Pls give me some opinion about the loop antenna

[Darren_pan]

i wanna design one horizontal monitor antenna,the request of frequency is from 40MHz to 400MHz,i select the small antenna,which means the loop circumference is smaller than one fifth of a wavelength. howerver,in term of calculation, the radiation resistance too small, i wanna make a big one to rise the radiaton resistance,but i'm worry about the pattern,maybe,there will be one null ont the horizontal plane.
i need you help,tks

 

[sergio mariotti]

The loop antenna, on the horizontal plane has two null on the direction perpendicular to the circle plane.
About low radiation resistance, it's true, but if you receive, don't worry, use a current amplifier.
Low radiation resistance, means that the power efficiency is extremally low, Power = V*I, but don't means that "current efficiency" is low too.
A small loop antenna is "current" sensitive and require a low noise current sensitive amplifier (low Zo) while small wire is a voltage sensitive and require a low noise voltage sensitive amplifier (Hi Zo).

I've worked with a 2 meter diameter loop antenna in order to receive VLF from 10Hz to 15 KHz and i've succesful received a multi thousand kilometers russian TX

 

[biff44]

Interesting. Could you give an example of a "current" amplifier that is available off-the-shelf. Most amps you can buy are designed for 50 ohm load. I have a project at 27 MHz where I need to use a small loop antenna.

 

[luqing]

To:sergio mariotti,
Can you give me further excuse about ""current" sensitive"? or give me some examples about that.
Thanks!
my email :bg_islang@163.com