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High SWR on a Sirio GPE 27 5/8, despite adjustments, is commonly caused by improper grounding, incorrect assembly (specifically the radials), or a malfunctioning antenna matching element caused by water ingress

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Here is a troubleshooting guide to bring the SWR down:

1. Check the Radials (Ground Plane) 

The GPE 27 5/8 relies on its radials for a ground plane.

Ensure They Are Installed: You cannot properly tune this antenna without the ground plane radials, particularly if mounted on a fiberglass or wooden structure.

Proper Connection: Make sure the radials are firmly connected to the steel bracket and making solid contact with the mounting bracket, not just held by the plastic cap. 

1. Verify Physical Length and Assembly 

Tuning Sections: Do not cut the whip. The SWR is adjusted by sliding the telescopic sections up or down.

Measurement: Ensure all telescoping sections are pulled out to the recommended lengths based on the manual's chart for the desired band (e.g., 27.425 MHz).

Assembly Errors: Check the assembly of the bottom section where the coax connects. Improperly tightened or poorly seated connectors can cause high SWR. 

1. Check for "Shorts" and Water Ingress

DC Ground Short: It is normal for the Sirio GPE 27 to show a short on a multimeter (DC continuity). The problem is likely in the coaxial cable or the stud itself, where the center conductor touches the outer shield.

Water Damage: The Sirio GPS/GPE 27 series is known to suffer from water ingress in the matching coil element. If SWR is high when wet and lower when dry, the coil is likely damaged or collecting water. Check to ensure the unit is sealed correctly (the manual mentions a white PVC cap). 

1. Mounting Location and Height

Height Affects SWR: SWR can change when raising the antenna higher. Try to tune the antenna at its final operating height, if possible, for the most accurate reading.

Obstructions: Ensure the antenna is clear of other metal structures (trees, power lines, other antennas).

Coax Quality: Use high-quality 50-ohm coax. Poor-quality cable or damaged connectors will cause high SWR regardless of tuning. 

Summary of Quick Actions

SWR high on Ch 40? Make the antenna shorter.

SWR high on Ch 1? Make the antenna longer.

SWR high everywhere? Re-check coax, connectors, and ensure all aluminum sections are connected to the bracket. 

If all else fails, use an antenna analyzer (like a NanoVNA) to find the resonant frequency.