How to reduce the length of antenna???

[ZengLei]

For example :there is a small radio station ,and its frequence is 30MHZ,if we use the antennas with 1/4λ,so the length of the antenna is 2.5M,and it's not very convenient to carry,and i want to reduce the length of the atenna without reduce the power the antenna transferred,any idears?????

Thanks~~~~~~~~~~

 

[vfone]

“There is no magic to make a small antenna act like a large one”
Always will be a compromise getting the gain and the efficiency.
To improve efficiency on small antennas have to make the current distribution as uniform as possible over the length of the antenna, and try to don’t bent or fold the radiating element in the high current areas.

 

[snkhan]

Hi
Vphone
"To improve efficiency on small antennas have to make the current distribution as uniform as possible over the length of the antenna, and try to don’t bent or fold the radiating element in the high current areas."

what happen if we bend the antenna at high current areas

 

[sundarmeenakshi]

Hi
Vphone
"To improve efficiency on small antennas have to make the current distribution as uniform as possible over the length of the antenna, and try to don’t bent or fold the radiating element in the high current areas."

what happen if we bend the antenna at high current areas

incrase the frequency so antenna size will reduce

 

[snkhan]

Hi
sundarmeenakshi

I have actually asked that what will happen if we bend the antenna at high current regions

Regards

 

[mfarhan1]

folding or bending antennas
well at low frequencies it dont creat problem but at higher frequencies the discontinuities themselve become radiating objects and radiations starts emitting from bends. also if you make a loop of antenna it will change the input impedance of antenna like in case of folded dipole.

 

[reza368]

use array antenna

 

[sinatra]

Normally antennas for this frequency range are made using an inductor in series with a reduced length rod.
Long ago I used to repair CB (citizen band)and other types of amateur radios. I was very much used to those antennas with size around 1.5m or less.
As you can see the lambda/4 is about 2.8m at 27MHz. So, in fact the antennas I used to have were approximately lambda/8 with an inductor in series.
There was also a screw to allow the antenna to be tunned, i.e., to adjust the length so that the antenna resonates with the series inductor and we get at the antenna input terminal a pure resistive load (imaginary part = 0).

An antenna shorted than lambda/4 behaves as a capacitive load (pure resistive load with a capacitor in series). An antenna slightly longer than lambda/4 behaves as inductive load. So it is expected that we use a series inductor to resonate a short antenna and a series capacitor to resonate a long antenna.

There is a closed formula to calculate that inductor. I have already seen the equation in a French book many years ago. The equations would give the equivalent irradiation resistance and the series inductor for a given antenna size and a given operating frequency.
Unfortunately I don't have those equations anymore. I may have this book somewhere but I can't find it.

And I believe the question asked by ZengLei could be well answered if somebody around could help us telling more about the closed formulas for radiation resistance and compensating inductor for single wire antennas.

Could anyone help us.

S.

 

[balaguru thavamani]

Hi,

Normally Inductor will be used in order to reduce the effective length of the

antenna and Capacitance will be used to increase te effective length.

 

[makiche]

To increase inductance, why don't you try coiling the antennaat the bottom. This will reduce the effective length. A capacitor at the top has similar effect.

 

[roykyn]

i have read this in some antenna book ... but i have nor tried it.....
by immersing the antenna in a dielectric the size of the antenna can be reduced to a very large extent...given by length = lambda/ε....ε permittivity of dielectric medium...

 

[heviraf]

Use microstrip array antenna

 

[rachel_zh]

high dielectric material or add slot to antenna pattern

 

[biff44]

You either resonate the shorter antenna with a base inductor or a capacitive cap at the end, or you make a non-resonant loop antenna and drive it with a 30 MHz current source.

You can play around with some funny shapes, like a pitchfork, to get the size down too.

To some extent, you throw away antenna efficiency, but you can do much better than a simple coil loaded whip if you try.

 

[heedavid]

To ZengLei,

I don't know if this is helpful for what you want, but perhaps you can try this reference in IEEExplore:

"Folded unipole antennas
Leonhard, J.; Mattuck, R.; Pote, A.
Antennas and Propagation, IEEE Transactions on [legacy, pre - 1988]
Volume 3, Issue 3, Jul 1955 Page(s): 111 - 116"

It seemed to me it is the basis of the Goubau antenna as well, as it is based on increasing the 'effective height' of and antenna by folding it 'a number of times'.

You cannot achieve much on material loading or microstrip patches as the frequency of interest is too low. And I concur with what some of the forum-readers mentioned about inductor/capacitor top-hat loading.

Do let me know if it works out!

BR,
Dave

 

[sinatra]

Radiation Resistance of short straight antenna radiating into free space is given by the following relation:

Rr = 80 PI (h/Lambda0)^2

Lambda0 = C0 / f

Rr - radiation resistance - this is equivalent to the real part or the antenna input impedance.

h - Antenna height

C0 - Speed of light

f - operating frequency


Can anyone give the equation for the imaginary part of the antenna input impedance?

Hope it helps.
S.

 

[fizi]

i think it's better to use other kinds of antennas ,like a radio you can use loops with ferrite core
as your frequency is low other kinds will have larg dimentions
regards