IK0YVV wrote: "Looking on the Low band bible by ON4UN and modelling the
30m vertical with eznec it seems that the antenna i'm using now has a
better efficency of the 30m one." ....
I don't get the same conclusion. Assuming your antennas are made from
wire and not from a tower section, the radiation resistance of the 18M
antenna with the top wires should be close to 9.7 ohms. The radiation
resistance of the 30M vertical should be close to 17 ohms. From that
you can see that ground loss is going to be lower with the vertical
(assuming the same radial system). The required L to match these will
be close to the same for both, so there will not be a lot of difference
in coil loss.
My simulation (making some assumption about your exact configuration)
shows that the vertical should be better by about 1.3 dB. That's
assuming a ground loss of 7 ohms. If the ground loss is 10 ohms, the
difference becomes 1.5 dB.
I'm not sure how you are tuning this antenna to a low SWR on 160 with
just an inductor. The feedpoint impedance at 1.84 MHz should be 24 to
27 ohms. If you are getting a low SWR, then there is some loss
somewhere that is unaccounted for in my simulation.
On 80 meters I see no significant gain difference between the two
antennas, however there is a very large different in the feedpoint
impedance. When tuned to resonance on 3.55 MHz using a series cap, the
vertical input impedance is about 150 ohms. The impedance varies a lot
as you tune to different frequencies. At 3.95 MHz the input impedance
is about 280 ohms. I don't think you are going to like the tuning on
this antenna on 80.
EZNEC note: When analyzing L or T antennas you should not use a Mininec
ground. That usually puts a long low horizontal wire over a Mininec
ground which is a rule violation. It usually results in a gain that is
too high and an impedance that is too low. In this case since the top
wire is fairly short, the error is only about 0.5 dB.
One more EZNEC tip: Always check for EZNEC errors using the Average
Gain number. It is very easy to build a model that will produce errors.
Sometimes those errors can be large.
IK0YVV wrote: "I do apologize with the gurus on the reflector if the
question is too easy,....."
Analyzing verticals, Ls, and T antennas with EZNEC is never easy.
Jerry, K4SAV
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