DK7ZB pisze:



If you use not-insulated mounting through the boom, you need a correction factor. The element must be longer, because a part of it is electrically shortened. The boom correction for some diameters is as follows:

Boom 10mm 15mm 20mm 25mm 30mm
Element +1,2mm +2,6mm +4,4mm +6,8mm +9,5mm

The boom correction uses the famous DL6WU-formula:

C = (12.5975-114.5 * B) * B * B

C=Correction factor for one element half (for full element x2) in wavelengths

B=Boom diameter in wavelength

If you realise an insulated mounting through the boom, take the half of the correction factor above

For very long Yagis with small bandwidth it is better to mount the elements insulated, because the factor must be seen in relationship to the reactance of the elements, that means another factor for the shorter directors. With a constant factor you can change the performance of your Yagi to worse conditions.