dipole resonance
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Re: dipole resonance
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.
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- Posts : 10
Join date : 2013-12-03
Re: dipole resonance
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
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- Posts : 796
Join date : 2012-11-25
Age : 64
Location : Praha, Czechia
dipole resonance
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…
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- Posts : 10
Join date : 2013-12-03
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