capacitive loading theory

[guo-hai]

Hi all, is there anyone know why capacitive loading can reduce antenna size? What is the theory behind this?
Thanks

 

[E Kafeman]

It is easier to understand with a bit modified logical thinking.
For a electrical short monopole antenna (<0.25 lambda), is it the opposite, antenna size grows if components are added.

A capacitive top-load increase a monopole antenna length, both electrical and physical.

Measuring a short monopole antenna impedance without any top-cap shows that antenna load at bottom feeding point is somewhat capacitive.
To compensate for that reactive load can a serial inductor be inserted at bottom of antenna.
If compensating instead is done 0.25 lambda away from this inductor location will a inductor become transformed to capacitor.
Top of the somewhat short monopole antenna is almost at that distance from bottom coil location, 0.25 lambda => at this location should a capacitor be placed to act as a bottom coil.

For same reason do a top capacitor not work very well as tuning element for a real short monopole antenna as the short antenna length is too short to transform top capacitor load to a inductive bottom load.

Monopole impedance, borrowed from https://www.aa5tb.com/efha.html

 

[chuckey]

Top loaded aerials improve efficiency by increasing the current in the vertical element. see :- https://commons.wikimedia.org/wiki/File:Jim_Creek_VLF_antenna.png
Frank

 

[biff44]

Top loaded aerials improve efficiency by increasing the current in the vertical element. see :- https://commons.wikimedia.org/wiki/File:Jim_Creek_VLF_antenna.png
Frank

how does that link have any applicability to capacitive loading?

 

[chuckey]

" It functions as a capacitively top-loaded monopole antenna; " I thought it was a good example, and the article provides further links on the subject.
It seems to throw more light on the subject then your contribution?
Frank