CAPTURE AREA

 

What is it? Does it mean the bigger the antenna the more performance it will have.

 

A very large antenna that has not been resonated can receive and transmit very poorly. If you resonate the antenna by a tuner or other technique, it will become quite affective. The size of the antenna will have an affect.

 

Not everyone can put up BIG antennas. Therefore, how can you determine if the antenna has sufficient capture area?

 

Keep in mind, this is an "area", not length. A wire has little area per foot. Making long skinny antennas will give the capture area needed. However, at the same time it needs to be resonant somehow or it will not perform well and your radio will reject it.

 

So, do you want to use sheets of 4'X 8' plate steel for this "area"?

 

If you want you could, but there is a better way to determine if the antenna has enough capture to be affective.

 

Start with a very small resonant circuit. A capacitor and inductor in series. I am referring to components you would find on a circuit board. The circuit is resonant just like an antenna. Why would you not put this on your 60 foot tower? Because it would not impress anyone that tries to see it. It would not be impressive to anyone trying to hear it either, why?

 

A series resonant circuit using small components is certainly doing the same thing electrically as the big antenna. Except for one value that you can measure. Radiation resistance. A small circuit will have a radiation resistance close to zero ohms. This means it is close to a dead short. Or, it means the circuit will have to operate at a very high current. This plummets efficiency.

 

Try making the circuit bigger. Use a bigger coil and an air spaced capacitor. You are also increasing the radiating area of the antenna. If you measure the radiation resistance, you will find it has increased. It will also start to radiate more efficiently.

 

Keep increasing the physical area of the resonant circuit and you eventually achieve 25, 50, 100 ohms and higher. When you start to get radiation resistance readings at resonance of these values, the antenna becomes quite efficient.

 

For example, you make the coils and capacitor large enough to develop 50 ohms of radiation resistance. This is convenient since most radios match to 50 ohms. If you do this using only an inductor and large plated capacitor your efficiency can be figured as:

 

 

Since your pure resistance is the resistance of the coil (using an ohm meter), it will be very close to zero or just a fraction of an ohm. This puts your efficiency almost to 100%.

 

Making the antenna bigger will offer little improvement since your efficiency is maxed out.

 

So, how big do you make it to have this 50 ohm radiation resistance?

 

It is sales pitch time. The sizes of the Isotrons are based on having enough "Capture Area" to develop 50 ohms of radiation resistance or more to provide a fully efficient antenna. It will be evident initially at the receiver, it will come alive. Putting the antenna in a good location will offer good transmit performance.

 

73,
Ralph WD0EJA

 

MARCH 2020