Building a homebrew 8-element collinear for 146 MHz using the WB3AYW design with phasing coils. Getting SWR of 2.8:1 at resonance instead of the expected <1.5:1. Using RG-213 hardline for elements, all cut to exactly 36.5" with proper phasing coil construction per the spreadsheet.

The frequency is spot-on at 146.52 MHz, but the impedance is clearly wrong. Should I focus on trimming individual elements or adjusting the spacing between elements? Also wondering if my 300Ω ladder line matching section (currently 17.25") needs adjustment. Ground plane is four 1/4λ radials at the base.

Any insights on where to start troubleshooting would be helpful.

High SWR with correct resonant frequency typically indicates impedance mismatch, not element length issues. Start by checking your matching section - that 300Ω ladder line length is critical. Try adjusting it in small increments first before touching the radiating elements.

I've built several of these antennas. Check all your solder joints on the phasing coils - a poor connection will kill your impedance matching. Also verify the coil inductance matches the design values. The elements themselves are probably fine if you're on frequency.

Are you measuring at the antenna or at the radio? That much coax can transform the impedance significantly at VHF. Try measuring right at the antenna feedpoint with a short jumper to isolate whether it's the antenna or feedline causing the problem.