At 02:57 PM 2005-09-25, Steve Flood KK7UV wrote:
>I am in the process of erecting a wire vertical. I could go with
>either an Inv-L or a T. In modeling the T design, I used a 70-ft
>top wire (which is my space limitation amongst the tree-tops), and a
>70-ft. vertical section with the base 4 feet above ground with 4
>elevated qtr-wave radials that slope up to 10 feet above
>ground. EZNEC gives me about 21 - j71. Using an L-network
>calculator, I come up with a low-pass series L of 6.37 uH and a
>shunt C of 205 pF. Any feedback or "better-way-to-do-it"
>comments? With either the Inv-L or theT, I am looking at some
>matching at the base. How about my MFJ949 outside at the feedpoint?
One alternative is to use a hairpin (beta) match. Add enough series
inductance to the bottom of the vertical wire to give 21-j25 ohms
(j71 - j25 = j46 ohms or 4 uH), then add a j42.5 ohm coil (3.7 uH)
across the feedpoint. Hairpin matching usually gives a slightly
wider bandwidth than an L-net. This is because the hairpin shunt
reactance rises with increasing frequency, which tends to reduce the
impedance step-up, de-compensating somewhat for the rising antenna impedance.
My model of Steve's T with the hairpin in EZNEC shows a 50 kHz BW
(<2:1). (I cheated and used a model with a shorted (ideal) 300 ohm
transmission line stub across the feedpoint for the hairpin, since
EZNEC doesn't allow directly adding a shunt inductor there.)
73, Terry N6RY
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