In this article I take the most promising of the 2-element loop arrays, the one made from narrow diamond loop elements, and turn it into a usable design. The objectives of the design include:
- Achieving an SWR close to 1.0 at the frequency where gain is maximum.
- Favour the CW segment, my favourite.
- Pattern switching, from one broadside direction to the other.
- Mechanically robust.
- Reliable performance.
Now on to the design specifics. I took the "raw" design from the survey article and modified it in the same fashion as I did for the switchable wire yagi antennas. The challenge was to move the array's resonant frequency to coincide with that for maximum gain, and to change the impedance to 50 + j0 Ω. While I did not show it in the survey article, with the maximum gain at 7.000 MHz the resonant frequency is 6.915 MHz (57 + j0 Ω).
The survey antenna and the one described here have an apex height of 15 meters. The supporting structure is assumed to be non-conducting or sufficiently non-resonant to allow it to be absent from the model. This is a matter we'll come back to later, as promised above. The boom length is 7 meters, which is the optimum length discovered in the survey article.
It took close to one hour of adjusting the design on EZNEC to get the frequencies of resonance and maximum gain on the same frequency. The antenna is optimized for CW. For a 2-element parasitic antenna using a reflector element this allow best performance for CW (lower part of the band) while still performing reasonably well higher in the lower part of the SSB segment.
Each loop is 40.46 meters in circumference (10.12 meters per side) when made from 12 AWG insulated copper wire. This is smaller than the 43.9 meters circumference of the survey antenna. As I indicated in the survey article the loop size in a switchable design would be smaller because of the inductive loading of the transmission lines running from each element to the switch box. With an apex height of 15 meters the bottom of the antenna is up 3.94 meters for interior angles of 120° at the top and bottom.
To keep the switch box close to the element feed points I assumed a rigid 7 meters long "boomlet" running between those corners of the elements. This allows for 3.5 meter runs (the minimum possible) of 300Ω ladder line (0.9 velocity factor). A switch box placed anywhere else, such as on the tower, requires longer runs of ladder line, thus smaller loops and poorer performance. The right angles between ladder line and elements also reduces the potential of common-mode currents on these lines. The boomlet not only supports the switch box it also serves to reliably hold the array's shape, as will be described later.
The resulting array has a maximum gain that is only -0.03 db inferior to that of the full-sized elements of the survey antenna, which is negligible. Maximum gain of 3.44 dbi at 10° elevation is set for 7.02 MHz. The maximum F/B is about 60 kHz higher, just as it was in the survey antenna.
The SWR at resonance is 1.1 (56 + j0 Ω) at 7.025 MHz, and slightly lower at 7.015 MHz where it reaches its lowest value. As can be seen the Q of the antenna is higher than that of a single loop antenna, and the SWR rises significantly away from resonance. Since the rise is quite sharp at lower frequencies it is best to set this frequency close to band edge. The SWR (and gain) degradation is more gradual at higher frequencies.
Unlike the 2-element yagi designs there is no need for a beta match or other matching network to match the antenna to 50Ω coax. The naturally high radiation resistance of a full-wave loop combined with the array's high Q takes care of the match for us!
To tune the reflector there is either a transmission line stub (again, using 300Ω ladder line) or a tapped coil inside the switch box. The length of the reflector stub, which is shorted at the far end) is 0.8 meters. It will have to be tuned when the antenna is first installed. Since the design, construction and use of the switching and tuning system is, aside from the removal of the beta match, the same as that for the switchable yagi designs I will point to that earlier article rather than repeat the material here.
The gain and F/B performance figures are nearly identical for the survey version of this antenna, right across the band. The plot stops at 7.2 MHz to capture the frequency range where the antenna performs best. To get both the gain and F/B on the same plot the gain is multiplied by 10. As mentioned above, the maximum gain is 3.44 dbi at 7.02 MHz (shown on the chart as 34.4).
Next, let's turn to the construction of the array. No matter how you look at it a switchable loop array is more structurally complex than a switchable wire yagi. On the other hand it is electrically simpler, mostly due to the ease of matching to a coax feed line. The feed point is also closer to the ground, and can be made closer to the shack by suitable selection of which side of the array to feed. Although transmission line losses are small at 7 MHz there is the expense of a long run to be considered.
The antenna boom is assumed to be tower mounted and trussed in much the same way as was done for the wire yagis. The boom is 1 meter longer and is not cluttered with a feed system. The element bottoms can be secured with a similar type of boom (boomlet) lower down the tower, though trussed from the bottom since the tension is upward.
The two corners of each element are more difficult to secure. While a simple tie-down rope can be used it must be run at an angle shallow enough to apply tension to both the lower and upper quarters of each loop, and to precisely position each corner. Symmetry and parallelism are paramount in a switchable array.
For the side containing the feed and switching system it is preferable to use a boomlet as described above. It can be used for the other side of the array as well. It should be both lightweight and rigid.
Consider the design at right. The boomlet is divided into 3 sections with truss ropes tied at each of the 4 edges. The centre third is aluminum tubing and the outer thirds are telescoping lengths of plastic or fibreglass. The aluminum tube and the rope truss ensure rigidity when the elements are tensioned.
The 4-point tying to a common point, then utilizing a common tie-down rope to a suitable anchor, is similar to that for the diamond vee wire yagi.
The switching and feed system can be mounted on the boomlet as shown at right. The coax can use one of the adjacent truss ropes for support (not shown, so reference the previous drawing).
Other construction details for the boomlet plus switching and feed system, and the tuning procedure, are as earlier described for the switchable wire yagis.
By feeding the array at one corner and the use of a common-mode choke in the transmission line there should be negligible interaction with the coax. Horizontally-polarized yagis mounted atop the tower may noticably interact with the 40 meters loop array, but the reverse is typically not a concern. However the tower and cables running along the tower can significantly interact with the vertically-polarized loop array.
How large an interaction we can expect? There is no simple answer since every installation is different. There is not only the resonance of the tower but the tower as loaded by rotatable yagis above the the tower. Even if the antennas are electrically isolated from the tower (e.g. Spiderbeam), the vertically-run feed line, and its capacitive coupling to the tower, can still interact with the loop array.
To gain some insight into the interaction I modelled a tower running up the centre of the array. This is an imperfect model for several reasons:
- The effective diameter of an open lattice tower is not the same as a solid cylinder.
- The tower may be directly connected to ground (lightning protection), but NEC2 does not support this configuration.
- Cables and yagis on the tower will, in general, increase the effective length of a tower since it all looks like capacitive loading.
What I found is that at 15 meters height the interaction was modest. The gain was not affected in the CW segment, gain dropped -0.5 db near 7.2 MHz, F/B was slightly reduced, and the SWR surprisingly improved above 7.15 MHz.
At 25 meters height the interaction was very minor, enough so as to not be a concern.
Between 16 and 23 meters height the interactions range from modest to debilitating. The peak effect (induced current on the tower -- see EZNEC current plot at right) is around a height (or effective height) of 18 to 19 meters. At these heights the impedance is a mess as the following SWR curve demonstrates. Gain is reduced by -1 db or worse and F/B is poor to non-existent across much of the band.
Unfortunately it is difficult to predict what to expect in any particular installation. With luck the effective height of the tower will be sufficiently raised by capacitive cable/yagi interactions to avoid the danger zone.
Update Feb 26: I ought to have mentioned that it is possible to detune a tower with a resonance on one particular band. I skipped over this possibility since I didn't have or know of any good references, and assumed it might be too difficult. One that I since found is this article by W8JI. It's worth a look.
At a height of 15 meters the 2-element narrow diamond loop array has ~1 db better gain than its structurally-closest wire yagi competitor, the inverted vee wire yagi. When account is taken of structural complexity, uncertainty regarding severity of interactions, or the ability to place either antenna at an apex height above 15 meters, the wire yagi may be the better choice.
Where the loop array excels are F/B and feed simplicity, which may sway one's decision. The choice is yours. As they say on the internetz: YMMV. I'll be carefully thinking this over before deciding what if any wire array to build when I am in a position to do so. That time may come later this year.