Many determined exponents of the art of 6M DXing have resorted to the stacking of Yagi beams. In the UK there are quite a few stations using a pair of stacked 6-element beams and sev-eral using stacked 4-element beams. This article takes an uncomplicated look at this technique to see why it works so well on 6M.

The Need for Low-Angle Radiation and Reception

Figure 1 - Reflection of Signals from F2 LayerFigure 1 - Reflection of Signals from F2 Layer

First back to basics! Figure 1 shows how 6M sig-nals from a transmitter ‘TX’ are reflected back to the ground from the ionosphere, in this case the F2-layer. As signals enter the layer at lower and lower angles of incidence they reach the earth at greater and greater distances from the transmitter. This applies to reflections from both the F2-layer and the E-layer. The height of the E-layer is around 100 to 120 km at midday while the F2 layer is around 200 to 400 km. It is clear therefore that much greater single-hop distances can be achieved by reflections from the F2-layer than the E-layer. Multi-hopping from the F2 also allows propagation over much greater distances such as to Australia and Japan.

Fig 2 - Angle of Incidence of Received SignalsFig 2 - Angle of Incidence of Received Signals

To maximise the distance your transmitted signal achieves for a given power (especially with the limited ERP in the UK), you must keep the angle of radiation from your antenna as low as possi-ble. This is especially important for extreme long-distance F2 propagation and weak-signal forward- and side-scatter propagation modes.
Figure 2 shows the reciprocal situation at the receiver. These graphs show the angle of incidence of signals arriving at your 6M antenna reflected from the E-layer and the F2-layer. From these it can be seen that signals at extreme distances arrive at angles of less that 10° in both cases. Consequently, it is important that if you want to maximise the strength of weak F2 DX in your receiver, your antenna should have maximum sensi-tivity at this angle or certainly less than 15°. This is not optimum for short-skip Es propagation as Figure 2 shows, and often not the case for meteor-scattered (MS) signals. It is also worth remembering that in practice, 6M signals often reach antennas by curious paths and angles, so reducing the angle of reception is not a panacea for improving the signal strength on all QSOs. OK, we all learnt all this stuff for the RAE - didn't we? How can we make use of this to improve the chances of working weak DX on 6M and beating the competition in pile-ups?

Single Yagi H-Plane Polar Diagram

Fig 3 - H-plane Radiation Pattern for a Single 6-Element Yagi at 1.0 Wavelength HighFig 3 - H-plane Radiation Pattern for a Single 6-Element Yagi at 1.0 Wavelength High

All of you, I am sure, are used to looking at an E-plane polar diagram of a horizontally polarised Yagi-Uda antenna showing such things as forward gain, front-to-back ratio and side-lobes on a decibel based scale. But with antenna stacking we are more inter-ested in the H-plane polar diagram. The H-plane lies at right-angles to the E-plane and shows the elevation or vertical radiation pattern of your antenna. Figure 3 shows the H-plane radiation plot of a typical 6-element Yagi beam at a height of one wavelength (about 20') above ground.

It can be seen that the main lobe peaks at an elevation of about 15° with the largest of many secondary lobes at 45°. This second lobe is only 6dB down on the main lobe so at least a quarter of your power is being ‘wasted’. If this and all of the other secondary lobes could be reduced then a lot more power could be sent to the DX rather than wasted. The reason as to why the main lobe lies on the -3dB line will become apparent later. As the antenna gets nearer to the ground the angle of radiation increases significantly to as much as 30° so this is why it is important to get the antenna as high as possi-ble above the ground and clear of all obstructions. The minimum height is probably about 1 and 1.5 wavelengths (20' to 30') before serious degradation of the radiation angle sets in. It can be estimated from the above plot that if a low-angle 8° F2 signal is received on an antenna such as this it could be down in signal strength by at least several dB compared to a better performing antenna and it could make all the difference between hearing a signal (or someone else hearing you) or not as the case maybe. For shorter skip Es propagation and meteor scatter work this antenna would be very effective with its large 45° lobe.

Vertical Stacking

It is often seen in amateur radio literature that it is possible to increase the forward gain of a Yagi system by stacking two antennas rather than adding extra directors. Theoretically, by stacking two identical antennas and phasing them correctly it is possible to in-crease the antenna gain by as much as +2.8dB. In practice you are likely to only achieve +2 to +2.5 dB. However, this is only the smaller of the advantages gained by stacking antennas. The greater advantage is the fact that the angle of maximum radiation and reception is considerably lowered thus realising an apparent gain on low-angle DX by as much as another +10dB. Take a look at Figure 4 below. The main lobe for the stacked antenna is not only showing +2.8dB of gain over the single Yagi (that is why the Figure 3 lobe was placed on the -3dB line), but more importantly the main lobe is lowered to a radia-tion angle of 11°. This angle is much closer to the received angle of long-skip, multi-hop, F2 DX. How's that for an advantage!

Fig 4 - H-plane Radiation Pattern for Two Stacked 6-Element Yagis at 0.75 and 1.5 Wavelength HighFig 4 - H-plane Radiation Pattern for Two Stacked 6-Element Yagis at 0.75 and 1.5 Wavelength High

On 6M it is very difficult to space Yagis 0.75 wave-lengths apart on a sensible sized tower. A much more practical spacing is between 12 to 16 feet (3.5 to 5 metres) with little degradation of performance. There is a price to be paid for the above advantages and this is reduced performance at middle-range angles of between 20° and 45°. Stacked antennas are not the best performers for short-skip Es and MS working. In reality though, the high signal strength of these propaga-tion modes on 6M means that you will never notice it. What are the important points to remember when stacking 6M antennas?

(1) Get your antennas as high and clear of obstructions as possible to keep the radiation angle low.

(3) Space antennas as far as part as possible, but if you can only manage 10' (3 metres) still do it!

(4) If you live in an electrically noisy environment lowering your angle of radiation might increase your noise level from local noise sources. It might also make TVI more prevalent, so beware!

(5) In my view it is better to spend your money on two smaller antennas and stack them rather than going for a single long-Yagi. Two stacked 4-elements is a good performer on 6M. Re-member also, two small antennas stacked could be far less visible than one gigantic long Yagi!

Overcoming the Downsides

Fig 5 - H-plane Radiation Pattern for Two Stacked 6-Element Yagis Fed In and Out of PhaseFig 5 - H-plane Radiation Pattern for Two Stacked 6-Element Yagis Fed In and Out of Phase

The one downside of stacked antennas is the loss of performance in mid- to high-angle radiation and reception. Although you will probably never notice it, if you are a perfectionist and an experimenter there is a ‘simple’ way around the problem. It is possible to electrically switch the radiation pattern between a low- and high-angle pattern. It is normal to drive the antennas in a stack with in-phase signals using a phasing harness such as described in the last SIX NEWS. This provides you with the optimum radiation profile as shown in Figure 4. If however, the antennas are driven out-of-phase by introducing an extra half-wave length of cable into one arm of the harness the radiation pattern switches high - up to as much as 30°! This is shown in Figure 5. By switching the half-wave piece of coax in and out with remote relays it is possible to experiment to see whether summer Es signals increase in strength when the antennas are fed out-of-phase - I am assured they do! Whether it is worth the extra effort and hardware will be left to your discretion! Another very simple way of overcoming the disadvantage is to wind the tower down as low as it will go thus increasing high-angle radiation!

The author's stacked 6 element beamsThe author's stacked 6 element beams.

Areas for experimentation

All of the above has been written on the assump-tion that the two stacked antennas are fixed to the same pole and rotated in unison. It is possible to experiment with other configurations. For example, the bottom antenna could be fixed in your worst direction, while the top one is rotatable. This would give you the benefit of stacking in your worst direction while being able to listen in two directions when beaming elsewhere. Thus you will never miss out on DX because it is on the back of your beam! Alternatively, you could switch between the top, bottom or both antennas for instant antenna direction changes. It is also possible to use two rotators, the one at the bottom turning both antennas in unison, while the one at the top just turns the top antenna in relation to the lower one!

It has been a general maxim in amateur radio for many years that if you have a spare pound to spend, spend it on your antennas first. This couldn't be more true than on 6M. So go and get stacked as I am confidant you will notice the difference on your first F2 opening with stacked antennas. To finish I can thoroughly recommend the following reading material if you want to know more about stacking antennas, the second reference is the ‘bible’ on the subject. Both are available from the ARRL and RSGB.

In summary

Sidelobe Noise Power Leaks Are Worst From Top And Bottom

If you use an antenna program like Yagi Optimizer with the horizontal (E-plane) plot you really don’t know what happens to the top/bottom lobes, since the 3 D-pattern is always at minimum toward the end of the dipoles. Therefore a smooth E-plane plot can show good F/B ratio, but it can hide big lobes in the vertical plane. Therefore I recommend to observe the H plane, and always optimise with that option activated. I only have the old 1988 version of YO but I have seen the newer one, and it is really a marvellous tool. I have also the Antenna Optimizer 6.35 (1994). In clear environments it may be worthwhile for best F/B ratio to slightly adjust antenna dimensions when running a stack instead of a single yagi. On a cluttered tower it’s hard to predict all the de-tuning unless it is completely in the model.

If you know the H-plot, you automatically know the E-plot, since it is the H-plot multiplied with the dipole pattern. Small H-plane sidelobes give still smaller sidelobes in any other 3-D direction. In total integrated noise power this means that the bulk of the galactic noise is trying to creep in from over your head, unless your preventive measures are rigid.

0.6 Wavelength Stacking Forgives Poor Designs a Lot (but not everything)

Look at the plot of two in-phase dipoles (in almost any antenna book) with about 0.6 wavelength spacing (like the two driven elements of stacked yagis). The pattern is compressed enormously to the sides, and gain increases to 4.9 dB over a single dipole. However, the 0 dB F/B ratio does not change.

With stacked yagis at 0.6 wavelength a poor F/B ratio is not forgiven, but anything to the top/bottom sides (and skew) is tremendously attenuated. H-plane sidelobes in wide cones around the Z-axis are virtually eliminated. However, the more complex mutual coupling due to parasitic elements limits stacking gain to lower values than with dipoles, but with very short yagis the 3 dB mark can even be slightly exceeded.

There are two simultaneous benefits: Overall parasitic noise power is greatly reduced, and nearly 3 dB additional gain is produced with the stack. The net S/N ratio gain with uniform noise from all directions is at least 5 dB, but with the cosmic hot spots much bigger contrasts occur. Remember also that the earth turns with a speed of one degree in every four minutes, so certain azimuths are not ‘jammed’ all the time. The sharper your main lobe, the shorter the suffering. We need someone with a good ‘electronic noise map’ (a sky noise data map on a PC) to calculate the advantage for a specific stack compared to a single yagi.

0.6 Wavelength And 1.2 - 1.4 Wavelength Stacks for Maximum Gain

Stacking at 0.6 wavelength always assures the noise power cancellation, but if booms get long (say longer than 8 m/ 27 ft), the stack starts to look as a single yagi gain-wise. The stacking gain may fall well under 2 dB (which is still a lot), but if the single yagi pattern is relatively clean it is better to go to for 1.2 wave stacking for moderate length yagis and 1.4 wavelength stacking for the long ones. This is just due to the mutual impedance: a 0.8 wave separation is always poor for gain and sidelobes (see the consistent gain dips in W2PV’s yagi book).

With four antennas it is superior to use non-uniform stacking: first stack 0.6 wave with each pair, and then allow the two bays to cancel the residual sidelobes of the separate bays (then there will be hardly any noise power from unwanted directions). For maximum gain shortcuts can be made, but due to the S/N issue on 50 MHz, I would never neglect the 0.6 wave basic cell as the building block for optimal listening. This is also why VE7BQH’s collinears on 144 MHz work so well. Close spacing and careful current balancing eliminates all parasitic noise leaks.

Combining Low/High Antenna Patterns with Wide Spacing

It is a fairly wide-spread misunderstanding that you can simply connect a low and high antenna to one cable and that the coverage is then the same as with each one separately. Remember that the pattern is a summation of field vectors and that the combination generally forms a zero field at at least one angle of takeoff where each antenna would launch a considerable field, when used alone. Also the result is 3 dB worse at an angle where one of the antennas has a pattern null due to ground reflection (since the power has been shared; half lost).

Also stacking of two antennas does not lower the takeoff angle, but it will correspond to the average height of the two (when fed in phase). Therefore the maximum field of the higher antenna will peak at a lower angle, but the higher gain of the two makes that the combination will still produce a stronger field at this same lower angle, though the peak strength of this lobe is a bit higher.

Varying the phase between the two yagis gives new orders of freedom, but it always weakens the field in an initial maximum (any new field maximum will be lower in strength and offset in elevation).

Terrain Analysis Is Not Sufficient in One Dimension

A relatively flat environment (or sea/lake) will produce a fairly neat analysis assuming that things are fairly homogeneous over the penetration depth of the wave. This is a completely neglected issue, since a thin layer of wet clay over rocky ground may not suffice as a real substitute on lower frequencies.

Most hilly landscapes tend to act like optical lenses, but it is hard to get into a clean focus. The problem about a focus is that when that makes a signal terribly strong in one place, it must be weak somewhere else. This is why a Fresnel zone is always an area and has to be considered in full for even a two-dimensional far-field analysis.

A nice example of the preceding is the story of VK3MO, who used to drive around in Australia (probably on his motorcycle), while listening to the BBC transmissions. When he finally found an area where signals peaked awfully high, he bought his new QTH at the hot spot. I remember working him on 14 MHz, while turning my power down to about 20 milliwatts, and still got a 57 report (loud and clear). My sea reflection was a big help too (though only 20 m asl at best, near perfect on 14/50 MHz horizontal -polarisation).

Pileups, S/N Ratio and One-Way Propagation

Most so called one-way propagation I tend to explain with local S/N ratio and the fact that signals from small antennas tend to drown in the noise, but big signals keep at least head and shoulders dry. A pile-up is the most brilliant example of that. Why is one calling for eight hours without results, and the other makes it with one shout, while both receive the DX at reasonable strength? It is not one-way propagation at the DXer’s end, and even less at yours! On receive, think about the pile-up as the cosmic noise and the weak DX as the new DXCC you need. Your stack pushes the noise floor down to get one more, and otherwise you would witness a dead band.

No one can understand the difference of a few dB unless he has worked a couple of years with a piece of wire, and is then allowed to work with a six-element monoband yagi for a day or two. The contrast will be clear when returning to his own setup - this is why newcomers should always play first with a simple antenna, albeit just for a while. Due to the inflation of S-meter scales, most reported differences of a few S-units are not more than those few dB, but in S/N ratio those values may often approach the truth.

Ordinary And Extraordinary Ionospheric Modes

With microwave equipment, Magic T’s and the like are used for one-way propagation (isolators etc.). These mostly require magnetic fields and coupling of different modes. In geophysics we may expect one-way propagation when the geomagnetic field is ready for the game. That is when the magnetic field is perpendicular to the movement of free electrons induced by the impinging radio wave. Therefore there are ordinary and extraordinary waves in ionospheric propagation, and the MUF is about 1.4 MHz (the gyro frequency) higher for the highest extraordinary mode. This may explain an effect like WD8ISK’s vertical beating a six-element yagi, but it could as well happen that the elevation of the yagi was such as to make a null for the vertical angle of arrival. A polarisation rotation is also possible, but will gradually continue to the other polarisation in time.

One-way propagation may occur when the extraordinary wave is excited to one path direction and inhibited to the other. The cause may be the initial state of polarisation. Since waves tend to gradually change polarisation in ionospheric layers (Faraday rotation effect like in EME) it is not that simple, and we may again play with the idea of crossing marginal S/N levels when it seems to appear. This means that the process is there but it is not completely on/off. The stack will again provide a rescue close to ‘off’ state.


Brewster and Polarisations

For poor ground the Brewster angle is high and this makes patterns of horizontally and vertically polarised yagis or groups very similar for the DX angles. Mainly at the ocean shore (‘liquid copper’), the maxima are interlaced to cover practically all angles, while switching between the two polarisations. Therefore, just switching the polarisation is not sufficient to cover all angles inland. Antennas are required at least at two different heights, and should also have provision to operate separately. The vertical reflection coefficient from poor ground is worst at the Brewster angle (with a 90 degree phase shift) and is always weaker than the horizontal coefficient.

Common Volumes with Scatter

With sliced patterns due to high antennas and good ground reflections, there will be a multitude of common slices in the cross section. The total common volume and the scatter angle determines average levels (height is a bonus, even on one side).

Tilting Mountain-Top Yagis

I remember a Boulder Foothills Field Day in Colorado with the W0DK group in 1985. It was a steep down-slope so ground reflection was not useful. The city covered most of the below horizon angles, and most noise was spiky man-made. Since the main lobe of a single yagi is fairly wide, tilting did not help much. Any 0.6 wave stack would have been the answer (and partly hiding behind the edge, if the top had been wide enough with a proper flat tilt for ground reflection gain).