Owen Duffy wrote:
On Tue, 10 Jan 2006 21:14:40 GMT, Owen Duffy wrote:
On Mon, 09 Jan 2006 15:50:20 GMT, Cecil Moore wrote:
Ricke wrote:
If SETUP right , There the best HF I've ever used.
Maybe the only one? :-) The G5RV, with tuner, is a pretty
good 80m, 40m, 20m, and 12m antenna. If the series section
is varied from 20 feet to 36 feet, it becomes a very good
all-HF-band antenna. With the addition of a parallel 1000pf
capacitor with the series section at 22 feet, on 75m my "G5RV"
has SWR of 1.3:1 and works as well as a 75m 1/2WL dipole.
Is this the antenna described at http://www.qsl.net/w5dxp/G5RV.HTM ?
I have made a mistake during my analysis, let me try again:
Now you tell me after I spent 15 minutes replying to it. :-)
The details are there so I won't repeat it here.
In that article, on 75m you model a feedpoint impedance of 36-j324,
28' of 300 ohm ladder line, for a Z of 15+j4 (seems to indicate 48.2
deg length of 300 ohm line with 0.007dB loss (optimistic)).
At that point, were 50 ohm coax connected directly, the VSWR at the
load end of the 50 ohm coax would be 3, however you shunt the 17+j4
with 1000pF to give a new Z of 17.3-j3.0 that results in a VSWR at the
load end of the 50 ohm coax of around 2.9, almost identical to the
case without the capacitor.
Presumably when you say that the capacitor improves the VSWR on 75m,
you mean the VSWR on the coax. Did I miss something, how does the
capacitor improve the VSWR on 75m?
What you missed is that the frequency must be changed to obtain the
benefit. The capacitor is *not* installed at the 17+j4 point. It is
installed at the 1/50 + j1/X admittance point. You can either increase
the length of the feedline past the 17+j4 point to the 1/50 + j1/X
admittance point or increase the frequency thus electrically lengthening
the feedline to the 1/50 + j1/X admittance point. You cannot keep both of
those values constant as you tried to do above.
You already know what I am trying to say. I must not be saying it
very well. When a parallel cap is used on a 75m screwdriver antenna
to achieve 50 ohms, the screwdriver is tuned to 1/50 + j1/X, i.e.
slightly inductive. When a parallel coil is used, the screwdriver
is tuned to 1/50 - j1/X, i.e. slightly capacitive.
--
73, Cecil
http://www.qsl.net/w5dxp