Thursday, March 24, 2016

3-element, 4-direction Vertical Parasitic Array for 80 Meters CW

Before describing this antenna it may help to declare my thoughts on the 4-square vertical array. This is an antenna that has become the "standard" for competitive stations on 80 and 160 meters. They are occasionally used on 40 meters where big towers and yagis are not feasible..

The attributes of the 4-square that I find unappealing are:
  • Complex and often finicky power division and phasing by use of a hybrid coupler. They can and do fail at the most inconvenient times.
  • Dump load that, ideally, dissipates well under 100 watts due to system imbalances. Imbalances can be due to weather, freeze/thaw cycles, aging, element and feed faults and other component issues. It's difficult to control.
  • Selected directions must be at 90° intervals. This is perhaps a small limitation since the -3 db beam width is greater than 45°.
  • Expansion of the array to more elements requires a total rebuild, and an even more complex power division and phasing system.
  • Performance and power handling requires a precision in element tuning and phasing that can be trying.
  • Typically requires 4 structurally independent 20 meter tall elements made from tubing or lightweight tower, guyed and electrically isolated from ground.
On the other hand the antenna has many positive attributes. These include:
  • Excellent F/B and compass coverage.
  • CW and SSB segments can usually be accommodated without tuning and little loss of gain and F/B.
  • Commercial products are available for the hybrid coupler and direction selection, which removes a significant technical challenge.
  • Lots of contesters and DXers use them so there is a community you can lean on for advice.
A parasitic array has its own set of issues, so the decision comes down to trade-offs and personal choice based on local circumstances and what one is comfortable with. These will become apparent as I proceed to describe the antenna.

My objective is to find an uncomplicated and relatively simple directional array for 80 meters to be used in my next station. The 3-element vertical yagi looks promising. Like the 3-element vs. 2-element wire yagis for 40 meters and 2-element vertical yagis I discussed some time ago, the use of 3 elements simplifies the physical and electrical design by introducing symmetry, and has better performance.

What you think is for you to decide. I suspect that most would choose to go with the tried and true 4-square. In fact I more than suspect it since I see that choice being made all the time by serious DXers and contesters for whom performance is a never-ending pursuit.

Where it comes from

This antenna is not my invention. It is a variation on a 160 meter parasitic array by K3LR, as described in ON4UN's Low-Band DXing on page 13-42 of the 5th edition. My aim is to delve more deeply than the brief description it received in that text. I believe it is worth a closer look. If you haven't read it or don't have the book, get it and read it. Don't stop there: read the entire book. It's a gold mine of information for those serious about high performance on the low bands.

I know one ham who has built the K3LR vertical yagi pretty much as described and I know he gets out very well on 160. But it does require a 40 meter tower with a base insulator! The challenge on 80 meters is not so severe, if one has the land and the motivation.

The driven element (tower) is tuned with an L-network and the parasites are tuned as directors with coils switched in to convert one element into a reflector. Additional coils and L-network adjustment are needed to cover both CW and SSB segments. Radiation resistance is low so it is critical that many radials are used to bring the ground loss as low as possible. Gain and F/B are critically dependent on tuning and radials. SWR bandwidth is not broad but can be made sufficient for most purposes.

Physical design

A light-duty tower can serve as the driven element and as a support for the parasitic elements. The DMX-52 tower I currently use could work well in this application with a couple of added sections to bring to 19 meters height. I might just do that. The height does not have to be tuned with exactitude since we'll be using an L-network at the feed point. A short mast projecting above the tower provides all the adjustment room we'll need.

Isolation from ground will need to be improved with plastic spacers and fibreglass or similar pins rather than screws to secure it to its base. Non-resonant guys are necessary. These should be separate from catenary ropes for the elements. Radials are not connected to the tower.

Each element base requires a ground anchor to support a switch box and to take the tension applied to the wire element. The tension comes from adjustment of the catenary rope, which is anchored beyond the extent of the radial field. The wire elements in this design are full height, which requires a tower extension mast to attach the ropes. Use good insulators such as ceramic eggs at the tops of the wire elements. The ON4UN design uses a T-top incorporated into the catenaries so that a mast extension isn't needed.

The mast, if used, must be non-conducting or metal with a fibreglass coupler to isolate it. It can serve double duty as the top of a 160 meter vertical using the same tower and radial field. But I will set that option aside for coverage in a possible future article. I have drawn in the 160 meter vertical wire in the manner suggested in ON4UN's book. A trap can be used though initial modelling shows that the SWR bandwidth on both bands narrows.

Geometry and trigonometry will help us calculate what length extension mast we need. If the mast extension is 3 meters and the rope anchor is 30 meters from the tower, for an element spacing of 10.5 meters (0.125λ at 3.5 MHz) the mast extension must be 10 meters. That is excessive. Longer rope or use of posts for anchors can bring the mast down to a reasonable length. I would aim for a maximum mast of 6 meters (20'); it'll be guyed by design and, as already said, it can be used for 160 meters.

The radials reach outward from all 5 elements. Where they intersect they can be tied together (as shown in an earlier antenna design) or a bus wire running between them terminates the radials from each side.

The tower driven element requires the same arrangement when intersecting the other radial fields. This is the design presented in ON4UN's book. A diagram showing this is at right. Notice that the overlap of radial fields is more pronounced than in the 4-square due to the closer spacing of elements.

Coax transmission line and a multi-conductor control cable connect to the tower (central) element. Control cables, but not transmission line, connects to the base of each parasitic element. For best results they should be buried and common mode chokes employed.

Total area of the antenna is a square with 55 meter sides, or about 0.75 acre. This is slightly less than the 4-square.


As with any ground-mounted vertical antenna the radial system is critical. It is easy to lose several decibels with an inadequate set of radials, and even more with this antenna due to its lower radiation resistance. Since the radial field of this antenna is too extensive for modelling I have reverted to an alternative technique that is widely used.

The model uses MININEC ground rather than "real" ground in EZNEC. In this treatment wires can be connected to ground (not possible in standard NEC2). That ground is a perfect conductor or, if you like, a zero-loss radial field. Real ground -- EZNEC medium ground in my model -- is only used to construct the far field pattern, including the reflection ground losses and low-angle field cancellation due to phase shift.

A real radial system is emulated by inserting a resistance load in the base of elements connected to ground. The resistor represents the net ground loss for that element. Since the radials are not present in the model the resistance value must be carefully chosen from measured values published in the literature for the number and length of radials to be used.

For the initial run through the model I assume that all radial fields have an equivalent ground loss resistance of 3 Ω. This is approximately the loss for 60 symmetric radials of λ/4 length (20 meters, for a total of 1.2 km of wire) over good farmland. The effects of different radial arrangements and ground quality will be addressed further below.

The parasitic elements (wires) reach 19.5 meters high, and are constructed of 14 AWG copper (2 mm). The driven element is 20 meters, with a model diameter of 250 mm (10"), which is an approximation of a light-duty tapered tower such as DMX. The driven element is adjusted for resonance at 3.55 MHz, however this is not critical provided it is kept well within the band edges if it is also to be used as a omni-directional vertical. The K3LR design includes this option, and I would like to have that as well.

Performance and adjustment

The antenna is a 3-element yagi. That it is comprised of ground-mounted verticals and radials does not alter that. It therefore has a narrow usable bandwidth, with respect to match, gain and F/B. In my opinion this is its primary disadvantage with respect to the 4-square.

In its simplest configuration it does well over 100 kHz. This is sufficient for CW (3.5 to 3.6 MHz) or the main SSB segment for DX and contests (3.7 to 3.8 MHz). To get both, or other segments of 80 meters, requires switching of loads to tune the elements.

The antenna as described here is for the CW segment. Direction switching and band segment selection is in the next section.

The azimuth and elevation patterns are unremarkable for an array of this type. The broad azimuth beam width is typical of vertical arrays since radiation cancel poorly off the sides. Horizontal arrays have sharp nulls to the side since the elements on edge. Instead it is the elevation pattern that is narrow in a vertical array.

The gain is quite good and is comparable or better than a 4-square near its best frequency. The difference is that the gain bandwidth is narrow this vertical yagi. The same assessment applies to the F/B. Even so this antenna does quite well over the entire CW segment.

Note: be careful how you compare this antenna to the 4-square since the efficiency of each is critically dependent on ground and radial system. I will not do so in this article since any comparison can be deceptive. Not only for that reason, but because the antennas behave differently over the same ground and radial system, as we'll see.

The plot goes as high as 3.65 MHz for the interest of those using digital modes. It is still usable though not a great performer above 3.6 MHz. The reason is the radiation resistance. It is ~21 Ω at 3.5 MHz and declines gradually before steeply falling above 3.6 MHz. With a superb radial system and good ground the gain will remain good through to 3.65 MHz. Otherwise the array can be tuned higher but not without affecting CW performance. Gain, SWR and F/B rapidly degrade at lower frequencies, in this case below 3.5 MHz.

Of the four wire elements only two are active at any one time. The other two must be left floating by disconnecting them with a switch or relay, otherwise the pattern will be substantially degraded. The director is grounded (connected to its radial system) and the reflector is grounded through a 2.4 μH coil. Although coil Q is not critical in this application it is still good practice to make the coil diameter at least 40 mm (1.5") and inter-turn spacing approximately the same as the wire diameter. Make the coil inductance greater than required and adjust it by use of a tap or spreading the windings.

If SSB is your primary interest you have only to shorten the elements by the desired scale factor. Another and better alternative is described below in the section on switching.

The L-network consists of a shunt capacitor of 1,000 pf across the transmission line port and a series inductor of 1.2 μH . Again, the coil Q is not critical since losses are small. The capacitor should be a low loss ceramic transmitting type or a variable capacitor of suitable size. If a fixed capacitor is used you should determine its optimum value with a variable capacitor. The L-network coil can be tapped for tuning.

Direction switching and additional modes

The transmission line and an 8-conductor cable connect to a switch box at the base of the driven element (tower). The 8-conductor cable is used for direction switching. Signalling is low-voltage DC to power relays. A 4-conductor cable runs from there to a box at the base of the four wire elements. The transmission line and switching cable should be buried and use common mode chokes at the driven element.

Inside the shack a switch box selects one of 5 directions, one of which is for omni-directional mode. One conductor is for DC ground. The remaining 2 conductors can be used to select a sub-band (e.g. CW or SSB) or a 160 meter vertical utilizing the same radial field and tower, but its own matching network.

The switch box at the driven element uses a diode matrix to tell each element what to do. For example, when the array is to be pointed northeast, the northeast element is grounded (director), the southwest element is grounded through a coil (reflector), the other two wire elements are floated (open) and the L-network at the driven element is switched in. The reflector coil is shorted with an SPST relay when that element is a director. In omni-directional mode the L-network is bypassed (two DPDT relays) and the four wire elements are floated (open).

When SSB and CW are included the directors are tuned (shortened) as a director for the SSB segment. Its modelled length is 18.54 meters. A coil of 1.85 μH is placed in series with the reflector 2.4 μH coil. For SSB operation the 1.85 μH coil is shorted with an SPST relay at all four wire elements. The CW performance of this dual band segment array is almost identical, suffering only a slight decrease in gain and F/B due to the shorter elements.

The L-network, using the same 20 meter tower height for SSB, requires a shunt capacitor of 760 pf and a series coil of 0.12 μH, configured as before. For the CW segment the L-network is slightly different due to the shorter elements. Use a 900 pf shunt capacitor and a 1.0 μH series coil.

When 160 meters is included by means of a separate vertical wire parallel to the tower (as described in ON4UN's book) the transmission line is switched to the matching network for that antenna element and all the wire elements are floated. Since the radial field has an approximate circular diameter of 60 meters the 160 meter ground loss should be quite low.

I have not bothered to design the switching matrix for this antenna since these are adequately described in ON4UN's book, chapter 11. The matrix is placed in the switch box at the base of the tower. If the default (unpowered) selections of float, CW/SSB and direction are different from that shown in the above element switching diagram you should use SPDT relays instead of the SPST relays shown so that you can wire the normally open or normally closed contacts per your choice. My choice would be to default to omni-directional, where are wire elements are floating. In this case the element wiring is as shown in the diagram.

Radials and performance

The efficiency of the elements in a vertical antenna is a function of the ground equivalent series resistance and the current flowing through that resistance. The loss of the array is the sum of the element losses. This efficiency loss is not to be confused with ground reflection loss that often plagues verticals over real ground at low elevation angles.

For our ground-mounted vertical elements the return current flows in the radials and the ground beneath. The more and longer the radials the less current flows through the lossy ground. It should be apparent that the local ground quality affects the antenna efficiency for all but the most extensive (and expensive) radial system.

I will assume reasonably good ground such as that found on or near farmland since that is most likely where this type of antenna is likely to be built. If your ground is rocky or sandy, or even just very dry, the antenna efficiency will be lower. All you can do is put down more radials or elevate the antenna. Elevating an 80 meter vertical array is rarely practical.

The following chart plots the current in each element across the band segment. For comparison the current in the driven element when in omni-directional mode (single vertical element) is 4.7 A for a power of 1,000 watts.

The magnitude of the current does depend total ground resistance (including the radials) but it does not vary abruptly. So the selection of 3 Ω in this case is not material. What I want to draw attention to is the current distribution among the elements and relative to a simple vertical. Recall that P = I²R, so for a doubling of current the loss is 4 times higher.

Since the radiation resistance is lower than in a single vertical the current in the driven element is higher. It increases with frequency as the array's radiation resistance declines. The current in the parasites is lower, which is typical of yagis. At the 3.55 MHz centre frequency the driven element current is 6.84 A. In comparison to a single vertical (omni-directional mode) the loss in the driven element doubles to ~200 watts from ~100 watts. Taken together the loss in the parasites is ~100 watts, for a total loss of ~300 watts, or about -1.7 db.

That's a lot but it gets worse for a typical radial system and for poorer ground quality. Not only that, the radiation peaks at higher elevation angles as ground loss increases. If you think you can escape this by retreating to the 4-square, well, think again. The loss is similar and can be even worse since the radiation resistance is low and the currents are more equal. To increase efficiency we will have to be creative.

One method is to exploit the current imbalance for a more economical radial system. We want the lowest ground resistance for the driven element since it contributes the lion's share of the total loss. Therefore we should install the maximum number of radials we can afford from the driven element base to the bus bar that rings the driven element.

Possibly better is a metal mesh filling that area. For a 6 meter on a side square inside that bus bar that amount to 36 m² (360 ft²) so it is not cheap or easy. Some hams use galvanized chicken wire mesh (1" squares or hexagons) that comes in widths of up to 3' or 4'. Bonding the edges and connecting to the bus bar and feed system is a challenge since soldering is impractical. Further, copper to zinc contact promotes corrosion and zinc (the coating used for galvanizing) rapidly corrodes when in contact with acidic soil. A dense set of copper wire radials is likely the best choice.

The radial fields under the parasites can be more modest since the current is lower. Unfortunately we can't use a sparser radial field for the lower current reflector since every parasite can be a director or reflector. Further, due to overlapping radial fields the return current for the driven element also flows in the radials for the parasites.

We can model the behaviour by adjusting the load resistances in the parasites and driven elements, making the resistance lower for the driven element. Without collecting and presenting lots of data from the models I ran, including assumptions for the resistances due to the complex radial system, I will only say that this appears to be a promising approach to reduce total antenna loss.

Here are a couple of articles references I know of that provide some background on estimating the resistance associated with radial fields over different ground types.
And don't forget to look at ON4UN's book and also the ARRL Antenna Book if you have it. There are many other useful resources you can find by looking around.

More directions and gain

Unlike the 4-square this vertical yagi is not as limited in the directions it can point. Although the director, driven element and reflector must be on a straight line the other two parasitic elements do not have to be at right angles; that is, direction intervals of 90°. Depending on your global location this flexibility can ensure you get the maximum gain or F/B for the directions of greatest interest.

You can even have 6 or 8 directions with more parasitic elements and increasingly complex switching and radial system. You must only ensure that the unused elements are floated. Of course this may be a little excessive since the -3 db beam width is so broad that the extra directions can only gain you 2 to 3 db. That is, unless you add more gain...

If it's more gain you want you can add a second director, either for all four directions or even just one. Although the latter would require L-network selection in tandem with direction. The added director increases SWR, gain and F/B bandwidth and adds about 1.5 db gain for a director spacing of 16 meters. The second director is 20 cm shorter than the other directors, whether the measurement for CW or the one for switchable CW and SSB band segments. The 2:1 SWR 150 kHz. The gain band width stays excellent except at the upper 25 kHz of the 150 kHz band segment.

But this becomes a very large antenna. The point is that you have this option. You don't get that with a 4-square.

The good news is that the element switching is simpler for the second directors because they have no reflector coil. They either float or are directors for CW or SSB. Even the mode coil can be eliminated if you use the second director for only one mode. The radial system can be more modest since the current is relatively low (see discussion above).

Perhaps in future I'll describe the 4-element vertical yagi in more detail, just for the sake of doing so since I believe few would build it. I find it helpful and interesting to discover what is possible if we are willing to put in the effort.

Final thoughts

I hope that the material in this article adds some insight into the antenna as described in ON4UN's book. That was my intention, if only for my own interest. If you have the land perhaps this antenna will interest you as well.

I could grow to like this antenna . The performance is good and it has an uncomplicated electrical and physical design that appeals to me. It is still a lot of work, though less than I'd expect for a 4-square and with some potential advantages. Supplementing it with a directional receive antenna is desirable since the F/B is not exceptional. But then you'd also want one for a 4-square, so the comparison in this respect is not unfavourable.

If for some reason the vertical yagi does not work out it is not very difficult to convert the antenna to 4-square. The tower would become a support for the wire elements and double as a 160 meter vertical. The element anchors and their associated radial system would have to be moved to achieve the λ/4 element spacing of a 4-square, which is 20 to 21 meters, rather than the ~15 meters of the vertical yagi.

To start off I would build the tower vertical and then, over time, install the wire elements and build the switching system. At first it could have only two directions, with the others added later. By comparison, you must build the 4-square all at once. In parallel I could add the 160 meter extension to the tower. It would be best to install the bus bars, wire element anchors and full radial system right from the start. The radial system can be sparse at first and more added when the antenna looks like a keeper.

Although CW is my priority for 80 meters I would build the elements for both modes. Switching the elements for CW and SSB does not require a switching matrix, and can be easily added when I am ready.

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