Tilted 80 Meter Inverted Vee

I previously had an 80 meter inverted vee on the 150' tower. I took it down to make room for work on side mounted yagis when the 80 meter vertical yagi became fully operational. I've been pretty happy since then having just that one antenna on 80. My opinion has changed for two reasons.

First, a vertical antenna is great for DX and not so much for short distances. I'm having some difficulty working the population rich US eastern seaboard and Midwest. It is not a problem in contests when I run high power, but it can be with low power or QRP in contests. That includes important contests like NAQP and Sweepstakes. It's a difference between being heard and completing QSOs.

The vertical yagi does not do well pre-sunset and post-sunrise when D-layer absorption is high and propagation favours higher elevation angles. Those "gray line" openings are when a number of otherwise difficult multipliers can be worked. The openings are brief and the competition to work the DX can be intense. The same is true of DXpedition pile ups.

The second reason is 30 meters. I have no 30 meter antenna and I need one to effectively work DX. The 80 meter vertical is a poor choice on 30 due to the low pass L-network, despite its omni-directional pattern on its third harmonic. A recent example was the difficulty working 3Y0J on Bouvet Island. In my experience, an 80 meter dipole or inverted vee works pretty well on 30. It's not quite the third harmonic so the SWR will be moderately high. That can be tamed with a tuner. 

Neither 80 meter antenna is very good on 30, and I intend to have a resonant 30 meter antenna, eventually. It is not a priority in 2023.

The 80 meter inverted vee is up and working. We had several days of unseasonably warm weather in late February that I put to good use for this and other projects. The antenna was pulled from storage and I constructed a new tower bracket for the wires and common mode choke. The bracket is identical to the old one which found its way into another project along the way.

This is my only antenna on 80 meters until I repair the malfunctioning vertical yagi. With no spare ports on the 2×8 antenna switch I disconnected the yagi. I need warmer weather to work on the antenna, hopefully later in March. I did only a cursory inspection since the radial system and other critical parts are still encased in snow and ice.

Although I cannot do an A-B test between the antennas for the time being, it is worthwhile to review the design and the electrical and mechanical behaviour of the inverted vee. The antenna is interesting because the wires do not lie in a vertical plane. This is done to keep the support lines out of the hay field. Otherwise I'd have to remove the antenna during the summer when my neighbour harvests the hay. 

Tilting the antenna raises questions about the antenna pattern and mechanical challenges, topics I only briefly touched on when I originally installed the antenna on the big tower. There is also the matter of interactions since it is mounted 2 meters below the lower 5-element yagi of the 10 meter stack and 8 meters above the TH6.

Real antennas are never textbook perfect. Elements curve, the environment intrudes and there are interactions with pretty well everything near and far. Being deliberate about the deviation, as with this inverted vee, adds another variable. The deviations can be modeled and studied, though few would bother. As I've previously written about skewed wire yagis and skewed vertical wire arrays, deviations can be made advantageous. 

In this light it is very worthwhile to see what effect deviations have on this inverted vee, and whether it meets my objectives to work nearby stations (within ~1000 km) and DX under gray line conditions.

Model

The model was done with EZNEC Pro/2+ Version 7 and the NEC2 engine. The 150' tower to which the antenna is attached is modeled as a straight wire. The model does not include tower attachments, yagis and guys. The guys, at least, are sufficiently segmented that there is negligible interaction.

When the legs of the vee are symmetrical with respect to the vertical tower (including the cables running along it), the induced currents cancel. The model confirms this. The model does not include the coax or common mode choke since, in this configuration, the antenna is effectively isolated.

To set a baseline for comparison, I've plotted several patterns for the inverted vee with its apex at 30 meters and 120° interior angle. The elevation patterns are for broadside and end fire, starting on the left. The rightmost plot is the azimuth pattern for an elevation angle of 45°. By removing the total field from the plot, we can better see the horizontal and vertical polarization patterns.

As the legs tilt outward there is a gradual change in the polarization and pattern of the antenna. For nearby contacts it is important that the horizontal component remain strong. For an inverted vee with the legs in a vertical plane, the radiation is horizontal in the broadside directions and vertical in the end fire directions. 

For my antenna the broadside directions are north and south, and it is south that is most important since it covers all of the US eastern seaboard. North may be helpful for working Asia during the post-sunrise opening. Vertical polarization in the end fire directions may prove useful for nighttime openings to Europe and the Pacific, however that's redundant with the vertical yagi. It worked well enough in the recent ARRL DX SSB contest to log numerous Europeans with just 5 watts.

Pattern impact of slant

I'll plot the patterns for tilts of 30°, 45° and 60°. Although the latter value is unrealistic for an inverted vee up so high, I am including it to demonstrate the trend of pattern change. As above for the non-tilted antenna, the plots will include horizontal and vertical polarization components, and not the total field, to better illustrate what is going on. 

The interior angle of the vee will be kept to a constant 120°. As a general rule the interior angle should be no less than 90° to keep the radiation resistance high. For more acute angles there is substantial field cancellation between the two legs. Loss increases as the radiation resistance decreases, and the antenna may require a matching network to get an acceptable SWR in a 50 Ω system. 

Boring, isn't it? The impact of tilt is surprisingly small. The nulls are less deep but there is otherwise little change in either the horizontal or vertical polarization patterns. Sometimes boring is good. I can rest easy about hauling those long support lines to the edge of the hay field, thereby making the installation permanent. In the final configuration of the inverted vee the tilt is about 35°.

I did not plot the impedance. At a reasonable height (relative to wavelength) and interior angle, the feed point at resonance can be quite close to 50 Ω. It is often possible to adjust these antenna parameters to get that desirable result. That, ground quality, environment and height dominate the impedance, and can be unique to every station. I'll describe how I dealt with it in the following section.

Interior angle

The support ropes are very long. Not only is the apex high at 30 meters, tilting the antenna and increasing the interior angle of the vee pushes the ground anchors further outward. The mechanical challenge will be addressed further along. 

I've drawn the approximate span of the antenna legs and support lines to the ground anchors (trees!) at the edge of the hay field. Yellow was my first configuration, and blue was the final one. The yellow lines are symmetric, but appear skewed because the Google satellite view isn't from directly above. Imagine the antenna apex at the tower base to correct the perspective. The opposite is true of the blue lines, with the right (east) support line longer than the left (west).

The angle between the yellow lines is close to 90°. That is not achieved in practice because the antenna wire and support rope sag. This is the usual catenary issue that I went into in some detail in a previous article about my overhead cable run. I went wider with significantly longer ropes and dragging the left (west) line through tree branches (see picture further down).

The interior angle between the yellow support lines is less than 90°. My guess is 80° (left). The antenna was resonant at 3650 kHz with a resistance of 35 Ω. After two more trials, I ended up with the blue support lines and an interior angle of about 110°. Resonance is now 3580 kHz with a resistance of about 42 Ω. A lower resonant frequency and higher impedance is what to expect by increasing the interior angle.

The SWR bandwidth is typical of a dipole or inverted vee. It cannot cover 3500 to 3800 kHz with a low SWR. I can live with the reduced SWR bandwidth since my primary interest is CW. The 80 meter vertical wire yagi is similarly optimized for CW. However in that case there is a matching network that improves the already good SWR of its omni-directional mode up through 3800 kHz. That's as high as I need for contests and DXing. I'll keep it simple with the inverted vee by using the rig's ATU or manual amplifier tuning to use it on SSB.

I was a little surprised that the impedance in its final configuration wasn't closer to 50 Ω or even higher since I achieved that with an intermediate tests with the interior angle of about 100° and resonance at 3600 kHz. There are enough variables involved that there was no obvious solution. In any case, a better match at resonance does not improve the SWR bandwidth. SWR at the edges is dominated by the rapidly increasing reactance.

Long support lines

That west support line was not easy to set in place. There are lots of trees to choose from, but the further I went the more branches of the forward trees got in the way. What you see in the adjacent photo is the best I could reasonably accomplish. That's why the final configuration is not symmetric.

Dacron rope can only last so long when battered by tree branches and foliage; there's no foliage now but it's coming. If I keep the antenna as it is I would like to replace the lowest span of rope with steel. I have ample ⅛" aircraft cable that is due for retirement from winch service. It can have a second life in this less demanding role. It should survive battering from the trees for many years, and it is far enough from other antennas to pose no interaction risk.

The catenary equation is unforgiving. Tension must be doubled to halve the sag. The lines must be longer than can be drawn on paper, and the tension higher, to achieve the desired interior angle. There is a trade off between resonant frequency, impedance, line length, tension and material that can withstand the tension and the weather.

If you keep the tension constant as you length the support line, you don't get the increase in the interior angle that you expect. Sag increases and that reduces the interior angle. You will likely have to increase tension as you increase the length of the support lines.

Small diameter rope and copper wire have modest tensile strength. In most cases ⅛" woven nylon and or Dacron is adequate. Black Dacron has good UV resistance, but I've had good success with white woven nylon surviving many years. Do not use polypropylene rope. 

Soft (annealed) copper wire for the antenna itself can stretch under excess tension. That will lower the resonant frequency and gradually convert the wire into hard drawn copper. Unfortunately that stretch will be at least 5% before the copper becomes sufficiently hard to stabilize. It may break first. In any case you'll have trim the antenna length as it lengthens. If that's a concern, build wire antennas with hard drawn copper or copper plated steel. The are less flexible and, for the latter, there is a risk of erosion and rust.

Rope and wire are thin and light. As you can see from the photo above, that doesn't avoid sag on long lines. Even if you keep the tension well within the ratings of the wire and rope, the weather has other ideas. All of that area adds. A 1' length of ⅛" wire or rope has a projected cylindrical surface area about 0.01 ft². Although that's a comfortably small number, the line is long. For 100' (32 m) of wire and rope that is 1 ft². For the 200' in my case that is 2 ft².

The approximate force of a 135 kph (85 mph) wind on 200' of ⅛" wire and rope is 40 lb. That will greatly increase the tension, which risks breakage and increased sag due to wire stretch. Don't be surprised to see the wire and rope bow out horizontally in a strong wind: 40 lb can be 10× the weight of the rope and wire. Ice is of course a further threat. I have seen long support lines collapse to the ground under an ice load.

Heavier and stronger rope and wire will increase the tensile strength can be far more expensive, and the weight and diameter will cause more sag and wind/ice load. There is no easy solution. You'll have to juggle the various parameters to come up with a design that works for your station and means. I used 12 AWG stranded THHN wire for this antenna since that's what I had lying around at the time.

I've already had to re-tension the support lines of the inverted vee that been up for 3 weeks. I'll probably have to do it again. Rope and stranded steel cable "relaxes" under tension. The resonant frequency hasn't changed so the wire hasn't (yet) stretched. Since being raised the antenna has weathered winds of 80 kph and a minor ice storm.

30 Meters

The third harmonic of 3.580 MHz is 10.740 MHz. That's 600 kHz higher than the narrow 30 meter band. It doesn't have to be ideal to work so I put it to the test.

The minimum SWR at about 10.850 MHz is 100 Hz (1%) higher than the arithmetic third harmonic. That's typical for dipoles and inverted vees. The SWR within the band would be high regardless of that additional 1%. That said, it's been working pretty well in the brief time I've spent on 30 meters since raising the antenna. The rig's ATU can handle the SWR, though I expect the tuner loss to be on the order of 10% (-0.5 db).

The pattern is obviously skewed. The model tells me that there is more gain to the north. Perhaps that will come in handy for working Asia. About the only operating I do on 30 meters is to chase DXCC countries so I can live with this antenna for now. Eventually I'll need a proper antenna for this and the other WARC bands.

Interactions

I did not evaluate (model) interactions before installing the inverted vee. It has to go somewhere, and I don't expect serious problems based on my previous real world and modelling experience. I could be wrong. Refer to the pictures above for an idea of how the antennas and guy wire are positioned relative to each other.

Guy wires are a negligible risk. This is because the segments they're broken into are very small relative to the 80 meter wavelength. The longest segments are a fraction of 41 meters, which is ½λ. All the antennas on the tower have similarly short elements. Any interactions that might exist are rarely detrimental to a single element antenna like an inverted vee. The danger is more applicable to directional antennas such as yagis.

There is a 10 meter yagi a few meters above the inverted vee apex. In general, a yagi above a wire antenna with the elements at a sharp downward angle will interact too little to cause more than a negligible pattern distortion. I could have lowered the apex had there been a worry. The 10 and 40 meter yagis at the top of the tower are too far to be affected and they are not resonant at the inverted vee's odd numbered harmonics.

The greater danger is to the side mounted TH6 at 22 meters height since the inverted vee elements pass behind it. They are far enough apart and the angle of the inverted vee legs sufficiently vertical that any interaction will be minor. Even that possibility is reduced since the limited 130° rotation of yagi between about 145° and 275° true bearing places the inverted vee at its rear or side. The TH6 never points at (through) the inverted vee. By pointed away from the inverted vee, the tuning of the yagi is minimally affected for the same reason that a yagi can be tuned by pointing it up, with the reflector quite close to the ground.

The SWR of the 10 meter yagi and TH6 are unchanged. However, that does not mean there is an absence of pattern distortion. The degree of interaction has to be quite severe before the impedance is noticably affected.

Hooking it up

All ports on the 2×8 antenna switch are in use. For now the inverted vee is connected to the 80 meter port. Once the 80 meter yagi is repaired I will face a small dilemma since both antennas can't be connected to the antenna switch. An auxiliary switch will need to be built and integrated with the station automation system. The hardware and software is ready and I've collected the parts for the switch.

More about the auxiliary antenna switch will be coming in the next month or two after it is built and installed. For now I happy to have an 80 meter antenna that works. Two or more antennas per band and redundancy are valuable assets when one antenna is lost before a contest or DXpedition.