Hf Antennas For All Locations Moxon Pdf

[Melchior Dow]

TheMoxon Rectangle comprises of 2 elements, a driven element and a reflector element. The tips of each element are bent towards the other with the ga being filled by an insulator to help support the ends of both elements. This results in a rectangluar shaped antenna with directional properties and very good F/B ratio.

G0KSC has taken the original design and applied modern computer optimisation techniques to talior performance to suit specific bands (these are not simply scaled from one band to another) whcih results in exceptional, band-specific performance.

Our antennas are constructed with the best quality materials in order that the best mechanical construction can be achieved, not the cheapest and most profitable! Even a digital caliper is used (with an accuracy of 1mm) to measure the elements during production to ensure they are within 1mm of what they should be, this ensures performance is delivered.

''Hi Justin thiught i would share this took my mini beam down and put up one of your 15 mt moxon ant so impresed by the 17 mt one aready worked kenya and south africa this morning next plan is aa 10mt one, Dave''

I am writing to tell you how pleased I am with both the mechanics and electrical performance of the Mox 14, two element beam antenna. The quality of the parts is outstanding with no burrs and everything fit together as it should. I am particularly impressed with the robust mechanical design of the element-to-boom attachment using threaded plates as well as lock nuts. I noticed the boom was bit bigger than in the specifications on line and the antenna bit heavier but that will make it hold up even better against snow and wind here on the eastern end of Lake Erie.

The antenna has been operational for the last three weeks and is mounted on a 35 foot, Penninger aluminum mast which allows me to raise and lower the mast and antenna with a hand winch. I would love to have it up 60 or more feet but the presence of power mains along the rear of my yard and certain building codes limit the height I can use. Still, the antenna outperforms my doublet array (3 non-resonant, selectable 55' doublets at the same height.) The 4 dBd gain helps a bit but it is the great front-to-back ratio that really improves my receive capabilities due to the improvement in signal to noise ratio. The antenna has a very low VSWR across the whole 20 meter band and the front-to-back ratio is still about 18-20 dB at the band edges and appears to exceed 30 dB near the band center Unlike other manufacturer's antennas, this model meets its published specifications. I did not want to compromise gain or front-to-back since I already had to compromise on antenna height and also wanted a very light weight and rugged antenna that would really improve my 20 meter performance. The MOX 14 exceeds my expectations in this regard.

This antenna is made with tapered element sections starting at 1'' tube and finishing at 1/2'' tube. It also has fully insulated elements which will ensure continuous, high performance and ensures corrosion will not impact performance. Marine grade stainless steel is used throughout. M4 terminal coaxial termination is provided on this antenna. The boom is 1.25'' inch square 16SWG aluminium to ensure strength and rigidity and has a safe wind speed handling of well over 100MPH.

No figures are made up here as they are in some Ham Radio adverts, all performance figures are verified in the very latest software simulation packages with some antennas being professionally confirmed on an antenna range.

The Moxon rectangle is a 2-element array using dual coupling between elements toproduce its nearly cardioidal pattern. Because it depends upon both the mutualcoupling between parallel portions of the elements and the coupling betweenelement ends, it is not amenable to the addition of further elements forincreased gain. In other words, a Moxon rectangle is not expandable by theaddition of director in the manner of a standard Yagi.

An alternative route to increased gain is the stacking of like antennas andfeeding the antennas in phase. At HF, operators employ stacks for a number oftasks. Combined, the antennas deliver the highest gain available. However,separately, the two antennas in the stack exhibit different take-off (TO)angles--or elevation angles of maximum radiation. Having a choice among the potentialsallows the operator to elect an elevation most suited to a given propagationpath. At VHF, additional gain is normally used solely for the purpose ofincreased gain.

To answer these questions, we shall divide the stacking question into two parts:the vertical stacking of horizontally oriented rectangles and the verticalstacking of vertically oriented rectangles. Since the latter type of stack isnormally used only at VHF and above, we shall begin with the more general case.

Fig. 1 shows the general outline of a 2-stack of (or stack of 2) Moxonrectangles. We normally begin for convenience with two identical antennas,although it is not at necessary that we do this. With sufficient patience, wemight customize each antenna for its position in the stack. However, the addedbenefits of such tedious work rarely outweigh the design and construction effort.

We must decide upon a base height for the array. Normally, we decide the baseheight of the lower antenna in the stack based upon task specifications andconstraints. Since this discussion aims for some general ideas rather than atask-specific design, we shall arbitrarily select base heights of about 1wavelength and about 2 wavelengths for the exercise. These heights translate at146 MHz--the center of the 2-meter amateur band--into about 80" (6.67') and 160"(13.33'), respectively. Above a base height of 2 wavelengths, about the onlyparameter of operation that will change is the TO angle. Hence, the selectiongives us a fair representation of performance.

The next decision involves the distance between the lower and the upper antennasin the array. We shall have more to say about this variable at the end of ourMoxon discussion. However, in general terms, arrays with low element counts tendto show that the separation required for maximum gain from the array and theseparation required for retention of the high front-to-back ratio that is ahallmark of the Moxon rectangle are not the same distances. In fact, they areso far apart that we obtain very different patterns for the two conditions.

With a base height of 1 wavelength, as shown in the patterns in Fig. 2, themaximum gain pattern shows a considerable vertical elevation lobe that resultsfrom spacing that is just above a half-wavelength. The maximum front-to-backpatterns result from a separation of just over 1 wavelength. The azimuthpatterns, taken at the TO angle, show the differential of front-to-back ratio,but do not reveal the forward gain differential of just over 1 dB. The elevationangle for the maximum gain configuration appears normal, that is, similar to thepattern of a single array at a height that is about 2/3 the way between the twophysical antennas. However, the maximum front-to-back stack shows oddities thatdo not appear with a single antenna, including a high variability in the strengthof secondary elevation lobes. The number of lobes is a function of the addedheight of the top antenna and the fact the that elevation structure is a functionof the lobes produced by both antennas. Since the radiation combines at adistance from the pair of antennas and since it also includes ground reflectionsas well as direct radiation, the pattern of radiation lobes and nulls can beerratic to the eye, even if predictable and calculable in straightforward ways.

Fig. 3 repeats the same modeling exercise using a 2-wavelength base height.Between the azimuth patterns in this set and the one for a 1-wavelength height,there is little to choose except for a slight addition to the forward gain forthe higher pair. However, the elevation patterns, even for the maximum gaincondition, begin to show some of the variations in lobe strength that we saw onlyin the maximum front-to-back pattern at a 1-wavelength base height. We find moreelevation lobes as a simple function of the greater overall height of theantennas. However, on either side of the most vertical lobe, we see some"suppressed" lobes, that is lobes that are weaker than we might expect from asingle antenna. These are functions of the complex combinations of direct andreflected radiation from each of the antennas composing the stack.

The maximum front-to-back elevation pattern is especially interesting. It breaksthe single large forward secondary lobe into 2 lobes, with additional strong andweak lobes, relative to the comparable pattern in Fig. 2 for a 1-wavelength baseheight. We can see that the stacked array offers strong radiation either at verylow or very high elevation angles.

Now let's overlay the patterns in our minds and note the similarities ofcorresponding elevation patterns for both base-height levels. In the maximumfront-to-back elevation pattern, we note a low angle main lobe, at least onestrong lobe at mid-elevation angles, and a moderate strength vertical lobe--orperhaps "bubble." The maximum gain patterns for each base-height level showincreasing radiation strength below about 35 degrees, with a single vertical lobeof considerable strength. These pattern similarities are more than accidental.

As we increase the height of an antenna or a complex array, such as our stacks,we find that elevation pattern properties tend to repeat themselves every 1/2wavelength, allowing for the addition of new lobes with every significant heightincrease. Since the pattern pairs in Fig. 3 are almost exactly 1 wavelengthhigher than those in Fig. 2, we should expect to see the overall outline of thepattern repeated. The phenomenon is perfectly general. Hence, you may beginwith an antenna at any height above about 1/2 wavelength and then check thepatterns at half-wavelength intervals above that. A single antenna will do forsuch a modeling exercise, but the principle applies as well to complex arrays.

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