VHF/UHF & Free Space Propagation – A primer
In the case of VHF/UHF communication that is nowadays commonly trending amongst radio amateurs, quite often most of these modifier effects rarely come into play. The reason being that over the last 30 years or so, a large section of radio amateurs conduct VHF/UHF communication either on short-distance local point-to-point circuits or use repeaters which are at times trunked using hybrid methods like the internet based Echolink or similar platforms. Moreover, the general trend is to use FM radios. This makes propagation on these bands rather mundane and elementary and one is deprived of experiencing the finer nuances that could show up while pursuing DX on VHF and UHF.
VHF/UHF terrestrial radio DXing which at one time was quite prevalent is now perhaps a dying art. The modes like SSB, CW, Amateur Television (ATV) are not so common anymore. Having said that, I would like to state that VHF/UHF DXing is a very exciting aspect of amateur radio communication. When we speak of DXing, we certainly do not mean the long distances that are possible to cover on HF, thanks to the lack of ionospheric propagation on the VHF/UHF bands but communication ranges spanning over several hundred kilometers are often possible.
Basic VHF/UHF propagation methods in general use
In the case of the usual urban and suburban radio communication on these bands using either handheld transceiver (HT) or base station rigs with FM modulation, a large amount of traffic is routed via repeaters that are set up at strategic locations. The rest of the traffic is simplex using either fix station or mobile radio equipment.
Under such scenarios, other than Line-of-Sight (LOS) direct wave propagation, a few other physical effects also play up to either enhance or more often adversely affect communication. These are typically the presence of buildings in the city as well as the nearby geographic entities like hills and valleys. For instance, a cluster of buildings, especially the modern structures made of steel and concrete quite often adversely affect VHF/UHF communication coverage, They absorb RF energy and act as nearly opaque barriers to the line of propagation. However, on a few occasions, these structures may also act as reflectors and enhance signals in certain favored directions.
More importantly, perhaps there is another phenomenon that always comes into play in all forms of terrestrial radio communication, yet not duly appreciated by most people, is the effect of Fresnel Zone. In fact, the first Fresnel Zone clearance is vital to establish a point-to-point LOS contact. The Fresnel Zone is an ellipsoidal barrel-shaped region between the antennas at point A and point B of a radio circuit.
To get a greater insight into the concepts of Fresnel Zone, check out this article on Ground wave propagation where we explain it in more detail.
There are multiple Fresnel Zone regions but it is important that the First Fresnel Zone in its entirety must be clear of any obstructions within its barrel-shaped region, or else, communication breakdown might occur. It is not only important for the direct ray path between points to be clear but the entire first Fresnel Zone region must have no obstructions. VHF/UHF and microwave terrestrial communication circuits must take this factor into account to establish reliable communication. Hence, the height of the antennas above ground becomes a very important factor. This is primarily the reason why operators on the ground with hand-held TXRs rarely achieve long-range communication capabilities… More on this important aspect later in a separate article.
The net effect of various factors on urban terrestrial VHF/UHF communication is a mixed bag. Signal ducting along streets surrounded by rows of buildings may often enhance RF signal strength along the street. Corners of buildings may cause knife-edge diffraction to allow signal illumination around a bend. Multi-path propagation on account of reflection from the various structures may often lead to signal fading. This is quite predominant in the case of vehicle-mounted mobile communication that could result in a rapid change in signal strength resulting in fading and flutter.
Terrestrial DXing on the VHF/UHF bands
DXing on VHF/UHF bands is an interesting activity but unfortunately, there aren’t too many DX operators on these bands. Usually, CW and SSB with either voice modulation or narrow-band digital modes are used to establish DX contacts. Unlike the more popular FM mode (either directly or through repeaters) that typically use vertical polarization for communication, the DX operations on VHF/UHF bands are typically conducted with horizontally polarized antennas. Although the polarization compatibility of antennas is inconsequential for HF band DX, it is vital to have identical antenna polarization for terrestrial VHF/UHF DX circuits. Incompatible polarization can practically lead to 20-30dB additional circuit loss and hence ruin the possibility of establishing DX contacts.
Usually, we find that DXers on these bands use high gain horizontally polarized antennas and stacked arrays. It is quite common to find serious VHF/UHF DX operators using large rotatable Yagi, Quagi, Cubical Quad, or stacked arrays deployed at good height above ground on towers. They invariably use low loss transmission lines and often have mast-head pre-amplifiers at the antenna for receiving purpose. With a setup of this kind, the real fun begins.
It is not only that DX operators leverage the antenna height above ground to reach out further beyond the horizon, but they also use other naturally occurring phenomena to greatly enhance DX coverage. As and when these natural physical phenomena occur on account of atmospheric processes, long-range VHF/UHF terrestrial communication possibilities present exciting prospects.
The most prominent atmospheric phenomena that play up from time to time are tropospheric super-refraction, tropospheric ducting, troposcatter, etc. A couple of ionospheric effects also often manifest themselves to enhance the VHF/UHF communication range. These include the Sporadic-E, meteor scatter, etc, and a few other geographical region-specific phenomena like trans-equatorial propagation due to clumping of electron clouds on both sides of the equator formed by a phenomenon called the Equatorial Electrojet.
All the above phenomena are relatively sporadic in nature, however, they manifest themselves often enough to make them exciting and useful for amateur radio communication. The occurrence of most of these phenomena is dependent on the time of day, the seasons, the warm and cold air layer profile of the lower tropospheric region, the position of the sun vis-a-vis the equator, etc. We will dwell thoroughly into all these phenomena individually through dedicated articles under this section of our website.
- 6m Band – This is often called the magic band by radio amateurs though there is nothing magical about it. Every nuance of propagation behavior on the 6m band can be explained by well-established scientific phenomena. The beauty of this band is that being at the cusp of the HF and VHF bands, it inherits some of its behavior from both the HF and the VHF bands. Under moderate to high SSN conditions the F2 layer as well as the E-layer propagation occur from time to time over preferred directions. It also works with Sporadic-E, meteor-scatter, atmospheric super-refraction, and tropospheric ducting. 6m band DXers find these propagation methods quite attractive which at times may even yield inter-continental radio contacts.
- 2m band – This band falls in the middle of the VHF spectrum and is ideally suited for frequent DX using almost all the modes available to the 6m band plus the manifestation of trans-equatorial propagation mode across the equator that might cover DX distances in the order of 6000-8000 Km.
- 70cm band – Most of the propagation methods cited above are also applicable to this band, however, the probability of robust and reliable Sporadic-E as well as Trans-equatorial openings get significantly reduced due to the lack of sufficient ionization density required at these frequencies. On the other hand, the tropospheric scatter mode becomes more frequently available due to the reduction in wavelength.
- 23cm and beyond – Many of the above-cited modes of communication begin to become less relevant as we increase the operating frequency, however, the tropospheric scatter becomes more predominant. Atmospheric absorption due to the presence of various gases, dust particles, raindrops, etc tends to increasingly accentuate attenuation. The buildings, architectural artifacts, and natural geographic topological entities start affecting propagation even further. The foliage comprising of trees, plants, etc also contributes to signal attenuation through absorption. By and large, these bands are more suitable for classic point-to-point Line-of-sight (LOS) communication.
Let us now take a brief look into the underlying concept of Free-space propagation. It is the basic building block and the foundation of all modes of propagation. A reasonable understanding of Free space propagation is vital to appreciate the effects of various propagation phenomena that we will subsequently cover in many of our website articles.
Free Space Propagation
Let me start by mentioning that although the Free Space propagation mode is very significant while dealing with VHF, UHF and Microwave propagation, generally on its own, it is rather an inconsequential mode for HF terrestrial propagation. However, we will dwell upon several very important fundamental concepts while discussing Free Space propagation. This will be very handy when we eventually dig into other relevant HF propagation modes later through a number of articles on our website. Some of these important concepts are Inverse Square Law, Power Flux Density, E-Field, H-Field, Antenna Aperture, Path Losses, etc.
We have mentioned Free Space Propagation earlier in the article. For all who have a background in science at the school level, the concept should be easy to follow. It is quite intuitive. Just like the light that spreads around in all directions and the intensity of illumination becomes weaker as we move away from the light source, the EM waves (radio waves) also behave in a similar manner. The fact of the matter is that both light and radio waves are EM waves. They are fundamentally the same animal. The only difference between the two is their frequency or wavelength. Radio waves have a much lower frequency (longer wavelength) whereas light waves have extremely high frequency (very short wavelength). Just as a good antenna designed for the specific radio frequency band efficiently picks up radio signals, the retina in a human eye is designed by nature to respond to the frequency band of light and is essentially an antenna at the frequency of the light wave.
Radiation from Isotropic Source
If light were to be emitted from a small light source uniformly in all directions in a three dimensional (3D) space it will spread out with equal brightness and intensity in all directions. Such a light source that emits equally all around in 3D space is called an Isotropic Source. If there was a large globe-shaped lampshade made of translucent (frosted) glass with an isotropic electric bulb at the center, we will notice that the brightness at any point on the surface of the lampshade is equal.
If the diameter or the size of the lampshade is very big we find the glow on the surface of the frosted glass will be less compared to a lampshade of a smaller diameter. The larger the diameter, the surface glow will be less bright. In other words, the longer the distance of the illuminated surface from the point of the light source, the intensity of illumination will be weaker.
The next vital question is how much weaker does the intensity of illumination becomes as we move further and further away from the light source? Is it inversely proportionate to the distance? The answer is no. It does not change or reduce linearly with the distance but in accordance with the square of the distance. Therefore if the distance becomes twice then the intensity becomes one-fourth, if the distance becomes three times then the intensity is one-ninth, and so on so forth.
This is exactly how radio waves also behave. We can replace the light source with a radio transmitter and an isotropic antenna that radiates uniformly in 3D. Since we cannot see the radio wave intensity with our eyes, we may now remove the globe-shaped lampshade and place a radio wave sensor (antenna) at a distance where the lampshade surface was. Hence we have exactly similar situations using radio signals as we had with light.
For those who might like to dig a bit deeper, I would suggest that you should continue to read on. I will try to present some of the very basic, yet important concepts with the aid of elementary mathematics. These concepts will help in acquiring a better understanding of some of the fundaments. Please read on…
What we have discussed so far is applicable directly to Free Space propagation and Line of Sight (LOS) propagation conditions. All these concepts are also essential in understanding various other modes of propagation that we will introduce in the following sections.