ANTENNAS & AERIALS 3 - Amateur Radio by M0MTJ

In my own installation, my loop is erected as a diamond shaped loop hung off a 33 ft pipe mast. It 'looks' to the North and South and is fed at the bottom, horizontally polarised with high quality 300 ohm feeder as a tuned line, and of course a good quality 4:1 balun and ATU is necessary. One aspect of the Quad Loop's operation that I find particularly valuable is its low noise 'receive' profile. As a person who has been plagued at this QTH by RF noise of many kinds, but mainly power line sizzle over many years, the low noise profile of the loop is much appreciated and the apparent cancellation of power line sizzle has to be heard to be believed!. It is much 'quieter' than dipole type antennas. Although operating well out of its resonant frequency range, it is also an excellent low noise receive antenna for general short wave listening applications, where the low noise pickup is also evident and appreciated.

Practical considerations: As one wavelength of wire is used in any given loop design, available space is a limiting factor and indeed 20 m is the only band where available space allows its construction at this QTH. It does require a fair bit of space. People who successfully build large rotatable Quad arrays have my complete respect! Thankfully though the diamond shape is easily supported off a simple pipe mast and installation is quite straightforward assuming sufficient space is available. I used 2.5 mm plastic covered 'earth' wire to make up my 20 m Quad Loop and it works fine. According to various references the actual loop 'gain' is somewhat dependent on loop shape, with a circle giving the highest gain and the 'delta' triangle shape the lowest. The square or related diamond shape is not too far off the circle in terms of inherent maximised gain. That's nice to know!

The Quad Loop in operation:

I guess this part gets a little subjective but there is something a little unique about even a single Quad Loop, in my opinion. As was mentioned earlier the low noise receive pick up profile is a major plus, in my opinion. Interestingly enough this aspect hasn't been mentioned too much in the antenna books based on my observations. The experience that has 'led me back' to the loop was the realisation of matters pertaining to directivity. Although a simple dipole, inverted V or Quad Loop is 'broadish' in its radiation pattern there are nulls off the ends which are actually somewhat more pronounced with a Quad Loop. Thanks to some compass checks I finally realised why my inverted V was not performing as well as expected. It was not actually pointing in the correct intended direction. It was in fact almost in the nulls of the antenna. Earlier evaluations with my Quad Loop suffered from the same positional flaw, and owing to the deeper nulls of the Quad Loop 'off the ends', this actually showed the Quad Loop up worse than the inverted V! This basic error took many years to realise! D'oh, as a certain TV cartoon character might exclaim!

In actual operation the Quad Loop is one of those highly dependable and consistent performing antennas 'putting out' a signal much better than it should, based on the minimal complexity of the design! S meter readings are always well up on the meter and I feel this is a HF antenna ideal for QRP HF duties. A highly recommended HF antenna! 73, Felix vk4fuq. 20/2/08.

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'To Err Is Human' or 'When One Can't See the Forest for the Trees!'

Yes there is an aspect of 'humble pie' to this story but I have decided to tell it anyway, a simple story of not being able to 'see the forest for the trees' or the inability to realize a very basic long term error! For some years I have had weekly skeds with an Amateur friend of mine in vk3 on 20 m. Over the years we have had many excellent skeds with good signals in both directions despite both of us using basic single element antennas. However in recent times this 'path' has become poor and many of our weekly contacts have been essentially non- existent!

As a consequence I have been giving much thought to improving my 20 m antenna set-up. One recent Saturday after lunch whilst having a doze and staring at the Amateur Radio Map of the World in front of me, it occurred to me that perhaps my inverted V dipole was not really 'looking' at the vk3 area at all, as I had previously thought that it was. Well, it wasn't! It was firing essentially to the SE and NW from this location in Northern vk4 and was skirting the edge of the vk3 area, a long way away from the intended target area in vk3, confirmed by a couple of basic compass bearing checks! I quickly ventured down to the backyard and reorientated the direction of the inverted V to 'look' due south and due north. Before I did so I noted the signal strength of Radio Australia on 15.240 Mhz from the Shepparton transmitter with the 20 db attenuator in circuit as a 'reference' signal . It was 5 db over S9. When I returned to the shack a few minutes later after repositioning my inverted V, the Radio Australia signal was now at 15 db over S9, a definite improvement!

I sat there for a few minutes as the significance of this sank in. In a flash of inspiration I decided to put my wire 20 m Quad loop back up again as now in this 'new' position it was completely clear of obstructions unlike its previous position in the same location as the inverted V in its original position when it was used for comparison testing with my inverted V, where it had a frequent unfortunate tendency to get caught up in our family's clothes hoist! The Quad loop was hauled up the mast in a few minutes ('Murphy' was obviously away for the day), and I did more receive tests. Radio Australia was still at around 15 over S9 on the Quad loop. I made a quick phone call to my friend in vk3 and arranged a contact on 20 m.

The result: Despite poor and disturbed propagation (a 'K' index of 5), a good solid contact at the 100 watt SSB power level with no problems resulted with the S meter indication at both ends 'well up' on the meter throughout the lengthy contact. I was quite happy, as one might expect! The whole incident has been a real eye opener as I had not properly appreciated the importance of correct aiming, even of a basic single element antenna. One does not ordinarily think of a single element antenna as a 'directive array'. An 'array', perhaps not, but 'directive', yes! I had always assumed that a single element antenna's radiation pattern was broad enough to be relatively non critical! Well, yes it is but there is a limit! The Quad loop actually does have a slightly sharper pattern with deeper nulls 'off the ends' than a dipole making optimised positioning even more important.

In the time since that fateful day I have rethought many aspects of this general situation and why I had not properly appreciated it before. My other antennas for HF are separate inverted V dipoles for 40 m and 80 m. As they are physically quite long, in this QTH they fit in the backyard pretty well as dictated by the available space. For this reason they run essentially in the N to NE and S to SW directions, firing broadside to the wires, yet they give excellent coverage to the desired vk coverage area as evidenced by my weekly 40 m Sunday morning WIA news broadcasts. This, despite being positioned poorly and firing mostly E (out to sea) and W, in theory anyway. Why? Well, it is worth remembering that inverted V antennas tend to have a somewhat more 'omnidirectionalised' pattern than true horizontal dipoles, and combined with the fact that dipoles on low HF frequencies and close to real 'Earth' tend to exacerbate this tendency! A big blob of radio frequency energy going just about everywhere and up is the main result. 20 m is a band where perhaps these 'omnidirectionalising' effects are substantially absent, making more precise positioning more important and practically mandatory. Well, it certainly works out that way in practice at this QTH!

Well my 20 m Quad loop is still up and whilst I'm still not totally convinced that it is noticeably better than my half wave inverted V for various reasons, such as 'effective height' considerations, it is working very well in practice and I have to concede that it does have some other unique virtues such as excellent noise cancellation on 20 m and also when used as a general coverage 'receive' antenna which as a keen SWL, I find a most desirable quality! Especially on the lower short wave frequencies and the AM medium wave broadcast band this noise cancellation and the resultant improvement in 'listening' S/N ratio has to be heard to be believed! For this reason as well as its good performance on 20 m, I think I might leave it up! I admit to having a considerable aversion to noise of all kinds!

Although in some ways I'm a little annoyed at how silly this error was, thankfully the antenna still worked quite well despite its non- optimum positioning into the vk3 area. Sometimes I think that it might be preferable if things simply didn't work at all when things aren't right rather than 'sort of working, after a fashion'. But perhaps I shouldn't be too annoyed as this sort of thing happens in life all the time, or so I'm told. Hi. Felix Scerri vk4fuq. 6/7//2/08.

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History of Cubical Quad

as reported by W6SAI and W2LX in "All About Cubical Quad Antennas", Radio Publications Inc. 1972

In the year 1939 a group of radio engineers from the United States travelled to the South American republic of Ecuador to install and maintain the Missionary Radio Station HCJB, at Quito, high in the Andes mountains. Designed to operate in the 25 meter short-wave broadcast band with a carrier power of 10,000 modulated watts, the mission of HCJB was to transmit the Gospel to the Northern Hemisphere, and to tell of the missionary work in the wilds of Ecuador. To insure the best possible reception of HCJB in the United States a gigantic four element parasitic beam was designed, built and
erected with great effort and centered upon the heartland of North America.

The enthusiasm of the engineers that greeted the first transmission of Radio HCJB was dampened after a few days of operation of the station when it became apparent that the four element beam was slowly being destroyed by an unusual combination of circumstances that were not under the control of the worried staff of the station. It was true that the big beam imparted a real "punch" to the signal of HCJB and that listener reports in the path of the beam were high in praise of the signal from Quito. This result had been expected. Totally unexpected, however, was the effect of operating the high-Q beam antenna in the thin evening air of Quito. Situated at 10,000 feet altitude in the Andes, the beam antenna reacted in a strange way to the mountain atmosphere. Gigantic corona discharges sprang full-blown from the tips of the driven element and directors, standing out in mid-air and burning with a wicked hiss and crackle. The heavy industrial aluminum tubing used for the elements of the doomed beam glowed with the heat of the arc and turned incandescent at the tips. Large molten chunks of aluminum dropped to the ground as the inexorable fire slowly consumed the antenna.

The corona discharges were so loud and so intense that they could be seen and heard singing and burning a quarter-mile away from the station. The music and programs of HCJB could be clearly heard through the quiet night air of the city as the r-f energy gave fuel to the crowns of fire clinging to the tips of the antenna elements. The joyful tones of studio music were transformed into a dirge of doom for the station unless an immediate solution to the problem could be found.

It fell to the lot of Clarence C. Moore, W9LZX, one of the engineers of HCJB to tackle this problem. It was obvious to him that the easily ionized air at the two mile elevation of Quito could not withstand the high voltage potentials developed at the tips of the beam elements. The awe-inspiring (to the natives) corona discharges would probably disappear if it were possible to operate HCJB at a sea level location. This, however, was impossible. The die was cast, and HCJB was permanently settled in Quito.

What to do? Moore attacked the problem with his usual energy. He achieved a partial solution by placing six-inch diameter copper balls obtained from sewage flush tanks on the tips of each element. An immediate reduction in corona trouble was noted, but the copper orbs detuned the beam, and still permitted a nasty corona to spring forth on the element tips in damp weather. Clearly the solution to the problem lay in some new, different approach to the antenna installation. The whole future of HCJB and the Evangelistic effort seemed to hinge upon the solution of the antenna
problem. The station could not be moved, and the use of a high-gain beam antenna to battle the interference in the crowded 25 meter international short-wave broadcast band was mandatory. It was distressingly apparent to Moore that the crux of the matter was at hand.

The Birth of the Quad

In the words of W9LZX, the idea of the Quad antenna slowly unfolded to him, almost as a Divine inspiration. "We took about one hundred pounds of engineering reference books with us on our short vacation to Posoraja, Ecuador during the summer of 1942, detrmined that with the help of God we could solve our problem. There on the floor of our bamboo cottage we spread open all the reference books we had brought with us and worked for hours on basic antenna design. Our prayers must have been answered, for gradually as we worked the vision of a quad-shaped antenna gradually grew with the new concept of a loop antenna having no ends to the elements, and combining relatively high transmitting impedance and high gain."

A Quad antenna with reflector was hastily built and erected at HCJB in the place of the charred four element beam. Warily, the crew of tired builders watched the new antenna through the long operating hours of the station. The vigil continued during the evening hours as the jungle exhaled its moisture collected during the hot daylight hours. The tension of the onlookers grew as a film of dew collected on the antenna wires and structure, but not once did the new Quad antenna flash over or break into a deadly corona flame, even with the full modulated power of the Missionary station applied to the wires. The problem of corona discharge seemed to be solved for all time.

The new Quad antenna distinguished itself in a short time with the listeners of HCJB. Reports flooded the station, attesting to the efficiency of the simple antenna and the strength of the signal. In his spare time, Moore built a second Quad antenna, this one to be used in the 20 meter band at his ham station, HC1JB, in Quito.

At a later date, after Moore had returned to the United States, he applied for a patent covering the new antenna. the fact that the Quad-type antenna radiated perpendicular to the plane of the loop was deemed by the Patent Office to be of sufficient importance to permit the issuance of a patent to Clarence C. Moore covering the so-called Cubical Quad antenna.

To understand the characteristics of the antenna, it is convenient to borrow the description of the Quad element given by W9LZX - "a pulled-open folded dipole."

This interesting account is taken from William I. Orr's book, "All about Cubical Quad Antennas". In it, technical details are addressed at length. However, there are only two pages devoted to the four element, full sized quad. Orr calls it the "Monster Quad". We know why.