Electronics > RF, Microwave, Ham Radio

questions about antennas and sleeve baluns

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Agent Smith:

--- Quote from: radiolistener on February 14, 2025, 02:38:14 pm ---just buy NanoVNA and you can measure ...

--- End quote ---
Right, i could remove the working cores and put on these ones just to check the difference. As soon as i lower the mast.
Thanks

Agent Smith:
I have one more question just out of curiosity: now the collinear antenna project is completed, working on my roof, i took a note of the measures with nanovna, it looks fine tuned:
Swr 1.016:1
R jX 49,96  0,78Ω
Phase alost vertical line within 5 MHz
The only thing i can't understand is why i should have made the whip 94 mm long, while the working frequency is 869.5 MHz, so the wavelength is 345mm, quarter wave 86,25mm, multiplied by the velocity factor, let's say 0.84, should be at least shorter than 86,25mm. Maybe there is another combination between elements length?
Should i guess an element electrical length different to a quarter wave means lower efficency?

Geoff-AU:
Element length different to 1/4 wave will change the radiation pattern.

Velocity factor applies to the surrounding media, so a coax cable has a velocity factor of 0.66-0.85 on the inside (ie centre conducter, and the inner surface of the shield) and your velocity factor is 1 otherwise (whip, and the outer surface of the shield).  VF of air is close enough to a vacuum.

An element longer than resonance is inductive, it may be compensating for extra capacitance in another part of your antenna.  Or, if you are measuring at the end of a feedline, then the situation is different depending on the length of the feedline.  R+jX impedances change continuously with feedline length, performing a complete inversion of the feedline's characteristic impedance every 1/4 wavelength.

Measure with an exact 1/2 wavelength (or multiple) of feedline, or do a port extension adjustment on your VNA so you're seeing the correct result and not the effects of the feedline.

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