Topband: Inveted L Matching

[Terry Conboy]

At 04:28 PM 2006-12-03, Ron Real WR5RR wrote:
>My Inverted L is 53 ft high supported with a 3 ft arm, and the 
>horizontal extends 70 ft to a tree at a 30 ft elevation. So far, I 
>have about 9 random length radials on average ground.  The shortest 
>is about 40 ft, and the longest about 90 ft.  The 90 ft radial is 
>tied to a metal fence.  The vertical section is secured with an 
>insulator, and the coax connector is mountedin PVC fittings.  This 
>PVC housing is attached to a driven 8 ft ground rod, and the coax 
>shield is connected to the top of the ground rod, and the radials 
>are attached to the rod via a ground rod clamp.  I have connected a 
>1 Meg ohm 1/2-watt resistor from the coax center connector to ground 
>to disipate static.     My question is how to match this antenna for 
>better SWR.  I have a fairly broad 2:5 swr b/w from 1845 to 1955, 
>but the SWR at the center of this range is still 2:1.  My MFJ 259 
>reads the lowest swr at 1895 and 100 ohms impedance.  Most 
>everything I have read indicates I should have a low resistance, a
>  nd match with a capacitor.  I realize the loss is not great, but 
> is the Unun, or some other inductor what I need to add?  I know I 
> need more radials, and I will add more as I can.  The ground is 
> covered with 8 inches of snow at the present time.

You didn't say what was supporting the vertical part of your Inverted 
L (tower? Yagi?), nor what type of wire you used, but it's likely 
that your antenna is resonant just under 2 MHz.  With your modest 
ground system, you should see an impedance around 20-25 ohms (over an 
excellent ground system, if would be about 12.2 ohms at 1.97 MHz with 
bare #14 wire).

You could match this with an un-un, but a hairpin inductor is much 
simpler, has low loss, and will also move your system resonance lower 
in the band (where the antenna will supply the series capacitive 
reactance needed for a virtual highpass L-network).

Note that if you have any AM broadcasters within a few miles of you, 
be skeptical of your MFJ readings, since it has a broadband detector 
that can be fooled by signal pickup.  Check your SWR with your 
station TX at low power to see if it agrees with the MFJ.

73, Terry N6RY