Hi-Z Antennas HIZ-PC-4A Podręcznik - Strona 9
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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
12
34pf
16
43.6pf
18
48.4pf
20
53.1pf
24
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