# Active Antennas

1. Dec 16, 2008

### Jdo300

Hello All,

I have a question related to the understanding of how an active antenna works. From my *extremely limited* knowledge of the subject, I hear that an active antenna is able to recieve a stronger signal from a transmitting source by transmitting a wave of the same frequency such that the EM waves of the active antenna cancel out the incoming waves? And that this action actually increases the effective apature (assuming I'm using the right word there) of the antenna.

If this is true, I want to know if one could create the same effect using a couple of coils in the following situation: For this example, I consider the case where I have one coil with an RF signal in it, and a second coil with a DC current flowing through it. What I would like to know is if the coil with the RF signal in it, can actualy absorb some of the magntic field energy from the DC coil as its magnetic field periodically cancels and un-cancels the DC coil's magnetic field (assuming that the field strengths are matched to allow this cancelation to happen.

I know this sounds like a weird question to ask but this is related to a thought experiment that I am working on. Any comments and/or criticisms welcome.

Thanks,
Jason O

2. Dec 21, 2008

### Pumblechook

An active antenna is just a short antenna with an amplifier which acts as an moderately successful impedance matching device and therefore broadens the bandwidth of the antenna when used with a length of coax which might be many wavelengths long. The output of the amplier is roughly matched to the impedance of the coax (50 or 75 ohms) so long lengths of coax could be used. The input will be HI-z and there is no attemp to match to the short antenna. Provided the coax is not too lossy it is as if you are using a short telescopic on the set itself but you are able to have the antenna further away from sources of electrical noise. They are a poor substitute for a proper sized and impedance matched antenna. Apart from impedance matching, amplifiers serve no useful purpose unless they can improve the noise figure of a a receiver. A very short antenna (in tems of wavelength) will have a small aperture.

I don't follow the bit about generating a local signal. There would be no point. You would 'jam' the signal you are trying to receive.

As far as I know a static magnetic field has no effect at all on RF. The proximity of a coil could de-tune an RF circuit but it wouldn't need any DC flowing in it. If there is a feromagnetic core involved that is another matter and DC can alter its properties esp if it pushed toward saturation but it is the feromagnetic material changing its effective permeablility and not the magnetic field acting directly on the RF in any way.

YIG (Yttrium,Iron and Garnet) Oscillators produce a wideband swept output by using the later process.

Last edited: Dec 21, 2008