Quarter-Wave Antenna Formula Slate Coaster

A stylish and practical gift for radio enthusiasts, this natural slate coaster is laser engraved with the classic quarter-wave antenna formula: L(m) = 75 × VF ÷ f(MHz). This formula is used to calculate the approximate length of a quarter-wave vertical antenna or radial, where L is the antenna length in metres, VF is the velocity factor of the material, and f is the operating frequency in megahertz.

For ham radio operators and short wave listeners, antenna length is one of the most important factors in getting strong, efficient reception and transmission. A quarter-wave vertical is a popular antenna design because it is simple, compact, and effective across many bands. Whether used as a shack coaster, workshop reference, or conversation piece, this engraved slate design combines radio heritage with genuinely useful technical information. Its dark natural stone and crisp engraving make it ideal for any radio room, desk, or operating position.

These coasters are supplied with four soft adhesive feet so they do not mark surfaces. Putting four or six together would make an ideal gift.

Product Features

• Precision laser engraved design
• Crafted from natural grey slate
• 10cm diameter and 4mm thick slate coaster with a rustic bevelled edge
• Soft sponge feet included

Please note slate coasters are not dishwasher safe.

Design Copyright Notice

All designs are the intellectual property of Primary 9 Designs and are protected by copyright.

These products are supplied for personal use only. The designs may not be reproduced, copied, distributed, modified, or used to create products for resale without prior written permission from Primary 9 Designs.

Purchase of this item does not transfer copyright or grant a commercial licence to reproduce or sell the design in any form.

Large Orders

Looking to order larger quantities? We are happy to provide bespoke lead time and shipping quotations for bulk orders, schools, clubs, care homes, events and organisations. Please contact us and we'll help find the most cost-effective shipping option for your order.