The principle is pretty easy. A normal dipole in free space normally has a 72 Ohm feed point impedance. A quarter wave ground plane is half this...about 36 Ohms.
By extending the antenna from a quarter wave to a three-eight wave, the maximum current point moves from the base to further up the radiating element. This has the effect of lowering the angle of radiation which is a plus.
The longer three-eight wave antenna now has a reactive feed point (inductive) as it's too long. This is tuned out with a series capacitor and it should be able to get close to 50 Ohms.
This antenna would suit someone who can only put up a quarter wave ground plane for 10 metres and wants better performance.
The article from the March 1983 edition of Short Wave Magazine is shown below...
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2 comments:
The mmana model of 16 feet vertical with a capacitor of 6 pF at basement and four radials provides input 28.2 mHz impedance of 1700 + J92 Ohm
The model is not functional. An another delusion of pre-computer era engineering....
73 VR2ZQZ
T
It would an appropriate to apologize for my previous comment.
The length of the vertical should be not 16 buy 13 feet.
In this case MMANA model of 20 mm tube of 3.73 m with 21 pF capacitor at basement shows 112 Ohm impedance at resonance of 28.5 Mhz and covers 28.0 to 29.2 with swr less than 1.5.
It can be feed by 50 Ohm coax via a quater wave transformer with excellent result.
73 VR2ZQZ
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