Antennas interact with their environment. In one sense that is obvious since that is the essence of what an antenna is all about: electromagnetic coupling to free space. That is a "good" interaction. There are also "bad" interactions. By this I mean interactions that cause unwanted effects (EMI, coupling, etc.) that degrade antenna performance from the objectives set out in its design.
Many hams are often unaware of or outright ignore interactions. Once the first QSO is made with a new antenna all thoughts turn to operating and away from the antenna itself. This isn't surprising. While we like to promote our hobby as one promoting technology and technological expertise we know that we are in it for the operating. The technology, antennas included, are a part of the journey, not the destination.
As you might guess since I am writing this article is that there is good reason to stop for a moment and consider the antenna's interactions during its design and siting. It can prevent many problems and achieve better performance.
What I will do here is talk about some of those interactions in regard to the nested 20, 15 and 10 meters delta loops antenna I described recently. In the following model the antenna is mounted on a mast at the apex of my roof, which is site 'A' described in the site planning article written earlier. Although, as I said in that article, I rejected that location I am being lazy and using an interaction model I had already created for that site.
The antenna view at right includes a model of a coaxial transmission line, consisting of wires 10 through 12. It is approximately to scale but I made only a modest attempt to be
precise about direction and length. My purpose was to get a
sense of the interactions rather than to make the coax part of the
formal model. You'll have to imagine an outline of my house to see that wire 10 angles down to the edge of the roof (tied to the eaves trough that is my no-antenna antenna). The same goes for the remaining wires, with 11 dropping down to the back edge of the lower roof, and 12 takes us down to near ground level, close to where it will enter the basement.
Two more things about the model: there is only one coax drawn but there will in fact be two, one for the 20 and 15 loops; and, the top of wire 10 ends between the two feed point (V1 and V2) and is connected to nothing. This is obviously simplified though sufficient for our purposes here. By not connecting the coax to the antenna I am modelling the presence of a high-resistance common-mode (current) choke on the coax. This ensures the coupling to the coax is by mutual conductance only, and not direct conductance.
Using a high-resistance choke is important to avoid turning the coax into part of the antenna, with a negative consequence on pattern and match, among other difficulties. Mutual inductance is bad enough, so don't make it worse. I won't get into chokes here, so instead I will refer you to a truly excellent document on the topic by K9YC (PDF). If you have May 2013 QST you'll notice this work referenced in the article on making a common mode choke.
Notice in the antenna view I am showing the plot of currents when fed at 21.1 MHz. This is the worst case scenario for induced currents on the model coax. The currents are much lower on 10 and 20 meters, with lesser impact on the antenna performance.
The currents, especially on wires 10 and 12, are enough to cause a significant change to the far field plots I presented in the antenna article, where I modelled none of the environment. The coupling occurs even though I kept wire 10 nearly perpendicular to the antenna plane. I did not model the aluminum eaves troughing or the house wiring in the 2nd-floor ceiling, all of which are in play when it comes to interactions.
Compare the far field plots shown here for 21.1 MHz to those in the earlier article. It is no surprise that the pattern is no longer symmetrical. The major changes are in the azimuth omnidirectionality and the increase in the amount of horizontally-polarized radiation. The SWR is not shown since it isn't much different, and is besides easily remedied during tuning when the elements are adjusted after installation on the mast.
Are these interactions bad? That depends on your objectives. The increase in azimuth asymmetry (~5 db rather than 3 db) will be noticable though probably not by much in actual use. The elevation pattern, while lopsided, is not too far off the ideal model.
An antenna like this is bound to have unwanted interactions of this sort since it is dimensionally large and close to the house, including all of its many embedded conductors. The difficulties are less for a yagi atop a tower where the coax (and tower) are orthogonal to the antenna plane. Less, but still there. Chokes will at the very least protect loss of depth in the front-to-back and front-to-side nulls.
These are not the only reasons to assess whether the interactions are acceptable. There are others that give me pause and that is why I am still thinking about whether I should build an antenna that is otherwise suitable for my purposes.
Coax currents are reciprocal as they are in any antenna: they act the same on receive as on transmit. With the antenna so close to the house (and neighbouring houses) there are interactions that could be more serious than on the far field pattern or the match.
First, there is the obvious matter of EMI. Using QRP as I am at present the risk is low. That will change should I increase power to 100 watts. With a low antenna where the main field intersects houses that is already a problem. Add in radiation from the coax and the probability of EMI increases. In the above model there are radiating surfaces all the way down to ground level, adjacent to inhabited areas. While a tower and yagi might appear more problematic to neighbours, the EMI risk is often greater from low antennas and all transmission lines.
On receive there is also risk of EMI to us. Our homes are filled with computers and computer-driven phones and other appliances, switching supplies in wall warts and light fixtures, and more. Their radiation will make its way into our sensitive HF receivers via the exterior of the coax. With an eaves trough for an antenna I currently have this problem in spades. In my house I have at least 3 Ethernet-based devices within a few meters of the eaves trough and there are more further away, including my next-door neighbours. If the coax radiates, even if its current is a tenth of what is on the antenna proper, the noise becomes a problem on at least parts of the HF bands. It is easier to install a choke one run of coax than to choke ten appliances.
Yet even with the common-mode choke at the antenna feed point there can still be radiation on the coax, as I've already demonstrated. If it is necessary to go further to solve problems we will have to install one or two more chokes along the length of coax, ensuring that the length of coax between chokes is non-resonant on all bands of interest. If you concurrently operate on more than one radio or band this will include radiation from antennas for those other bands since they will happily couple to coax runs of other antennas.
Many hams do not realize the true source of EMI problem, often blaming it on high power or just bad luck. Yet it can often be cured or at least reduced with more and better common-mode chokes on their runs of coax. Ignorance is not bliss. Just because one is unaware of the root cause of EMI, or poor antenna performance, it will not go away.
Interactions between antennas and all conductors in the vicinity is an unavoidable consequence of the laws of physics. We can ignore those interactions but they will not ignore us.