Topband: T-vertical matching

[Terry Conboy]

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