SP3L's Antennas - SCV - Self-Contained Vertically Polarized Antennas

SCV - Self-Contained Vertically Polarized Antennas

The term self-contained vertically polarized wire antenna, or SCV in short, was coined by L.B. Cebik, W4RNL. The SCV does require neither a system of ground radials nor a system of elevated counterpoises to complete the antenna. We may say that the counterpoise is already included or embedded in the antenna itself. In this sense, this is a complete (self-contained) antenna. W4RNL examined quite a number of SCVs in a series of his articles (the links to these articles are provided at the end of this page). I wanted to demonstrate that except for the designs already analyzed by ingenious L.B. Cebik, there were other interesting CSVs you can build and use.

Warning - this page is very loooong!

But before starting the overview of the SCV antenna designs, I would like to draw your attention to the potential current imbalance that one can encounter when feeding an SCV. Figure below shows two antennas: the end-fed Zepp, and a C-pole. Both have vertical polarization and operate without connection to ground, so they are both SCVs. However, there is an important difference between them. The end-fed Zepp has its feed point located in the current maximum (Imax) spot, and the C-pole has its feed point off the spot.

If the feed point is not in the Imax spot, it may be difficult to reduce imbalance of the currents flowing in the antenna feed line. In other words, it may be difficult to get rid of the common-mode current flowing on the coax shield. Placing a 1:1 balun (a.k.a. a common-mode choke or a line isolator) between the feed point and coaxial cable will not be enough unless the asymmetry is small. In such cases, you may get: radio or TV interference and/or RF voltage in the shack during transmission and/or significant power loss in the baluns/chokes inserted in the coax connection. This collection of antenna designs is limited to the SCVs having their feed points located in the Imax spots. In such antennas, it is much easier to ensure that the currents in the antenna feed line are balanced.

Majority of the presented designs are brand new and have been simulated but not built and tested. There are too many of them for a single ham. I have constructed two antennas from this collection and was very satisfied with their performance. All the antennas presented in this article were modeled with lossless wires of 2 mm diameter. All of them were simulated in free space because such approach makes it easier to compare performance of different designs. The antennas were designed for 20 m band and resonated at 14.175 MHz. Of course, every design can be rescaled for another frequency.

Be aware that the presented designs are not ready-to-go instructions for constructing antennas. But gathering them in one webpage, allows you to compare different SCV designs and select the one that fits best to your particular conditions and expectations. The best way to go, after you choose your favorite SCV, is to simulate it again over the ground at the intended installation height and with wires of expected diameter and material. Finally, you will need to choose or design the matching network.

I divided the SCVs into three classes: SCSVs – self-contained slim verticals, SCDVs – self-contained delta verticals and SCRVs – self-contained rectangular verticals.

SCSVs - Self-Contained Slim Verticals

In order to compare different SCSV designs in a fair way, their dimensions were so adjusted that every SCSV fits into an imaginary cylinder of a fixed diameter 18.4 cm. Such value was chosen because the first SCSV that I actually built and tested had such dimension. 18.4 cm equals 0.0087 wavelength (less than 1% wavelength) at 14.175 kHz. There is nothing special in this dimension, and you may want to change it. But be cautious. A smaller diameter cylinder entails closer wire spacing. Very closely spaced wires may cause the NEC-2 based antenna simulators to become inaccurate, so your simulation results may not agree very well with the real antenna. Very close wire spacing may also create voltage breakdowns between the wires in the real antenna. Finally, when you bring wires closer to one another, you usually decrease feed point impedance and antenna bandwidth. So, you should not go too far in making SCSV slimmer.

SCSV-1: end-fed Zepp

This classic antenna consists of an open-wire line quarter-wave transformer (also called a stub) connected to the end of a half-wave dipole. So, the overall antenna height is ¾ wavelength. If designed for 14.175 MHz, the SCSV-1 overall height is 15.62 meters. Figure below shows the antenna shape and dimensions. Note: this and all following simulations were run with a 100 W signal source assumed, hence such values of the antenna currents in the legend.

The SCSV-1 resonance impedance is rather low (28 ohms) and its SWR ≤ 2 bandwidth is not very impressive and equals 360 kHz. However, its free space gain in XY plane is about 2.1 dBi what is virtually equal to the gain of a center-fed dipole.

The SCSV-1 resonance impedance can be increased (for example, to 50 ohm) by shifting the feed point a little higher in the quarter wave transformer (stub). Such a trick is used in the J-pole antenna, which is a slightly modified version of the end-fed Zepp. Unfortunately, this hardly improves the bandwidth. So, let’s examine the other possible antenna designs.

SCSV-2: folded Zepp

If you take an SCSV-1, add a copy of it and short-circuit the loose ends of those two antennas, you will create a new antenna. The process is similar to creating a folded dipole from a basic dipole. Let’s call this antenna SCSV-2. Changing SCSV-1 to SCSV-2 increases the SWR ≤ 2 bandwidth from 360 kHz to 420 kHz. The resonance impedance goes up to 63 ohms.

The antenna view and dimensions are presented below. Please mind that the wire spacing had to be decreased to 13 cm in order to fit into the 18.4 cm diameter cylinder.

The SCSV-2 radiation pattern is slightly skewed. But in the XY plane, the maximum to minimum gain ratio is so small that we can call it quasi-circular. This small asymmetry of the radiation pattern can be cured. See the next antenna.

SCSV-3: symmetrized folded Zepp

When creating SCSV-2, we used two identically oriented SCSV-1s. But what if a copy of the first SCSV-1 is rotated by 180 degrees around its Z-axis? Such an antenna is shown below. Let’s call it SCSV-3.

Note: the shorting link connecting 5 m long stub arms runs between the 15.44 m long radiators.

SCSV-3 has middle values of resonance impedance (46 ohms) and bandwidth (390 kHz) when compared with SCSV-1 and SCSV-2. But it excels in the radiation pattern shape. It is perfectly circular.

All three SCSVs described above are very tall. So, let’s check if we can fold this or that antenna element to shorten it.

SCSV-4: folded-stub Zepp

If we take the SCSV-1, fold its quarter-wave transformer (stub) in half and rotate it so that it embraces the main half-wavelength radiator, we will get the SCSV-4. The SCSV-4 design is shown below. It is only ½ wavelength tall and the length of its folded stub is only 1/8 wavelength. The antenna is shorter than the previous designs (by about 33%), maintains the same gain and has significantly greater bandwidth (640 kHz)!

This antenna has been described in the 2020/7 issue of QST, in the article “A Vertical End-Fed Dipole with a Folded Stub”. You can see its photograph below. The antenna performed very well despite some unexpected interaction between the radiator wire and the fiber glass pole material which was used as a supporting structure. You can read all about this in the article mentioned above.

Due to perspective distortion, its upper part seems to be shorter on the photograph than it actually is.

SCSV-5: partly-folded stub Zepp

The SCSV-5 differs from the SCSV-4 in that one of its stub legs is not folded in half but goes upward up to ¼ wavelength height, see figure below. This apparently small change increases antenna bandwidth to an awesome 855 kHz. If physically rescaled for 10 m band, the antenna will cover the whole band with SWR less than 2:1. The radiation pattern and gain are virtually equal to a center-fed dipole. Because one of the stub arms goes now up to ¼ wavelength, the antenna needs an extra support 5.1 m above the base. But the increased bandwidth might be worth such constructional complication.

SCSV-6: triangle-base partly-folded-stub Zepp

The main half-wavelength radiator in the SCSV-5 is not located in the imaginary cylinder axis. If you consider it to be an esthetic or constructional imperfection, you may prefer the SCSV-6. This is in fact the same antenna, but its base occupies an equilateral triangle, the main radiator is located just in the center of the triangle and the rest of wires in the vertices of the triangle. See figure below. Except for the relatively small decrease in bandwidth, the antenna performance is very similar to the SCSV-5.

SCSV-7: folded-stub Zepp with a bent end

If even ½ wavelength height is too much for you, there is a simple way to make it shorter. You can fold back the top end of the main radiator. If you do that with the SCSV-4 you will create a shorter antenna - SCSV-7. The longer the folded back wire portion, the shorter the antenna becomes, but the antenna bandwidth and its feed point impedance become smaller too. So, this is a trade-off. The model shown below was calculated so that its feed point impedance was decreased from 75 ohms to 50 ohms. The antenna height decreased from 10.3 m to 8.88 m (by 14%) and its bandwidth – from 640 kHz to 395 kHz (by 38%). The gain was only marginally affected by 0.1-0.2 dB.

SCSV-8: bent-end modification of a SCSV-6

Similar height reduction can be done to the SCSV-6. This time, the top wire is folded back in such a way that it almost touches one of the stub arms (10 cm air gap is left) as shown below. The folded back main radiator end and the stub arm end are on the same potential – so no risk of voltage breakdown between them. The antenna height is reduced from 10.32 m to 8.37 m (by 19%) and the bandwidth from 805 kHz to 515 kHz (by 36%). This antenna is better than the SCSV-7 because it is shorter and has greater bandwidth.

SCSV-9: Zepp folded in half

Instead of folding the stub, you can fold in half the whole antenna. If you do that to the SCSV-1, you will get the SCSV-9 shown below. The new antenna is somewhat taller than 50% of the SCSV-1, but nevertheless, the reduction in height is significant. The disadvantage of the antenna created in this way is its rather small bandwidth – only 210 kHz. The base of the antenna has the shape of an equilateral triangle with a 16 cm side.

SCSV-10: Zepp folded in half with a doubled wire

The previous antenna had rather small bandwidth. You can improve it by shunting the main radiator with a parallel wire. Two parallel wires behaves to some extent like a single thick wire, and it is a well known fact that antennas made from thick wires have greater bandwidth. Such a “thick wire” needs to be a little longer than the thin one in order to resonate at the same frequency. Using this approach, we can convert SCSV-9 to SCSV-10 presented below. Instead of a single 8.05 m long wire, it has two parallel wires 8.73 m long each (an increase by 8.4%). The bandwidth increases from 210 kHz to 450 kHz (by 114%). Due to the additional wire, the antenna base is no longer triangular but square.

SCSV-11: multifold loop

All the antennas described so far were taller than 8 m. So it is time now for something shorter. The SCSV-11 is only 7.23 m tall. And it has quite acceptable bandwidth - 415 kHz. As you can see below, the antenna is a loop of wire folded a few times. The SCSV-11 has a hexagonal base and because the antenna needs to fit into an 18.4 cm diameter cylinder, the side of the hexagon is equal to 9.2 cm. The antenna gain is slightly below a dipole (-0.15 .. -0.25 dBd) what is quite understandable because it is only 0.34 wavelength tall. When modeled in free space, its resonance impedance is 48 ohms.

SCSV-12: multifold loop with a drop wire

You can shorten the previous antenna if you add a drop wire between the longest radiators. A drop wire between the 6.44 m long radiators acts like a bent end in the SCSV-7 or SCSV-8. See the figure below. The SCSV-12 height is only 6.44 m, so no wonder that its gain is by 0.35 dB smaller than the gain of a dipole. The feed point impedance decreased, and the bandwidth also decreased to 295 kHz. But if you take into account the reduction of antenna height, you may still consider this design as interesting. Especially, if your transmitter is equipped with an antenna tuner. Having somewhat bigger SWR at the band edges might be an acceptable price for reduced height.

SCSV-13: vertical with a folded back counterpoise

This is probably the simplest SCSV and the one requiring the shortest length of wire. You can consider this antenna as an open-ended twin-line quarter wavelength transformer in which one leg has been folded back. The base of the antenna has the shape of an equilateral triangle with a 16 cm side. The antenna is shown below. Although very simple, this antenna has very small bandwidth and very small feed point impedance.

SCSV-14: vertical with two folded back counterpoises

If you take a ¼ wavelength GP with 2 horizontal counterpoises bend both counterpoises up and then fold them back down, you will create the SCSV-14 (figure below). The main radiator will have to be extended a little bit to 6.52 m. The antenna has wider bandwidth than the SCSV-13 but its feed point impedance 9.2 ohm is rather small, what makes matching more difficult.

SCSV-15: vertical with folded up counterpoise loop

Let’s take a ¼ wavelength GP with 2 horizontal counterpoises and fold back both counterpoises. The ends of the counterpoises are at the same RF potential, so let’s connect them together. We have now a vertical with something that I call a counterpoise loop. Let’s move the feed point from the bottom end of the vertical to the beginning of the counterpoise loop. An interesting observation is that moving the feed point virtually does not change distribution of the RF current in the antenna – see figure below. However, the feed point impedance is 4 times greater now. Such a trick would be impossible if we did not connect together the ends of the original counterpoises and did not create the counterpoise loop.

Having the antenna like in the figure above, you may go further and transform it into an SCSV. In order to do that, you need to twist the counterpoise loop by 180 degrees around the Y axis (what will create a wire crossing below the main radiator). Then, you fold the ends of the twisted loop up until the counterpoise wires become parallel to the main radiator. The final antenna view is shown below. Please note that there is an insulation gap between the apparently crossing wires at the bottom of the main radiator. The wires in dark blue color are below the wires in yellow color. And only the wires in yellow color connect to the bottom end of the main radiator. The antenna has almost identical parameters as the SCSV-14, but has 4 times greater feed point impedance.

I have built a real antenna based on the SCSV-15 design. You can see its photograph below.

Before building it, I repeated the simulation of this antenna, but this time not in free space but 1.5 m above the ground. This has increased its feed point impedance to around 50 ohms and increased SWR=2 bandwidth to 490 kHz. When I changed the main radiator thickness to something closer to the aluminum tubes I was going to use, I noticed that the accuracy of the model in the NEC-2 based simulators (4nec2 and EZNEC) became very poor. It is a well-known shortcoming of the NEC-2 engine that simulating parallel wires of different diameters positioned in proximity leads to significant inaccuracies of the results.

I also simulated the SCSV-15 in the MMANA-GAL which is based on the MININEC engine and the final antenna dimensions for 20 m band were significantly different from those achieved in NEC-2 based simulators. When I built the real antenna, it turned out that the MMANA-GAL predictions were correct. The final main radiator is 6.8 m tall and the ends of the counterpoise loop go up 2.55 m above the antenna base. The real antenna has SWR=2 bandwidth equal to 550 kHz when measured with a very short coax (2 m long). It is fed via 1:1 current balun. When I compared it to a commercial multiband GP, it performed slightly better. Usually, its signal was 1-2 dB better when received on the distant WebSDR receivers. The difference was not significant, but the main feature of all SCSVs is not their high gain but minimal foot print.

An article describing this antenna was published in 2020/11 issue of RadCom Magazine. For the purpose of the article, I called it "A vertical with upright folded back counterpoises". I know - the name is terrible, but a self-contained slim vertical does not sound good either...

SCSV-16: vertical with folded up counterpoise loop Mk. 2

A year or so after I had built the SCSV-15 a somewhat simpler version of this antenna occurred to me. In this version, the wires do not cross under the main radiator like in SCSV-15. That's why I think it is slightly simpler to construct this version. Its performance is essentially the same, as you can see in the drawing below.

The feed point is again located not at the bottom of the main radiator but in the beginning of the counterpoises loop. This time, however, the loop is not twisted.

SCDVs - Self-contained delta-shaped vertically polarized antennas

Antennas belonging to this group, as the name implies, are triangle shaped. I am omitting the classic deltas made of a 1 wavelength long loop of wire. It is hard to add anything interesting after W4RNL visited the topic years ago. Instead, I am focusing on the new antenna ideas discovered when working on slim verticals. I had initially rejected them as the antennas of not small enough footprint, but on a second thought, I came to a conclusion that they could still be an interesting option. Their footprint is not as small as the SCSV, but they are shorter, and that may be an advantage in some circumstances. So, let’s go.

SCDV-1: delta with overlapping wires - type 1

This antenna is a broken delta in which the loose-end wires overlap along the whole triangle base. Figure below shows the antenna shape, dimensions, basic parameters and current distribution. As you can see, the feed point is located in the current maximum spot.

Although the SCDV-1 is vertically polarized, its radiation pattern is rather oval than circular in XY plane. It has the highest gain broadside, along the X axis. Its narrowside gain is 1.7 dB weaker. I would not call this antenna bidirectional though, but rather not-so-perfectly omnidirectional.

The length of wire you need to construct this antenna is more than 1 wavelength (about 15% more). Generally, the antennas having their wires overlapped need some extra wire. Nevertheless, they are still more compact if compared to the regular closed loop deltas. SCDV-1 occupies significantly less footprint than a GP with two counterpoises designed for the same frequency, and is only slightly taller.

The antenna feed point impedance is a little inconvenient to match, but this will likely change a bit if you simulate it over the ground. Its 550 kHz SWR=2 absolute bandwidth is more than acceptable.

SCDV-2: delta with overlapping wires - type 2

If you shorten the triangle base and bend up the overlapping wires, you will get the SCDV-2 antenna as shown below. I adjusted the antenna dimensions so that the feed point is still in the maximum current spot, but its impedance is now close to 50 ohms. Please note that a short horizontal wire (20 cm) at the top of the antenna model was added there to avoid NEC-2 simulation accuracy problem that arises when wires join at an acute angle. In the real antenna implementation, both sides of the delta can meet in the same point at the top and create an acute angle.

A decrease in feed point impedance usually leads to a decrease in bandwidth. And it has also happened in this case. The absolute bandwidth has dropped from 550 kHz to 445 kHz. Nevertheless, it is still quite acceptable. Because the sides of the delta are now closer to one another, the radiation pattern has become quasi-circular rather than oval, with only 0.3 dB difference between the maximum and minimum gains in the XY plane. The short delta base makes it easier to fit the antenna in space restricted locations, and the relatively short distance between the overlapping wires (10 cm) simplifies the construction.

SCDV-3: delta formed from a twisted loop

If you look at the previous antenna, you will notice the both ends of the antenna wire are at the same RF potential because they are in fact the ends of the 1 wavelength long wire. So why not to join them together? The SCDV-3 was created just in this way. See the figure below. The dimensions of this antenna as well as its performance parameters are very similar to the previous one.

SCDV-4-75: delta formed from a twisted loop with a bottom drop wire - the 75 ohms version

Another modification to the antenna is shown in the figure below. This is SCDV-4-75 antenna. The center of the horizontal wire of the SCDV-3 has been pulled down and fixed between the bottom wires. Additional short wire (10 cm) has been added for trimming purposes, as you can see in the following figure. Thanks to that, you can fine-tune the antenna by changing the length of a wire at the bottom of the antenna. The term “75” in the antenna name means its feed point impedance is close to 75 ohms. Please note that the bandwidth has increased quite a bit. That’s because the spots of voltage maxima (current minima) are now at larger distance from one another.

SCDV-4-50: delta formed from a twisted loop with a bottom drop wire – the 50 ohms version

If you shorten the delta base even more – to just 1.4 m, you can reduce the antenna feed point impedance to 50 ohms. Such SCDV-4-50 antenna is shown below. Naturally, the SWR=2 bandwidth has suffered a little, but 435 kHz is not a bad result at all. The SCDV-4-50 radiation pattern is almost perfectly omnidirectional, with only 0.1 dB difference between the maximum and minimum gains in the XY plane. The 10 cm long trimming wire is still where it was in the 75 ohms version.

SCRVs - Self-contained rectangles vertically polarized antennas

Every self-contained delta described above can be turned into a self-contained rectangle (SCRV) by simple modification. The rectangles radiation patterns are less omnidirectional – they tend to be rather oval than circular, but the SCRVs are shorter than SCDVs. This may be advantageous in some applications. Figures below show the SCRV-1 through SCRV-4 antennas in the order corresponding to the order of the SCDVs. The SCRV-1 is the shortest one, but its bandwidth is not impressive. Fortunately, the SCRV-2 through SCRV-4 are more broad banded.

SCRV-1

SCRV-2

SCRV-3

SCRV-4-75

SCRV-4-50

Comparison of different SCV designs

Twenty-five SCV antenna designs are presented in this page. You can compare various parameters of all the designs in the table below.

Conclusion

The SCV antennas operate without connection to ground nor a ground plane created by a system of horizontal counterpoises. This can be an important advantage in the locations where it is impossible or impractical to bury radials in the ground or install long horizontal counterpoises. So, it seems that it was worth investigating different SCV designs and extending the designs already examined by L. B. Cebik.

Contemporary antenna simulators make it possible to design antennas with their feed points located in the current maximum spots. Although it adds extra effort to the antenna design process, the correct feed point placement reduces the risk of unwanted phenomena associated with common-mode current flowing on the coax shield. I heartily recommend doing that.