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.

Monday, February 25, 2013

Antenna Objectives

When it comes to antennas it has always surprised me how much can be accomplished with very little. My very first antenna in 1972 was a 20m dipole running from the edge of the roof to the top of a clothesline post. The average height was no better than 4 meters. With that and an 807 I worked the world.

The eaves trough antenna (mentioned yesterday, and feed point shown in the adjacent picture) is at least as good, and it's up higher: 6 meters. Of course it also helps that I'm no longer in the radio black-hole that is VE4-land.

The antenna farm I took down in 1992 consisted of a 19-meter tower sporting a TH6DXX for high bands, long-boom yagis for 6m and 2m stacked above it, a 2-element 40m inverted-vee wire yagi and a half-sloper for 80m. Toss in a 30S1 and I was competitive in any pile-up (and I could generate a few myself).

My plans for 2013 are substantially more modest since this time it's for fun, not to be competitive. As I got down to planning last month I made a list of the primary objectives my next antennas would have to meet.
  • No tower: In fact, no new permanent structures of any kind, whether stand-alone or attached to the house. I will make do with what I have or with new, non-permanent supports. The trees are not suitable as antenna supports but I do have other uses for them. I do intend to make use of my house's height, which is just over 8 meters above local grade.
  • No rotator: Antenna will have to be fixed and at best electrically steered. This means there must be an emphasis on omnidirectional antennas.
  • DX performance: DX requires low-angle radiation. Considering the heights I have to work with there will have to be some bias towards vertical polarization.
  • QRP friendly: With little power coming out of the transmitter there is little tolerance for loss in the transmission line, matching networks and antennas. While I don't think I'll need to bring the Heliax out of storage, loss will need careful attention.
  • Minimum impact on aesthetics: Putting antennas in my back yard is best but there is no way around visible impact if I use the house roof. This means the antennas will be visible and I'll have to do my best to not make them the centre of the neighbourhood's attention.
  • Survival: Wind, ice, sun and temperature gradients are all concerns. Non-permanent structures and antennas can be made robust and worry-free, just as permanent structures (as I and many others will attest) can be fragile as a result of poor design and unjustified optimism.
I did not list low cost since that is not an objective. However if I can make it inexpensive that's certainly a positive outcome.

I have been designing supports and antennas during the past several weeks. I upgraded my ancient version of EZNEC from W7EL (actually EZNEC+) and put it to work on my quad-core desktop. Ideas are gelling into solid plans.

I'll talk more about these antenna designs in future posts. Right now I'm being distracted by the background sound of an unruly pile-up on XT2TT. This one may be out of reach until my antenna plans come to fruition in the coming months. Spring is in the air.

Sunday, February 24, 2013

No Longer QRT

Contest...DX...contest...DX...tower...contest...DX...DX...tower... (ad nauseum).

By 1992 I was getting burnt out in the amateur radio hobby and it was time for a change. That came when I had to dismantle my station and, most importantly, remove my tower, stacked yagis and wire antennas. The reason was that I was demolishing my old decrepit dwelling to build a new house. This was a great way to increase the property value but not good for radio.

The reason the tower had to come down was that the grade had to be raised well over 1 meter to meet modern DOE regulations for septic systems. This is an odd neighbourhood since although it is well within the city we have no sewer service. We like it that way, but it does constrain property use.

I very nearly did put up a new tower in 1993. I went to the trouble of gathering all the required engineering data, putting a design to paper, getting quotes on the tower and antennas, and even discussing it with the city. However when a decision needed to be made I found that my heart wasn't in it. That was the start of my 20 year hiatus, one that I fully expected to be permanent. My interests and work responsibilities had changed and fully occupied my attention.

To my surprise there was a rekindling of interest in late 2012. I don't know why, it just happened. First I had to determine if the interest was permanent or simply nostalgia for the old days. I unpacked my ancient and venerable, and only partly operational FT-102 and turned it on. Suddenly I was hearing signals again. To my surprise I had little difficult copying moderately-fast CW.

I played with reception for a while, just to get the taste of the bands. The antenna was nothing, just a length of wire pressed into the antenna jack. For a few days I listened.

The FT-102 it is an interesting transceiver. It is one of the last hybrid transistor-tube designs, produced for only a few years in the early 1980s. The receiver is exceptional and the transmitter is clean and conservative. It was the receiver that attracted me to this rig, which became the centerpiece of my station from 1984 to 1992. Its one glaring flaw is the quality of the relays. There are many of them and are not build to last. This can be remedied with some effort. That needs to be done to make it usable once again

My more immediate concern was the RF pre-amplifier. One of the transistors had gotten fried many years ago when I was attempting to repair one of the relays. After some research I decided to replace both long-obsolete 2SK125Y transistors with J310 JFETs. It took some work since they are hard to access and the pin-outs of the devices differ. After fixing one cold solder joint the RF pre-amp came alive. I don't yet know how the performance compares, although I expect it to be at least equivalent.

I needed an antenna. To draw little attention to myself for no good reason and to avoid significant effort I decided on stealth. I drilled a hole in the aluminum eaves trough just outside the window of the guest bedroom, secured a wire to it and thread it a few feet around the window seal to an L-network tuner I found in the basement. I ran a counterpoise along the inside wall and a short length of RG-58 crossed the room to a desk. It isn't much but it works.

Deciding that the FT-102 repair would have to wait I started shopping for a more modern rig. After looking at the usual suspects on the used market I chose to buy a new radio, a QRP rig, the Elecraft KX3. Low power suits my new minimalist leaning and the receiver is very good. I ordered it just before Christmas and a few days later I was assembling the no-solder kit.

I will come back to this and other adventures, including my plans for the future, in coming posts. For now I will say that in the subsequent 8 weeks I have worked 80 countries on CW using from 3 to 10 watts on 40 through 10 meters to my eaves trough antenna. The one thing that hasn't changed is that I am still a DXer. That means I must do something about antennas.