antenna-theory.com

[ConwayB]

Hi All,

I am a newbie and have little experience in antennas and their theory, so please excuse my ignorance on this subject.

I need to make a radio transmitter's output legal. Because the output is measured as EIRP in mW, I figure I can make it legal by reducing the EIRP from 250mW to 25mW. The signal is a video signal.

How can I do this (and please answer as if I was a 10 year old).

1. Can I shield the antenna in order to achieve this reduction in output? I would probably do this by affixing some sort of shield to the antenna.
2. Is there an easy way for me to measure the EIRP so I can check it for legality prior to using it?
3. Am I right to assume that I can just increase the gain of the receiving antenna in order to overcome the reduction in output strength of the transmitting antenna?

The application is for radio controlled aircraft and the legal power output for the frequency range in this country is 25mW.

All help is appreciated.

Thanks in advance.
Conway

[bigSteve]

Shielding the antenna won't be reliable. You are essentially trying to reduce the antenna efficiency by 10 dB (i.e. if your antenna efficiency is -3 dB, you want to change it to -13 dB). I would look for a commercially available attenuator. Basically, if you assume your radio impedance is 50 Ohms (it probably is) and your antenna is about 50 Ohms (or close enough), then if you add something like 500 Ohms in series with your antenna it will knock down your efficiency. (just google 10 dB attenuator and find the right connectors for your setup.)

The unfortunate thing is you are degrading the receive signal by 10 dB as well (when you really only want to reduce the transmit signal). The better way to do it then is to reduce the output power on your chip, if you can do that.

Measuring EIRP is not easy.

As for overcoming this loss with a higher gain receive antenna on the other end, yes this works, but adding 10 dB in gain will require a larger antenna, and you'll want to know exactly what direction the signal is coming from (i.e. you can't have gain without antenna directionality)

[R. Fry]

[bigSteve]