dipole resonance

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Re: dipole resonance

Post by Bernie on Tue Jan 06, 2015 9:19 pm

Thanks Ivan,

I will do some other tests as you suggest, firstly coiling the feedline near the feedpoint, but also later placing it higher- The 2 mts was an easy start.

Bernie

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Re: dipole resonance

Post by Ivan on Tue Jan 06, 2015 8:25 am

Hi Bernie,
you did not use a balun on the dipole ? You never eliminate the outer side of the coax shield thoroughly in such case. I suspect the resonance at 16,5 MHz is a basic resonance of the coax stub capacitively loaded by the dipole. Do the strange dips move when you change the length of the coax ? Can you do your GDO measurements directly at the center of the dipole ?
BTW the height of the dipole (2 m) seems to be too small.

VBR from Ivan

Ivan

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dipole resonance

Post by Bernie on Mon Jan 05, 2015 3:12 pm

Yesterday I put up a dipole for 10m, on the almost final location (about 2 meters high and reasonably clear of other objects). And tried to see where it was resonant at, for later making it resonate at 28.25 Mhz.

The 150/f  ½ wavelength was first multiplied by 0.95 (for end effects, etc), then cut 10% longer to allow for trimming.
While connecting everything at the center insulator, a couple of knots and the splicing may have reduced the length aprox 10cm. Final length unknown but near 5 mts

The feedline was 10m of coaxial cable, 0,82 vf, which I estimate to be 1.2 wavelenghts at 28.25 Mhz (I read is the right length to minimize outer side of coax shield radiation)


Resonance was checked with my all-purpose GDO and found the following dips:  16.5 – 30.7 – 42.0 – 54.5 – 68.0 Mhz, all read from the “hand-calibrated” scale.

The 30.7 and 68.0 (which I expected near 63 Mhz not 68) I presume are the fundamental and 2nd harmonic for the ½ wavelength. (dipole ended up too short?)


But I am in doubt with the 16.5  - 42.0 – 54.5 Mhz dips.

Can the 16.5 Mhz dip be the dipole resonating as a quarter wavelength?

I searched the internet for sample antenna analyzer plots but all I found seem to show the dipoles resonate at fundamental and harmonics, never sub-harmonics…

Bernie

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Re: dipole resonance

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