inductor to which a variable capacitor is connected at the feed point to achieve resonance. However, the radiation resistance is extremely small, on the order of milliohms and therefore the efficiency is very low.

Unfortunately any small loss severely affects the antenna efficiency, such as losses in the capacitor, wires, interconnections, solder joints, surrounding objects, ground plane, to name just a few. In fact, this antenna can be tuned and get a wide bandwidth thanks to all the losses. The maximum radiation occurs upwards when the antenna is installed vertical to the ground plane, so some have suggested installing it horizontally. We should emphasize, however, that it is a popular design due to its ease of installation and small size, but we must be careful because high voltages can be expected, especially in the tuning capacitor.

This AN-SOF model consists of a frame of 50 cm on each side (0.00625 of the wavelength) with 7 turns of wire. This is another example where we need to simulate with very short wire segments, very close to each other and bent at right angles.

In these results we can see the effect on the input resistance of adding losses in the ground plane and surrounding objects, such as a wall,

Perfect ground: **4 milliohm**

“Cities industrial poor” ground: **1.3 Ohm**

“Cities industrial poor” ground + wall: **49 Ohm**

This design can be easily transformed into a multi-band antenna by shorting turns of wire, much like a variable inductor, to make it operate on the 40, 30 and 20 meter bands.

##### ISM 433MHz Helical Antenna

Small helical antennas for the 433 MHz ISM band are a good example where we need advanced software that has the ability to model:

* **curved wires** with exact description of the helix contour

* **very close wires**, spaced a fraction of the wire radius

* **horizontal wires** almost touching the ground plane

* **bent wires** at right angles or less

* **short segments**, below 0.001 wavelength

* **thick wires** by means of Exact Kernel instead of thin-wire approximation

AN-SOF includes all these features.

In this example, a perfect ground plane is used to get the input impedance: 48 Ohm, very close to the 50 Ohm of the antenna datasheet. We must subtract 3 dBi from the gain to obtain the gain in free space. It is a simple model, but it coincides very well with reality.

##### Array of Snowflake Quads

Nathan Cohen in the US fractalized the quad loop based on the Minkowski square and invented an array of two elements. I think the biggest advantage of fractal antennas is that we get a wide bandwidth with a small size.

This simulation shows that we can almost double the bandwidth with a 3-element array. It has a reflector, a driven element, and a director. These are the results for the 20 m band (~ 14 MHz),

Quad size ~ 290 x 290 cm (0.14 x 0.14 of a wavelength)

Element spacing ~ 280 cm

450 KHz bandwidth (VSWR < 2) around 14.5 MHz

6 dBi gain

10 dB F/B

80° beamwidth

Vertical polarization

It has less gain than could be achieved with a 3-element Yagi, but it has a relatively large bandwidth and is a very compact antenna. It does not need a matching circuit and its impedance is 50 Ohm at resonance, so it could be fed directly through a 50 Ohm coax.