With LTSpice and a loop antenna model, you can easily compare wideband loop amplifier performance. Here is my evaluation of several popular circuits.
When you are building your own active loop antenna, the first thing you do is go searching for amplifier circuits. There are a dozen or so such circuits easily found on the web. How well do they work? It’s one thing to evaluate a circuit in isolation; it’s another to evaluate performance connected to an antenna.
With my LTSpice model of a loop antenna, it is relatively straightforward to simulate both loop and amplifier as a system. Just add the amplifier circuit to the LTSpice model of the antenna equivalent circuit. So, that’s what I did.
I selected four popular designs to compare wideband loop amplifier performance.
- Perhaps the easiest way to add an amp to your loop is just to buy a wideband low noise amplifier on eBay. This approach has been used by PA3GZK with the INA-02186 module, available on eBay for around $6.
- Some years ago, G8CQX provided a common emitter differential amplifier for loop use. It’s a fairly simple design with a couple of NPN transistors.
- Slightly more complicated is the Wellgood loop amplifier. George Smart M1GEO tried to reverse engineer the circuit used in the popular Wellbrook loop.
- Finally, you will find a lot of loop amplifier designs from LZ1AQ. He provides a common base differential amplifier that you can build or buy.
I was able to find a SPICE circuit for the INA-02186. For the other three amps, I just entered the circuits by hand into LTSpice models.
Compare Wideband Loop Amplifier Performance – Results
You can see the simulation results in the above graphic. I fed a signal with 1 μV/m field strength into the loop model and measured amplifier output in dB relative to input signal.
The INA-02186 module provides tons of gain over a very wide bandwidth 100 kHz to 2 GHz. Gain is 20-30 dB, depending on voltage provided. I bought a few of these modules on eBay and tried them with my loop. (Working with SMA connectors is painful.)
Results were disappointing. While the simulation looks great, there is too much gain. Reception was plagued by much IMD from local broadcasters. If you want to try this amp, add attenuation.
You will find the LZ1AQ design to be the best performer. It provides decent output and is relatively flat across 100 kHz to 30 MHz. The Wellgood design is pretty good in the low-HF range but output drops off considerably above and especially below. Finally, the G8CQX circuit seems to work well for such a simple design but is less sensitive. With the Wellgood and G8CQX amplifiers you need to rely a lot on receiver gain.
With my LTSpice loop model, you can do your own experiments to compare wideband loop amplifier performance before selecting an amplifier.