ATS MINI V4B frontend thoughts

[DB9JG]

I recently reported on the RF frontend design of the currently available ATS Mini V4B.

Based on the existing PCB layout, I have developed some ideas to improve shortwave reception and eliminate the remaining traces of the “Theremin effect.”

Please take a look at my proposed modifications (marked in blue), which focus mainly on achieving an extremely low capacitive load at the SMA input.

This should enhance sensitivity without introducing cross‑modulation and also allow the use of a telescopic whip or other high‑impedance antennas, for example directly connected capacitively tuned magnetic loops.

The currently used JFET is certainly not the best choice for this application, and replacing it could further improve overall performance.

The revised circuit design now closely resembles that of the well‑known MiniWhip antennas.

My solution will probably not improve the FM path, and the headphone‑antenna path is completely omitted. However, my main focus for this mini receiver is shortwave performance.

The modifications can be implemented on the existing V4 PCB; I have successfully made similar changes on a V3S before.

Many thanks for your valuable feedback!

[Yuradol1972]

It seems to me that such a scheme would be more correct.
Schematic_ATS-MINI_2025-11-30.pdf

[DB9JG]

Thanks for suggestion.

I see your intentions:
-No diplexer but a switch for AMi / FMI
-No theremin effect as the FMI is not at SMA while shortwave listening
-A proper bandpass for shortwaves

That's perfect for a 50Ohm-input communication receiver, but probably not for improving the input circuitry of the MiniRX being a portable RX

But:
-Your LPF_HF is made for a low-Z input and output not for a capacitive probe antenna of a portable MiniRadio

EDIT: SORRY, your LPfilter is at the OUTPUT of the emitter follower, so NO filter at all at the gate!!

• Your gate is (unintentionally) not properly conneceted. This is a dualgate FET; one up, one down?? - The FET may require more than 3V3 at Vcc to work properly with your source circuitry.

-The drain coil works as a HPF and limits the possible lower frequency margin, a series resistor is missing. (And that would again require a higher VCC.)

• If the DC circuitry would be corrected: The circuit shows a gain of >>1 at an unnecessary low output impedance <50ohm, whereas the SI4732 has 7k||7pF at AMi

• The drain coil drags the base of the source follower to VCC. Emitter is at VCC - 0.7V The emitter-follower can not work properly. Emitter should be at ~1.5V

• Why this amplifier at all? The SI4732 has the appropriate impedance for your approach to work without an additional impedance changer. The amp adds a unnecessary gain>1 and noise and distortion. Would be usefull if a long coax is driven to a distante receiver.

BUT: The MiniRX/Si4732 doesn't need more gain at its input, at all. It needs very,very high impedance and low parasitic capacitive loads for its active antenna probe to work best.

Take a look at the PA0RDT Miniwhips where the JFET has a 15pF probe at the gate, and works with a emitter follower just to drive a 50ohm coax cable.