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
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