Is there another piece missing from this jigsaw - the length of the
Taking Jim's figure for Rr of 35 Ohms, we get a VSWR of 2.1 in a 75 Ohm
system. BUT if the feedline were an electrical quarter wavelength that
35 Ohms would become 160 Ohms at the measurement point and that would
show as a VSWR of 3.2 in a 50 Ohms system. No amount of series
capacitance would improve it!
Jim Brown wrote:
> The radiation resistance (Rr) of a quarter wave antenna is going to be
> something like 25 ohms. Add 10 ohms for the ground system, and it's 35
> ohms. You're making it longer, which will raise Rr a bit and add some
> series L, which you're tuning out with the capacitor. That's good, and it's
> what I'm doing. My guess is that your Rr is about 35-40 ohms. When you
> switch to RG8, you'll be able to get the SWR below 2:1.
> 2:1 is a fine match on 160 if the coax isn't very long, because coax has
> very little loss at that frequency, and any decent antenna tuner should be
> able to match it.
> Adding radials will INCREASE the VSWR, because you're lowering the
> resistance of the ground system. That's a GOOD thing -- it improves
> efficiency. This is yet another simple example of why VSWR is NOT a good
> indicator of antenna performance.
> If you want to improve your match (and bring it to 1:1), you'll need to
> make the antenna even longer. That will raise Rr and increase L. Rr is the
> only GOOD resistance in an antenna -- it's a theoretical resistor that
> represents the power that is radiated by the antenna. My guess is that you
> need something 30-40 ft more top-loading to get there if you use 50 ohm
> coax. You can also get that length by making the top a Tee rather than an
> inverted L. That is, keep the length you already have and add 30-40 ft at
> the point where the top turns horizontal.
> Jim Brown K9YC
> TowerTalk mailing list
TowerTalk mailing list