Thursday, February 28, 2013

Radiating Off in All Directions

In my previous post I mentioned that my antennas must be omnidirectional since they will fixed (not rotatable). By this I mean omnidirectional in azimuth (compass directions), not elevation. For DX the elevation will have to favour low-angles.

Achieving a true omnidirectional pattern is not possible with a real antenna. The one that comes closest is an antenna with one vertical element. This is true whether the vertical is λ/4, λ/2 or other length. The pattern will however be corrupted by its immediate environment, ground (especially if ground-mounted), attachment to a transmission line and so on. Perfection is in any case unnecessary. This means we are free to choose other, theoretically-imperfect antennas to achieve and effective omnidirectional pattern.

My favourites in this regard include:
  • Inverted-vee
  • Delta loop (when fed at the side or corner)
The criteria I am using to whittle the list down to just these two (in addition to the vertical, as mentioned) include simplicity in construction and mounting (wire, weight, single support), able to be mounted on my house roof, and presence of at least some vertical polarization. Dipole arrays, as a counter-example, are more complex and fare no better than a simple inverted-vee.

I will focus on this short list of 3 antenna types in this post.

The inverted-vee is a very simple antenna that is too often presented as a compromised design. This is unfair. Consider its positive points: single support; height where it matters (at the current maximum, as show in the EZNEC antenna view); good match to 50Ω coax (the interior angle lowers the impedance below that of a dipole); inexpensive; and reasonably omnidirectional with some vertically-polarized radiation.

That it does actually have a vertical polarization component requires some discussion. Even a horizontal dipole has some vertical polarization if you look at it the right way. "Looking" at it is key: you can get a good approximation of an antenna's far field attributes by just looking at it. What you do is to travel outward along the ray at a specific azimuth and elevation, then turn around and look at the antenna. What do you see?

A horizontal dipole is only "invisible" precisely at 0° elevation and on the wire axis. In the same azimuth direction but higher angles the antenna has the appearance of a (stunted) vertical element. This is accentuated by an inverted-vee which is never invisible in the same way as a dipole. In other words, the gain and polarization is strongly correlated to direction. That is one reason why siting an antenna is important to meet personal communications objectives.

An azimuth slice of an inverted-vee is shown here, as modeled in EZNEC. This antenna is tuned for 14.15 MHz and the apex is at 10 meters above perfect ground. The antenna's gain peaks in the broadside direction (along the X-axis of the depicted antenna) at an elevation of 40°. The plot show both the horizontal and vertical far-field components as well as the total field. There is of course some gain from ground reflection and there is the expected peak in the vertical radiation off the ends. The inverted-vee is less "deaf" than a dipole off the ends but is still (in this configuration) down about an S-unit from the broadside direction.

While not shown here the peak in radiation angle off the ends regrettably does not gets appreciably lower despite the presence of some vertical radiation. It still depends on height to be a decent DX performer. I don't have that height in my plans and so this is not the best antenna for me.

This brings us to the 1λ delta loop, a variation of the more conventional square loop used in quad beams.

Like an inverted-vee it needs but a single mast support. However that mast has a minimum height (0.3λ) to stay above the ground (or roof) where the mast is mounted, will typically need to be at least somewhat higher. One can lower this requirement by flattening the triangle so that it is no longer equilateral or leaning it to one side (strategies not uncommon on lower bands such as 80 meters), but not without modifying its vertical and omnidirectional performance.

For the discussion note that the attached plots are for a delta loop resonant at 14.1 MHz fed on the side as shown, and the bottom wire 10 meters above real (medium) ground. I used real ground since with this antenna the low angle characteristics change dramatically in comparison to perfect ground. You should mentally add the NEC2-reduced gain figures by ~3.5 db to get the more-realistic far field gain, which is reasonably similar to a dipole or inverted-vee, but in all directions, not only broadside.

When fed 1/4-λ from the apex we get an intriguingly symmetric and useful current distribution. The second current maximum is at the mirror-image position on the other leg of the delta, exactly λ/4 distant. In the far field this all combines into an almost pure omnidirectional and vertical radiation pattern, with substantial radiation at low elevation angles. (This may be difficult to see on the plots since the vertical/red curve is almost coincident with the total field/black curve.)  The azimuth plot for the low angle lobe at 15° is not shown but is nearly identical to that of the stronger 45° lobe.

Other advantages include partial usability at the 2nd-harmonic (where it is a 2λ loop), broadband match (at its fundamental) and an impedance that isn't difficult to transform to 50Ω.

Among its disadvantages is its somewhat unwieldy structure, visual impact on neighbours, the peculiar feed point and the high-impedance points at the apex and the middle of bottom horizontal wire. The structure works on my roof, but only on 14 MHz and higher bands. The visual impact is a concern for me in my particular situation. The feed point is a problem since there is no easy way to run the transmission line to avoid coupling to the antenna, result in a common-mode problem and disruption to the pattern and match. Feeding it at a bottom corner is more convenient, at the cost of a modest impact on polarization and directivity.

The high-impedance at the apex and bottom should not dismissed. At the bottom there is not only a safety issue, the antenna will be detuned if the snow builds up under or onto the wire, even if it's insulated wire. If mounted on the ground, the antenna should in any case be high enough that the wire is well above people's heads. On the roof, it just needs to be high enough to cleanly avoid the winter snow line.

The apex is a different problem. Any conductor near the mast attachment point, including the mast itself will greatly impact the antenna performance. If at all possible the mast should be non-conductive, such as plastic or fibreglass. If for structural reasons at least the bottom of the mast must be metal, transition to a non-conductive section towards the apex and make sure the mast is sufficiently non-resonant that currents are not induced on it. The mast can be modeled as a "wire" in EZNEC and other modeling software.

As for vertical antennas, they are (as already noted) about as omnidirectional in the azimuth direction as any antenna can be. Rather than needed a mast to support it, it is its own mast, only needing guying for survivability in select cases. There are ample commercial products that suit the bill, in particular loaded, multi-band vertical dipoles that permit feed line connection at the bottom.

Getting all of that to work requires a lot of design work in the careful placement of traps, loading elements and matching systems, and then extensive real-world testing. While none of this has an appreciable impact on the far field pattern there are problems. All of that loading and matching lowers efficiency, reduces the low-SWR bandwidth on some or all bands, and there are endless compromises in getting good performance on all bands. The alternative of having multiple verticals increases cost and complexity.

It is also important to get verticals well off the ground in most instances since there is no magic in verticals that makes it immune to its environment. Consider all the buildings, trees and ground variation the radiation has to get through. This may be unavoidable on 80 and 160 meters, but for higher bands we need to do better. This means raised radials for a λ/4 vertical or a substantially greater standing height for a vertical dipole, even if shortened.

As you should guess by now, I am not enthusiastic about verticals for the higher HF bands. In fact I have taken them off my short list of antennas for 40 through 10 meters. However I did want to discuss my thinking about them that led me to this conclusion.

In coming posts I will revisit each of the inverted-vee and delta loop to review a little more about what they can do for me in my present situation. Again, I refer you back to the objectives I laid out in an earlier post. What I didn't state in that list is that I would ideally like the minimum number of antennas for all the bands of interest: 40, 30, 20, 15, and 10, with 6 and 80 of secondary concern. This has consequences.

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