The Center-fed Half-Wave Dipole Inverted V Antenna
The performance characteristics of an Inverted V antenna is not too different from a horizontal dipole. Therefore, I will not repeat what we have already discussed in the article titled The Ubiquitous Dipole Antenna. Readers are requested to read the article if they wish because practically all the concepts and features discussed in relation to a standard dipole are also applicable to the Inverted V. There is a general belief that a dipole antenna will typically produce a set of bi-directional radiation lobes in the azimuth plane, whereas, an Inverted V pattern will be nearly omnidirectional. This belief is certainly true for these antennas in either free-space or those deployed very high above the ground, but might not so much be, in the case of most of the typical and practical amateur radio antenna installations. This is one example of the many myths that prevail. We are going to bust some of them in this article and try to put matters in perspective.
The truth is, in the case of most real-world HF band amateur radio antenna deployments, especially at 20m or lower frequency bands, whatever performance difference one might have expected in comparison to a textbook style antenna installation, progressively starts to diminish on the lower HF bands or at low height above ground. As a consequence, both the standard dipole as well as the Inverted V generally begin to behave in a fairly similar manner. The distinction in their lobe patterns and most other characteristics including the gain begins to get blurred.
A Brief Synopsis of the Antenna Features & characteristics
In this article, I will try to cover various aspects of the half-wave resonant Inverted V dipole antenna including, its geometry, characteristics, performance parameters, the influence of practical surrounding environment where the antenna might be deployed, as well as transmission line interfacing. In addition to all the variables that are applicable to a dipole, the Inverted V has to also account for the Apex angle variation from one installation to another. This is the angle that eventually determines the slope of the Inverted V dipole elements with respect to the horizontal. Take a quick look at the summary below before we proceed further. The indicated Power Gain is TX mode gain that factors in the overall Radiation Efficiency of the antenna installation. RDF stands for Receive Directivity Factor and it characterizes the antenna receive performance. Radiation efficiency takes into account all structural losses and also ground reflection and absorption losses when applicable.
Going by the above summary, it is quite evident that a Half-wave Resonant Inverted V Dipole is potentially a good antenna that offers high radiation efficiency and also a substantial gain. Despite all these positives, most Inverted V installations that fail to deliver satisfactory performance are usually due to recklessly and thoughtlessly carried out installation and deployment.
We will try to find the reasons as to why the performance of a fairly good antenna like the Inverted V, so often, tends to get compromised and what could be done to mitigate these issues.
Half-Wave Resonant Inverted V Dipole Antenna Geometry
An Inverted V antenna has a fairly simple structural geometry. As noted before, it is just a variant of the standard horizontal dipole. The Invertd-V antenna is a center-fed dipole antenna with the feed-point that is at the highest point of the structure. From this central feed-point location, the two arms of the dipole slope downwards at an angle with the endpoints being closer to the ground level. The angle formed between the two sloping wire elements at the apex point is an antenna design parameter that is designated as the Apex Angle. One might note that a standard dipole is actually a special case where the Apex Angle is 180°.
Typically, most Inverted V structures are designed to have the Apex angle between 120°-90°. However, 150° as well as 75° angle versions may also be found in rarer instances. If the Apex Angle is lowered below 60°, the SWR begins to rise rapidly making the configuration unsuitable. The 90° variant produces a 1:1 SWR @50Ω which is not attained under any other Apex Angle condition, however, the trade-off is that there is a very slight reduction in the gain in comparison to a horizontal dipole. We will examine all these factors in further detail in the next section of this article.
Meanwhile, we need to know that although the technical specifications of a standard dipole in free-space (or very high above the ground) would indicate a marginally higher gain in comparison to an Inverted V under similar conditions, let us understand that despite the lower gain, an Inverted V might not be an inferior choice. Both the standard Dipole as well as the Inverted V are typically fixed and non-rotatable antennas, especially on HF bands under practical amateur radio deployment conditions. This is especially true for the HF radio bands with a wavelength of 20m or more.
If and when a Dipole is installed at a height of at least 1 λ or preferably even more above ground level, it might perhaps boast of a slight additional gain at the tip of the lobes on the broadside of the antenna, however, as we look in other directions, the gain begins to drop till there are notable nulls along the wire in the end-fire direction. On the other hand, though the broadside peak gain might be marginally less in the case of the Inverted V antenna, the drop in gain as we look around the azimuth is relatively much less. The end-fire direction nulls are obliterated to a considerable extent. However, this may not be entirely true when either a Dipole or an Inverted V for HF bands might be installed at far lower heights as found in the case of a large number of amateur radio installations. Under these circumstances, both these antennas behave quite similarly.
As a consequence of the above, an HF band fixed oriented Inverted V antenna is likely to offer a far more pleasant overall day-to-day performance in comparison to a standard dipole. Keep in mind that unlike a point-to-point fixed radio communication circuit where a dipole could be carefully oriented to leverage optimum performance, amateur radio requires one to listen and work other stations that might be located along any azimuth direction. Therefore, a non-rotatable antenna would definitely benefit from the relatively better (shallower null) azimuth pattern attributes of an Inverted V antenna.
The Inverted V antenna is a good antenna. Don't dismiss it as trivial... A carefully and thoughtfully installed Inverted V antenna can turn out to be a good station asset. Just as in the case of a standard dipole, an Inverted V antenna can be designed to be either a mono-bander or a multi-bander.
Typical Inverted V Antenna Characteristics & Performance
The basic trend of the characteristics of an Inverted V antenna is more-or-less in line with that of a typical standard dipole. Therefore, I will not dwell on the in-depth explanations of the specifications, parameters, and characteristics that are common. Please find more information in my article on dipole antennas titled The Ubiquitous Dipole Antenna. In this article, I will keep the narrative focused on the unique and special characteristics of the Inverted V antenna.
Most of the differences in characteristics arise due to the sloping elements of the Inverted V antenna. What are the effects of sloping of wire elements? Let us examine.
All the differences in characteristics are proportionate to the magnitude of the Apex Angle. The more it deviates from 180°, as in the standard dipole, the greater are the behavioral deviations. Of primary importance, there are four characteristic attributes that deviate with alteration in the Apex Angle. We already know one of them. It is the radiation lobe pattern that we have been discussing so far. There are three more. Let me list out all four of these deviations below before we begin to look into them one by one...
- The radiation pattern changes preventing deep nulls to allow better azimuth coverage.
- Marginal reduction in peak gain with the reduction in Apex Angle.
- Increase in antenna resonant frequency with lower Apex Angle thus requiring slightly longer element lengths
- Variation in feed-point impedance thus affecting lowest attainable SWR with change in Apex Angle.
Take a look at the table below that illustrates the typically expected variations in various performance characteristics with the alteration of the Inverted V antenna Apex Angle. Please note that in all the cases of Inverted V listed below, the antenna apex point height above ground has been maintained constant at 1/2λ. Only the slope of the elements change in each case with the change in Apex Angle...
Let us summarize what we see above. We have compared five different scenarios. This includes one horizontal dipole (180° apex angle), and four Inverted V configurations with different apex angles ranging from the nearly horizontal 150° to 60° at the other extreme. We examined and tabulated four performance parameters of the antenna in relation to the apex angle variations. All the antennas are assumed to be deployed under similar environmental conditions and installation heights.
Does Inverted V produce both Horizontal & Vertical Polarization?
Despite the fact that many radio station operators tend to believe it to be true, the categorical answer is a BIG NO!... A symmetrical Inverted V antenna that is a variant of a center-fed dipole with equal element lengths which are sloped downwards at equal and symmetrical angles from the apex point will always produce only horizontally polarized signal.
The exception would be if the inverted V is installed with asymmetric slope angles with respect to the horizontal. In other words, if the two sides of the dipole are drooped down w.r.t. the horizontal at different angles, then the polarization will become Oblique to an extent determined by the angular asymmetry. Similarly, another exception is an asymmetric wire antenna like the OCFD. In this case, despite the symmetric angle of slope on both sides, the antenna will produce Oblique polarization.
Here is a fun fact that many of us might be unaware of. There is hardly any amateur radio literature both on and off the internet web space that speaks about it. IF the slopes of the OCFD antenna that might have been installed with sloping elements is made asymmetrical so as to ensure that he end-points of the elements form a horizontal line, then the polarization will be horizontal.
I won't go into mathematical derivations to validate the above point at this juncture, however, here is the thumb rule... Draw an imaginary line between the end-points at both sides of the wire antenna. The orientation and the angle of an imaginary line passing through the two end-points of the wire antenna will determine its polarization. A geometric plane that would pass through the above-cited line and the propagation vector (direction of propagation) will fully determine the plane of polarization of the propagating wave.
BTW, there is nothing as mixed polarization. It is just a layman's way of describing the oblique angle polarization. In reality, linear polarization may manifest as either horizontal, vertical, or oblique. In the case of oblique polarization, for the purpose of mathematical analysis, the oblique vector may be split into a set of vertical and horizontal vectors. However, the split vectors are merely a mathematical notion.
Influence on Performance due to Deployment Environment
As we noted in the earlier parts of this article, it is true that a dipole and an inverted V in free-space would perform differently with significantly different all-round coverage. However, we, the radio amateurs in our day-to-day life would usually deal with these antennas under realistic deployment conditions. As a consequence, an inverted V antenna might behave as a very close cousin of a horizontal dipole when installed at low and medium heights above ground level.
Since a picture is said to be worth a thousand words, I will leave you with the following pictorial chart of the simulated radiation pattern of both the dipole as well as the inverted V installed at 35 feet AGL. The depicted radiation patterns display both the azimuth and the elevation planes sections of the 3D radiation pattern for 20m and 40m HF bands. Just see how similar both the antennas are. Check it out...