Skip to main content

After switching to brass legs for my 70cm dipole for strength, I started trimming and filing them down to get something working.

  • At ~132mm measured from the coax, it was resonating well at 424.5 MHz with an impedance of 44.24+j1.987 Ω and an SWR of 1.138.
  • At ~107mm measured from the coax, the impedance at 436.0 MHz was approximately halved with an impedance of 23.41+j7.66 Ω and an SWR of 2.200.

Between the feed point and opposing UHF connector, the coax is about 114mm long, which is very close to two wavelengths.

(%i1) 1.14 / (%c / 436E6 * 0.83);
(%o1) 1.9975262006020027

I'm getting the sneaking suspicion that the coax is doing unwanted impedance transformation, but I'm not sure. What do folks think?

#AmateurRadio #HamRadio

Dipole legs @ 132mm
NanoVNA: 44.24+j1.987 Ω @ 424.5 MHz (SWR of 1.138)
Dipole legs @ 107mm
23.41+j7.66 Ω @ 436.0 MHz (SWR of 2.200)
#hamradio #amateurradio
in reply to Neil E. Hodges

Neil E. Hodges
Neil E. Hodges
•
Another thing that makes me think it's the coax is that I was seeing an "extra" resonance when the dipole's self-resonance was at a lower frequency. #AmateurRadio #HamRadio
Self-resonance of the dipole itself: 74.97-j6.25Ω @ 420.0 MHz (SWR of 1.52)
Coax resonance?: 66.94+j5.595Ω @ 468.0 MHz (SWR of 1.359)
#hamradio #amateurradio
⇧