The vertical-L-Antenna

The starting point of the considerations was the question how much radials a vertical antenna actually needs. The version of the vertical 90 ° dipole described here is also very popular as an “up-and-outer” for portable fans. It only needs one counterweight. This is the reason to take a closer look at this antenna and to develop a version that covers the bands 10-40 m.

Theoretical considerations

The advantage of a vertical antenna is, provided you set it up correctly and have a free environment, that it offers 360°-radiation with a relatively flat elevation angle. This can sometimes be better for DX than a horizontal dipole, whose elevation angle is larger and therefore more unfavorable, primarily due to the lack of installation height. The minimum for perfect all-round radiation is two radials, which are arranged opposite each other with a 180 ° distance. This version, developed as an "inverted T antenna", was described by me in [1]. But it is even easier if you accept that the all-round radiation is not perfect.

The vertical L-antenna is obtained by bending the classic half-wave dipole in the middle and leading one section vertically upwards and the other parallel to the ground. In the free space, this angular dipole has an impedance of 45 Ohm, the length must be increased by approx. 3% compared to the strechted normal form.

With the horizontal part near the ground, usually characterized as a "counterpoise", you come to the vertical L-antenna. As the simulation with EZNEC [2] shows, just one height is sufficient. This design is probably also called because one leg goes up and the other goes out, “upper-and-outer” or shortened “up-and-outer”.
If you build them strictly symmetrical, i.e. with the same lengths for the vertical and horizontal parts, an interesting effect occurs. The capacitive load occurring through the ground leads to an asymmetry of the two mechanically identical antenna sections and the impedance drops. As a result, the 50-Ohm-point on the counterpoise moves electrically further outwards.

Analyzes with EZNEC

If a symmetrical antenna has an impedance <50 Ohm, you can optimize for a higher radiation resistance by arranging the feed point off-center. This is known from the Windom-antenna FD-4, in which the feed takes place at a point of approximately 300 Ohm and thus the antenna is divided into two sections of different lengths.

Here the vertical section is made longer and the horizontal section is shortened accordingly. This results in a ratio of approx. 55% to 45% for 20 m in order to reach the required 50 Ohm. This also results in a somewhat flatter radiation angle. The diagram of the antenna with the current distribution can be seen in Figure 1. Depending on the wire and insulation, the length of a normal half-wave dipole should be approx. 0.92-0.97 * lambda / 2. With the vertical L-dipole, it is almost exactly lambda / 2, i.e. significantly longer, which has to be considered when comparing. For the different frequencies, the ground effect (based on lambda) is correspondingly different, which necessitates new factors for each band and makes the adjustment not very easy. Especially at 30 m there were other conditions, which surprised me.

An alternative option is to mount the counterwpoise falling a little closer to the ground. Due to the larger angle, the impedance can be increased within certain limits. Systematic experimentation is important here, because wires simulated close to earth are problematic in NEC-II with regard to the calculated impedance. The dimensions given here were roughly determined with EZNEC, the fine adjustment was carried out experimentally.

Such antennas, which are fed off-center are called "OFC" antennas. Since they tend to common waves (like the classic Windom antenna too), they must be connected via a current balun.

All diagrams for 14 MHz

The azimuth diagram (i.e. viewed from above)is not circular, there is an attenuation of 5 dB on the side opposite to the horizontal wire. If you have a preferred direction in mind, the counterpoise should be aligned in this. The "gain" in this area is -1.5 dBd. As with other vertical antennas, this appears to be very little. This is compensated for by the flat elevation angle. In comparison, a horizontal half-wave dipole at a low installation height produces a lot of useless high-angle radiation, which is counterproductive, especially on the lower frequency bands.

The elevation diagram is therefore not symmetrical. Despite the low overall height, the radiation diagram is quite DX-compatible.

The up-and-outer can also be fed with a two-wire line. In this case, both sections should be 7 m long. This enables operation of 10-40 m, but only with an antenna tuner.

Practical structure

If you want to work from 10-30 m, a 9 m GRP mast (no carbon fiber material!) Is sufficient as a support for the vertical wire. If 40 m should also be used, a construction height of 12.5 m would be required. By inserting an inductor in the vertical section, the 9 m mast can also be sufficient for the 40 m band. Then only the counterpoise is extended further. Since, as expected, a lot had to be cut during the adjustment, I used inexpensive 1.5 mm² electric wire with PVC insulation. In fact, I used up a complete 100 m roll. Since this is easily available, the replica can use the specified lengths directly. With other wires, there are different dimensions that require readjustment!

In advance, I made tests with monoband antennas and then used the dimensions determined there for the plug-in version. For operation on several bands, the length of the two sections must therefore be changed when changing the frequency. The simplest solution, which however always requires the mast to be repositioned, is a plug-in system for the wires. I use connectors and couplings for automotive wire connections. Depending on the band, the connections are closed or interrupted as bridges.

For this I had assembled 50 mm long epoxy sections, through which two holes were drilled on each side for strain relief. The wire is pulled through as a loop. Surprisingly, the resonances, especially on the higher bands, were significantly below the previously determined lengths and the respective SWV minimum was also significantly worse than before. Obviously, the free hanging ends play a serious role and have a negative impact on the properties of the used band. Then I made the insulating pieces much longer and enlarged them to 120x20 mm.The picture shows this method with the “jumpers” used.

I then used a 2.5 mm thick carving board made of PE as the material, which can be cut with sturdy scissors. When opening the bridges, the ends of the wires must be bent as far apart as possible to prevent the undesired coupling mentioned. Now the result looked much better, even if the lengths are still slightly shorter than with the pure mono-band construction.

Correction Lengths/100KHz

Adjustment and operation

Basically, when using other wires, the ends should first be left slightly longer than indicated. In Table 2, I gave the approximate correction values per 100 KHz. By this amount, both ends must be corrected if the frequency is wrong. This should make the adjustment of the entire antenna no difficulties. An exception is the 40 m band, in which the adjustment is made exclusively on the horizontal section. For this reason, the double value must be set!

First, you connect the individual sections with luster terminals, so you can easily make length changes. Start from 10 m and then shorten the ends again slightly, when the metal sleeves of the couplings and connectors are soldered. Then you go to the 12-m band and proceed accordingly with all other bands. So you can put the best fit in the preferred areas. Up to 10 m, where I have selected 28.0-29 MHz, a balance is just below the mid band. The measured SWR gradients on the different bands are documented in Figures . Feeder cable was 10 m H-155.

With a ground socket the GRP mast can be set up, for the second leg one uses a tree or fence. If necessary, there are also other attachment points. If necessary, a short auxiliary mast is used for bracing. It is important that this half of the antenna, as shown in Fig. 17, is parallel to the ground at a height of 1 m and does not descend obliquely, because this results in shifting the resonance again. To change the band, the mast is folded over and the corresponding connectors are opened or closed. This is easier with the horizontal part.

However, one thing has to be pointed out: As with all antennas that are set up near the ground (the "counterpoise" is only 1 m high!), The impedance and resonance strongly dependent on the ground conductivities. On dry sandy soil can be compared to wet meadow floor quite different. That's the price for the simple construction. On the other hand, a correction within certain limits by height change or ground angle of the horizontal portion is possible. It should be noted that with a smaller distance, the impedance is shifted upwards, with a larger downwards. The resonance is exactly the opposite.