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Wideband Loop Antenna Implementation Begins

wideband loop antenna implementation

Enough theory. It’s time to start the wideband loop antenna implementation using real parts and circuits. 

Last fall, I described my vision for a complete diversity reception system using wideband magnetic loops. You may remember that the basic idea was to have two active loops spaced 100 feet apart, using CAT 7 cable to connect up to my Afedri dual channel coherent receiver. The physical loop antenna will be made using aluminum core PEX. I built a prototype in October. At the same time, I bought 100 foot lengths of shielded CAT 7.

More recently, my attention turned to understanding loop theory and how it is different for wideband mode. During this effort, I learned to use LTSpice simulation and built an equivalent circuit model for a one meter diameter loop. After modeling the loop performance with several readily available amplifier circuits, I decided to attempt an original circuit design.

The key to my wideband loop antenna implementation will be two circuits, the “head end” and the “base”. Head End will be a current to voltage converter located at the antenna. This transimpedance amplifier will take the small antenna currents created by the magnetic fields and produce a signal voltage. At the other end of the CAT cable will be the Base circuit. Base will sum the balanced signals into a 50 output to the receiver. Since I am after dual channel reception, Base will contain two identical amplifiers, one for each channel.

Should work nicely.

My next step is to design the Head End and Base circuits and model their performance. LTSpice provides me with the ability to draw the schematics and evaluate both frequency and transient response. You remember I want to cover roughly 100 kHz to 30 MHz with the loop system and I want enough dynamic range to handle signals up to S9+50.

My main uncertainty at this point is how the loops will be effected by strong local AM and FM broadcast signals. Since I live at the south side of Calgary, I am fairly close to several broadcast antennas.

Wideband Loop Antenna Implementation – Caveats

This promises to be quite the adventure. I am literally Making It Up as I go along. You see, I have never done any serious work with operational amplifiers at RF frequencies. I have never used surface mount components or built really complex circuit boards. Even worse, my knowledge of electronics is truly “amateur”.

But LTSpice and a few textbooks is giving me confidence to experiment with ideas and learn along the way. If I can get a simulation to work, perhaps the real thing will, too. One thing is for sure. We will both find out soon. Happy New Year!

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