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Adjusting HF Antenna Lengths

 

01 April 2011

 

 

No, it's not an April Fool's joke. It is very handy if you can find out how much to adjust the length of a quarter wave whip or a half wave dipole rather than the outright cut-and-try method. 

 

The values below are not "true space" calculated lengths, they have been modified by a value of 0.96 to compensate for the "end-effect" found in a physical antenna. Note that some configurations may reduce this even further, down to 0.92, thus making the physical antenna even shorter.

 

The values supplied are for a 100 KHz shift in frequency. If your shift is more or less, multiply the value given in the table by a suitable factor.  (eg shift of 130 KHz = 1.3, 60 KHz = 0.6)

 

Please remember that this is a guide only - it is impossible to take into account all of the factors attributable to your particular installation.

 

The process of using the table below is simplistic. 

 

 

 

 

The following is an example of it's use...

 

 

 

There are a few things to note when making up new dipole or whip antennas :-

 

 

 

 

This info should make your task just that little bit easier......

 

 

 

Calculation of length adjustments for quarter and half wave sections at various amateur bands

 

 

 

 

 

( 300/4 = quarter wave radiator * end effect adjustment ) :=

 

 

Wavelength ref

72.000

 

 

 

 

Quarter wave

Halfwave dipole

 

Qtr wavelength

variation per 100KHz

variation per 100KHz

Freq

Metres

cm

cm

50

1.440

0.29

0.57

28

2.571

0.92

1.83

24.9

2.892

1.16

2.31

21

3.429

1.62

3.25

18.1

3.978

2.19

4.37

14

5.143

3.65

7.29

10.1

7.129

6.99

13.98

7

10.286

14.49

28.97

3.5

20.571

57.14

114.29

1.8

40.000

210.53

421.05