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Mazzoni Stealth Antenna

This is a report about the Mazzoni Stealth Antenna.
Many thanks to HE9DJB for the opportunity to document his antenna.

Picture: HE9DJB

The Stealth is about 2 years old and has been exposed to the weather for 2 years.

At the top is a nice plate capacitor.
Aluminum plates welded and painted.

Rough measurement

Plate thickness	d_p	2.2	mm
Clearance	d_c	4.7	mm
Plate spacing	d_s = 2 d_c + d_p	11.6	mm
Stator plates	n_s	15
Rotor plates	n_r	14

On the sides of the capacitor, threaded inserts are press-fitted. The optional plastic cover can be attached here. The threaded inserts are magnetic and show fine traces of corrosion.

The matching commercially available cover for the capacitor still looks perfect even after so much sun.

The side plates are made of plastic. The spacing of the holes does not exactly match the aluminium bracket. Due to the constant force exerted on the plates, they have deformed over the years.
We enlarged the screw diameter from 8 mm to 9.5 mm to reduce the stress. It fits properly now.

The base plate is sagging significantly.

The base plate design: a relatively thin aluminum sheet bent into a U-shape with a plastic plate inside.
The loop is isolated from the base plate. The loop is colored red and the upper plate is colored blue.

Linear actuator for adjusting the capacitance.

Gamma Match.

Gamma match, roughly measured.

Here you can see the two M8x25 countersunk screws used to fasten the antenna to the base plate.
The screw is removed in the picture.

Problem ATU “Motor Fail No Load”

The ATU (device to tune the antenna from Mazzoni) can no longer tune the antenna. Error message: “Motor Fail No Load”
The motor can be moved with manual tuning.
If I connect a 50 ohm termination to the ATU instead of the antenna, the ATU measures an SWR of 1.1, which seems to work.
The cable to the antenna works.

We clean the plates. It has spider nests inside.
The whole antenna is slightly crooked. The black plastic parts on the sides look a bit bent.
It cannot be aligned. The hole for the hinge at the bottom seems to be slightly crooked.
We align the plates so that they have a slightly more even spacing.
It did not help; the ATU still cannot tune.

The Smith chart looks like this. There is a resonance, but the SWR is poor. The ATU therefore cannot find the resonance.

The suspicion falls on the stainless steel contact piece that electrically bridges the hinge at the bottom.

Between the stainless steel sheet and the aluminum it is heavily corroded.
Both sides are cleaned with sandpaper. The aluminum is quite badly corroded. Grease it and put it back together.

Bottom hinge with a stainless steel sheet as the electrical bridge.
Side note: for this measurement, the VNA was calibrated directly at the VNA, not at the antenna feed point. Accordingly, the attenuation is visible in the Smith chart.

The Smith chart now looks perfect. At resonance it goes through the center: a perfect SWR.
The ATU can tune the antenna again.

Cover to protect from corrosion.

The sensitive parts should not get wet.

Silicone is used to form a bead before the critical transition so that water does not run from the loop to the transition.

A 3D-printed part covers the transition and also covers the coax cable entry.

On the right side it has a drip edge. It is so narrow that it fits between the black side parts of the antenna.

If you also want to print such a cover, you can find the model here: Link to folder /step

We printed it in black PETG, completely filled, approx. 0.3 kg.

The cover fits.

Comprehensive measurement data for this antenna can be found here: https://petermaerki.github.io/magloop_field/src/antennen/mazzoni_stealth_HE9DJB/generated_antenna.html

Last Change: 2026-09-21 Zulu v4.0.4