Two Element 10 Meter Beam

TenMeters One of the easiest ways to have a better signal is to build a two element beam for the Ten Meter band using a driven element and a director. Why two elements? If we consider the dipole to be the fundamental antenna (everything is always referenced to it) the two element beam will give the greatest amount of gain per unit size of any antenna that I am aware of. Why a parasitic beam? The element lengths are not critical and the boom length is definitely not critical in order to have a functional antenna with around five dB gain. There are various charts in many books published about antennas and they do not all agree! I have read a good many of them and have tried to filter out a lot of the extraneous information for this article. I will try tobe practical and not too theoretical. Basically the two element beam is nothing more than two dipoles parallel to eachother.One is cut to the frequency in use and is connected to the feedline. The other is tuned either above that frequency (acts like a director) or below that frequency (acts like a reflector). There are several parameters of the beam that are important for us to consider and for the most part changing any one of them will change the others. These parameters are: Radiation resistance, forward gain, front to back ratio (F/B ratio), front to side ratio and overall pattern. Most users are interested primarally in forward gain. From an interference reduction standpoint the front to side and front to back ratios are important by products of obtaining forward gain. The boom length is a major factor in front to back ratio. There is usually always a significant null to the side of a parasitic beam antenna. The boom length is also a factor in forward gain and radiation resistance. The designer needs to make decisions that ammount to compromises depending on the design goal. Why use a director and nor a reflector? An important choice is will the parasitic element be a reflector or a director? In most cases the director is the best choice. As the spacing between the two elements increases the gain increases to just over 5 dB for spacings around one tenth of a wavelength. That is just over three feet on ten meters. The peak for a director comes just before the peak for a reflector and is just a little higher. The radiation resistance increases with spacing. Increasing radiation resistance is desirable but with a two element beam there is no need to sacrifice any significant gain in order to do this. When faced with a choice of the same gain either with a lower or higher radiation resistance I would suggesr that you opt for the higher. Front to back ratios are higher with a director with close spacing and decrease from approximatly 20 dB for very close spaings to around 10 dB for the spacing that gives maximum gain . For a reflector the boom length must almost twice as long for a 10 dB front to back ratio. Consequently for similar performance a parasitic beam using a reflector must have a much longer boom and the reflector element itself will be longer than the director and for the same F/B ratio will have less forward gain. The antenna will also have more wind load simply because it is bigger. From a practical viewpoint, a two element ten meter parasitic beam consisting of a driven element 16 feet 8 inches, a director 15 feet 8 inches and boom length from three to five feet will give five dB gain over a real dipole and a front to back ratio from 5 to 10 dB. The radiation resistance will be in the neighborhood of 20 to 25 ohms and very easy to match with a simple gamma match.Gamma match??A gamma match is a common way to match the 50 ohm impedance of standard coax to the driven element. To split the driven element in the center, insulate it and make it structurally strong so that it can be fed like a standard dipole is possible but there are so many problems I do not recommend it. In addition the the mechanical problem, the radiation resistance will be around 20 to 25 ohms and will insure a SWR of at least 2 to 1 unless some form of impedance matching is used. Sure the boom length could be increased untill the radiation resistance came up to near 50 but you will have lost most of your gain!For most of us it is much easier to use a gamma match. You do not have to understand it in detail. Theoritaclly it is complicated, practically there is not much to it.For ten meters all you need is a small box to house a small air variable capacitor (mount the box on the boom near the driven element) and a "gamma rod" which is a rod mounted parallel to the driven element about 2 feet and connected electrically to the driven element at the far end. The feedline (coax center conductor) is connected to the near end of the rod through the capacitor. Basically the gamma rod is connected to a point on the element that is a higher impedance point than the center and the capacitor compensates for the inductive reactance of the rod itself. I have included sketches of the antenna and the gamma rod. In most cases the rod can be mounted to the element with one uninsulated clamp providing both mechanical connection and electrical connection. If additional support is needed it will have to be insulated.How high?After the antenna is constructed it should be mounted at some distance from the ground.Since radiation at high angles (especially on ten meters) goes out into space and is not useful for communications we need to minimize it. The radiation angle is very much influenced by the height of the antenna above the ground. Any antenna half wave above or a multiple of a half wave above ground will have minimum radiation straight up. This is most desirable, but a minor problem is that the "ground" I am talking about is not necessarially at the surface of the ground. If the surface were salt water then we could measure half wave from the surface and we would have it. Earth ground gets more complicated. Let's just say the wetter and more salty the earth the closer to the surface of the real ground you will find the "antenna" ground. Because it is too hard for me to do I just measure from the real ground. Antenna heights of 33 and 66 feet are very good for maximizing the low angle radiation on ten meters. You will find, as many have, that 100 feet is too high for most ten meter work. A low antenna can beat out a 100 foot high antenna working Europe, Africa and the Carribean area from the east coast! Of course working a VU in India is a different story. For most operating you will find a two element beam as I have described mounted 33 or 66 feet above the ground (preferably in the clear) will be a very good antenna especially for its size and the small effort needed to construct and install it.

 

Sketch

A gamma capacitor with a max value of 50 mmf should work. Tune it for minimum SWR. Adjusting the length of the gamma rod may be necessary depending on the element diameter and the gamma rod diameter. \

My element diameter is 1 inch OD and the gamma rod is 3/4 inch OD with a center to center spacing of 2 1/2 inches. I need about half the capacitance available in a 50 MMF variable.

Gamma rod