Video
Introduction
Ham radio operators that operate on the HF bands understand that the distance our signal travels is largely dictated by the angle of the radiation, or the angle that the energy leaves. Our antenna energy is reflected or bent by the ionized layers and arrives back to earth at a far point. And quite clearly, the lower the angle, the greater the distance. It doesn't always mean to say that the lowest angle is the best distance, because the best angle is the angle that enables the signal to arrive at the destination we're aiming for. But we don't really have too much control over that, because once we've erected our antenna, it's all predetermined.
The Importance of Antenna Angle
Now, the vertical antenna is a prime example of that. It gives very low angle radiation. But what perhaps we don't always realize is how effective that low angle radiation is, and in fact, whether it's actually going anywhere at all. So I thought I'd do a little bit of investigation and see if I can come up with some facts and figures that may indicate what we actually experience, particularly in a small backyard or back garden.
Vertical Antennas and Radiation Patterns
For a long time, I've had my reservations about the low angle radiation properties of vertical antennas. There's no doubt that they should provide very good low angle radiation. We are told that in the textbooks. But there's certain facts that are missed out in many textbooks. And also in more recent times. Modeling software seems to ignore one very, very important fact, a fact that's so important that it could mean that a lot of this modeling software is not really telling us the truth.
Well, we all like to know the truth, don't we? And as ham radio operators, it's nice to ferret around and see if we can find out a bit more about what our antennas are really doing. So that's the purpose of this short blog, really, to have a look at the angle properties of vertical antennas and how perhaps we're missing something which we should know about.
The Brewster Angle Revelation
The Brewster angle, as it's called, has been around for a long time. Basically, it relates to light, the transmission of light, reflection of light and images and so forth. And because radio waves are not unlike light waves in the way that they travel and are reflected and bent and so forth, the Brewster angle has been applied to the transmission of radio waves from antennas.
And in fact it's something called PBA, pseudo Brewster angle. And it basically explains how the ground reflection from the radio waves that hit the ground actually bounce back and cancel out low angle radiation. Now, although it's been known for a long time, it's not often mentioned, and as I say, I first saw it in the 1969 RSGB handbook. It's appeared in more recent versions of the ARRL antenna book. But it seems to have disappeared when it comes to antenna modeling.
Implications of Brewster Angle on Antenna Performance
The fact that it's disappeared in antenna modeling doesn't actually mean it doesn't exist. It means that it's not taken into account. And it's a bit worrying really, because you see quite a lot of examples of the vertical radiation patterns of antennas, particularly vertical antennas. And basically, what it means is that what you're seeing is not actually what is happening in real life. And that's a bit worrying, isn't it?
The Brewster angle is basically the angle at which no interference takes place. It's the angle where the reflections from the ground don't have any effect at all, but it's below that angle where the ground reflections do have a major part.
Conclusion
So what is the point of all this? Well, the point of all this really is to explain that don't be disappointed if you don't get the result you expected from your ground mounted vertical antenna. We now know that there's an invisible item that's not shown in the modeling software, which is the Brewster angle, which is quite important and will have a major effect on low angle radiation.
There's no easy way of actually evaluating how your vertical antenna is going to work in your particular location, but it's a fair bet that you're not going to get anything below ten degrees, and you'll probably be quite fortunate if you get much below about 15 degrees if you take into account the urban situation that many of us have to operate from.
So take heart, because for a lot of DX operation, we don't need ultra low angle radiation anyway. A lot of DX contacts are made through multi hop transmissions, and I covered that in a video fairly recently, covering shortwave propagation. And you can work DX with fairly reasonable angles of radiation around about 25-30 degrees very often.
That's all you need. You don't need ultra low angle radiation unless you're a member of the Flat Earth Society and believe in a flat Earth. Perhaps these modeling programs, when they're illustrated, should really come with a health warning. Particularly when the discussion is about angles of radiation, perhaps they should come with a health warning.
In the meantime, thanks for reading this blog. I hope it's been informative. In the meantime, enjoy ham radio and take care.