Hi-Z Antennas HIZ-PC-4A Manuel - Sayfa 9

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HIGH PERFORMANCE HF RECEIVING SYSTEMS & COMPONENTS

Appendix A: Element Construction EXAMPLES

Over the years since the Hi-Z Antennas array availability announcement there have been countless

designs of the antenna elements themselves and the necessary mounting type of insulator. As it turns out the

length of the elements are not necessarily critical as long as all elements in an array are the same length and

physical construction. Recommended element length is a minimum 16 feet and a maximum of 24 feet for use

down to the 160-meter band. Arrays with few elements like the three and 4 element arrays can use the shorter

elements while the best 8 element arrays should use the 24-foot elements. This is due to higher combining

losses when combining the higher number of elements.

The mounting insulator requirements have a needed resistance value in excess of 2 Megohms

resistance and very low capacitance to surrounding grounded objects. The suggested size elements present

themselves as a series connected source impedance consisting of a very small capacitance in pico-farads

and a very low radiation resistance. The ground resistance of the needed ground post adds to the radiation

resistance to produce the source resistance value. For best performance the ground resistance of the ground

post used should provide some values less than about 50 ohms.

Length

inches

144

192

216

240

288

Accompanying Radiation Resistance for all these calculations were less than 1 Ohm.

These relatively low values of source capacitance available shape the requirements of the intended

amplifier used to convert the signal from this very high impedance to 75 ohms matching the intended

connecting cables. These low source capacitance values also shape the requirements for the element

mounting insulators. The amplifier input capacitance and the element insulator parallel capacitance both add

together as a load capacitance to the element. As an example, from the chart for 12 feet long 0.25-inch

diameter element the source capacitance is 34 pf. If the combination of the mounting insulator capacitance

and the amplifier input capacitance happened to add up to this 34pf then the available signal to the amplifier

would be ½ half of the open circuit element voltage. That is 6 dB or one S-unit loss due to capacitive division

of the voltage. This division makes the case for keeping insulator capacitance small.

Hi-Z has settled on 16 to 24 feet element length with at least a 7/8 inch OD starting diameter for the

elements. The longer length being our best recommendation. This gives us a source capacitance to work with

Typical Source Impedance Calculated Values

Length

.250 Dia.

Feet

34pf

43.6pf

48.4pf

53.1pf

62.8pf

.50Dia

.75 Dia

38.4pf

41.6pf

49pf

52.8pf

54.2pf

58.3pf

59.4pf

63.8pf

69.9pf

75.0pf

1.0 Dia

.040 wire

Dia

44pf

26.1pf

55.8pf

33.8pf

61.6pf

37.7pf

67.4pf

41.5pf

79.0pf

49.3pf