Producing Low Angle Radiation on all the HF Bands from a Suburban Garden BY G3TNO (SK)

Most radio amateurs would like to be able to erect aerial systems that produce good DX performance and at least be able to produce a competitive signal. Thoughts turn to large high, yagi or quad systems. However, lack of space and neighbours, who do not consider those aerials as things of great beauty, limit the possibility of using them. 

Many of the alternatives do not perform well enough to even be considered, i.e. low height doublets, dipoles and, probably worst of all, trapped dipoles, or GSRV types. In the main, on 160, 80, 40 metres, they produce little or no low angle radiation, that is so essential for good DX performance.

A number of operators use end fed long wires with the feed point and in some cases, the RF current maximum, actually inside the shack.  The RF current, will never be in the same position on the wire on each of the various HF bands. See simplified Fig. 1.

Advantages:

• Cheap
• Simple to erect
• Can be unobtrusive.

Disadvantages:

The feed impedance’s on the various HF bands is wildly different, on 40, 20 and 15 metres, I did find it rather difficult to obtain a decent match. This is because the feed impedance on those bands is in excess of 1000 ohms A further problem being that the ATU tends to flash over on bands with the higher feed impedances.

The performance on the various HF bands is very dissimilar, due to the fact that the current maximum is or, are (Note: on higher bands there can be more than one current maximum) in a different position on the wire and, the actual position of the maximum depends upon which band is being used and this consequently determines the actual angle of radiation.

My big wish over many years (36 actually) and having tried very many different aerials, was to find a system that would at least overcome some of the problems, while not being too visually obvious to my neighbours. 

During a Sunday evening QSO with G3KXF and G4FFE, discussing aerial systems, as usual! Adrian G4LRP broke in and asked if I had tried a Grounded Marconi? Adrian and I QSY’d to the telephone and he gave me the details. He told me that it’s design concept was very old I later found further information on the design in an old book, dated 1923. 

It can be seen from Fig 2 that this version of the Grounded Marconi is a large loop. The dimensions provide a most effective non-resonant aerial system. The dimensions are NOT critical, provided the total loop length is not a multiple of one or more half waves. This is to avoid very high impedances at the feed points A and B. These would cause high RF voltages to be present 
at the ATU output. I also found it best to avoid total loop lengths that are odd multiples of a quarter wavelength, i.e.,1/4 3/4 11/4 etc., which would result in fairly high RF currents flowing at points A and E. The dimensions I have given in Fig 2 seem to produce feed impedances in the approximate range 200 to 700 Ohms. I have had no problems matching the aerial to 50 Ohms on all bands using my home brew T-Match ATU. 

RF power is applied via an ATU (a smart tuner is perfect). However, a simple manually tuned ATU is fine and in my case is installed in my outdoor wooden shack. The ATU output is fed to the aerial at A and flows via B, C to D and then through the Heavy Conductor back to the ATU earth terminal B. Now, the clever trick of this aerial is that maximum current will ALWAYS be present in the lower part of the wire section to D. This is because point D on the aerial is firmly grounded and therefore, at Zero RF potential, but is also at maximum current. This condition is met on 
ALL bands. The high current flowing in section C to D will produce large low angle vertical polar diagram. From my trials of this aerial from two locations in the Worthing and Rustington areas, radiation appears to be almost non-directional, and I have found it to be a very effective performer on All bands from 1.8 up to and including 50MHz. 

NOTE:  The ground spikes care for lightning protection only. They have little or no effect on RF performance. To derive optimum performance from a Grounded Marconi, it is of utmost importance to lay as extensive a counterpoise and radial system beneath the aerial as your circumstances permit The more radials and or counterpoise wires the better the DX performance. 

Lengths of scrap UR67 cable with the inner conductor and screen shorted together, or areas of chicken wire, all bonded together, or what you have at hand, will improve the ‘ground’ 
conductivity under the aerial.

It has been shown by many separate experiments over many years, that above ground radials or counterpoises give far superior performance benefits over ground rods and buried radials. Thus, any conductors should be close to the surface of the ground, only being ‘pushed’ into the ground sufficiently and only where necessary to render them ‘invisible’ and to prevent tripping.

A to B : 58ft PTFE covered standard wire slopes from outdoor shack to pole on boiler 
chimney.  

B to C : 51ft PTFE covered wire  

C to D : 35ft. If a metal pole is used to support the aerial, the pole itself can be used or, if the support is non-metallic, then a heavy gauge conductor should be used for this section. If a wire down lead is used here, adding additional parallel wires has been shown to improve the radiation efficiency. 

D to E : 51ft. Very heavy gauge conductor; URM210 or UR67 are ideal. 

For its size and cost, it is truly amazing!! e.g. 55 countries on 160 metres in the first ten weeks of operation from my Salvington QTH. I also found that long haul DX performance on the higher 
bands has produced good DX contacts with places I had never worked or heard in my previous 35 years of operations. 

I would like to thank Adrian G4LRP for telling me about the Grounded Marconi in the first place. Also the great amount of help over the past 3 years with very many comprehensive tests by 
G3KXF and G4FFE as well as others near and far. Finally, thanks are due to Guglielmo Marconi for inventing the aerial many years ago.

Ed’s Note: Malcom G3TNO was a long-time member of HARC and was one of the small group of amateurs who reformed the club back in 1970. Malcom was one of life’s real characters, an avid home-brew builder (the story of his cat asleep in his nice warm HF linear PSU will have to wait until a later date!), experimenter and CW operator. Malcom passed away some 20 years ago, but is still fondly remembered. 

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