Change Aerial lengths without trimming them?

[Megamox]

I build alot of FM transmitters, and sometimes i want to limit the range temporarily ie to reduce transmissions interfering with other systems. Is there anything i can add to the aerial (just hook up wire at this point), perhaps some sort of ring that slides up and down and effectively reduces the aerial to the length its attatched to. Or any other suggestions!

Thanks in Advance.

Megamox

 

[chemelec]

Do a Search for "Antenna Tuners". Or Get a copy of the "ARRL" Radio Amateur Handbook from your library.

They are Typically just a coil and Variable Capacitor.

There are Two Configurations for these:
One Increases the Effective lenght of a Short Antenna.
The Other Decreases the effective Length of a Long Antenna.

Both try to tune the Antenna to the Required Frequency.

 

[stevez]

I presume you'd like to reduce the power radiated from the antenna - and one way to do that is reduce the power going in. What you probably dont' want to do is affect the rest of the transmitter - especially the frequency.

If you knew the impedance that the transmitter likes to work into then one way would be to add what's called a dummy load. 50 ohms or so is common but it's hard to tell with the simpler circuits. At any rate, if the output impedance is 50 ohms then the addition of a 50 ohm non-inductive resistor in place of the antenna should let the transmitter think its running into a normal load - a little bit will radiate but most will be absorbed in the resistor.

You could construct an attenuator. ARRL handbook shows these. For low power a simple T arrangement with non-inductive resistors will work. My VHF pals tell me that the old carbon resistors work well at VHF but the lead lengths must be very short. The attenuator is installed between the transmitter and the antenna. If done right the transmitter thinks it's seeing the right load (and it is) but much of the power is absorbed in the attenuator resistors - the rest in the antenna. The good thing about this is that it's predictable.

Using capacitors or inductors on the output certainly can affect the output and may provide acceptable performance - as long as the transmitter can tolerate the changes in the output.

 

[Megamox]

Thanks guys, will check those out