Yagi elements

[KJ4ZFJ]

I bought a sirio 4 element 11 meter antenna that can be tuned to 10 meters. The elements telescope out. The instructions say to cut to provided length for 10 M. Do I need to cut or can I just telescope to the shorter 'cut' length? Does the smaller diameter element inside the larger diameter element create a problem?  New at this; first directional antenna. Thanks!

[WN3R]

Simply adjust elements to proper length making sure the element length is the same on both sides of the boom. Never cut any tubing if the length can be achieved simply by pushing the smaller tubes into a larger one. Follow the directions. Good luck. 

[K3MRI]

Thanks Dick. As always, perfect answer!

[KN3U]

Dick explained exactly what you need to do. To answer the second part of the question -- a very good question, by the way -- RF current is concentrated near the surface of the conductor. For aluminum conductors at HF frequencies, most of the current is confined to the outer 100 µm (micrometers) of the conductor. The higher the frequency, the more shallow the skin depth, so at 30 MHz (10 m) the effective skin depth is about 15 µm. That's a surprisingly tiny fraction of the tubing cross-section, but then electrons are pretty small, so a lot of them fit in that tiny amount of area.

So, the part of the outer antenna element that is telescoped inside the larger-diameter tubing provides mechanical strength (and an easy way to tune the antenna to resonance) without having any effect on electrical performance. The current literally follows the skin of the antenna element over its entire length. The tiny jump where the tubing changes diameter has negligible effect at HF frequencies.

You can google "skin effect" for more information.