[Antennas] need suggestions on inverted L

[Terry Conboy]

At 06:38 PM 2004-03-08, Bill, W9OL, aka FireBrick wrote:
>I built a very skimpy inverted L.  Shack is at back basement wall of 
>house.  Less than 5 feet of coax to base of tower.  Braid to ground rods 
>with a few radials, way too few and too short.  But that's all I can do.
>
>Center conductor to wire that runs up the side of my Wilson Rotating 
>Tower.  A pivoting yardarm just under the yagi, supports the wire and it 
>then goes at a slightly downward slope to the back of my lot.
>
>Vertical wire is approx 50' and horizontal wire is approx 60'.
>Again I know this is insufficient but it's what I can fit.
>
>So here's the problem.
>I'm using a MT3000A tuner and I can tune it very well.
>No problem getting a 1:1 match (very narrow bandwidth as would be expected)
>
>but if I try to use my amp, it spits, arcs across the capacitors.
>This is NOT a California Kilowatt, 1000 watts at best.

The short inverted L presents a load with a low resistance and a moderate 
capacitive reactance.  This is a worst-case type of load for T-network 
tuners such as the Dentron MT3000A.  This is compounded by the relatively 
small tuning capacitors in the MT3000A.  They are only 120 pF max, where 
most tuners designed for use on 160 m have 250 pF or even 500 pF maximum 
tuning capacitors.  The smaller caps have higher reactance carrying the 
load current, which then develops higher voltages.

I agree with Bob, W3HKK - lengthen the inverted L if at all possible.  Even 
connecting more wire hanging vertically from the open end of the horizontal 
section will work well, since this wire will carry only relatively small 
current and won't have much effect on the pattern.  Adding 30 feet will get 
you pretty close to resonance, as well as slightly increasing the radiation 
resistance (which also lowers the ground losses).  With W9OL's limited 
ground system, the total feedpoint R will probably be fairly close to 50 
ohms.  Adding even more wire, to make the feedpoint impedance inductive, 
will further lower the internal voltages (and heating losses) in the tuner.

Especially on the 80 and 160m, when tuning a C-L-C T-network, use maximum 
capacitance and minimum inductance to let a T-network operate at it's 
lowest Q.  With low impedance resistive or capacitive loads, the output 
capacitor will probably be at maximum and the input C and shunt L can be 
adjusted for a match.  This will lower the tuner losses and should make the 
matched bandwidth wider, too.

73, Terry N6RY


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