[TowerTalk] Inverted L Question
n6ry at arrl.net
Sat Aug 30 12:38:13 EDT 2008
At 08:30 AM 2008-08-30, DENNIS BRICKEY N4DD wrote:
>I am measuring the SWR in the shack with the rig (Orion), an inline
>wattmeter (WM-1), and the Tentec 238 Tuner. All read pretty much the
>same. The feedline is between 250' and 300' in length. I don't
>have an accurate measurement.
All those instruments are calibrated for a 50 ohm system, so the SWR
they indicate on a 75 ohm cable isn't too representative of the
mismatch at the antenna feedpoint. For example, they should read
1.5:1 if the 75 ohm cable is terminated in 75 ohms.
For other load impedances, you would have to get a reasonable idea of
the electrical length of the cable to determine what reading to
expect on a 50 ohm instrument. I don't know the specifics of your
cable, but if the VF=~0.8, a 300 foot length is about 0.7 wl, pretty
close to an odd multiple of 1/4 wl, which would tend to give the
highest 50 ohm SWR reading on a 75 ohm cable if the antenna impedance
is less than 75 ohms.
You can get another valuable data point if you take the WM-1 out to
the antenna feedpoint and measure the SWR there. This would take the
impedance transformation of an unknown 75 ohm line length out of the equation.
Another issue that comes to mind are the effects of the tree(s) that
support the inverted-L wire. These can add a small amount of
capacitance to the top and to the open end of the wire (if it runs
through a tree there, too) that can modify the feedpoint
impedance. This is really hard to model with any accuracy, but it's
easy to observe the effect on the feedpoint Z.
As others have suggested, you can add matching components at the
feedpoint to raise the impedance. A simple approach would be to just
add a shunt inductor across the connection to the feedline. Combined
with the series capacitor going to the antenna wire, this would give
you an L-network that provides an impedance step-up. If you need to
lower the Z, the inductor should connect from the antenna wire to
ground, leaving the capacitor on the feedline side.
Because you want a 50 ohm load for your transmitter, you will have to
adjust the matching network to give a 1.5:1 SWR on the 75 ohm cable
to get a 1:1 SWR indication on your 50 ohm meters. The load Z will
need to be about 1.5 * 75 ohms = 112 ohms to present 50 ohms to the
TX through an odd-multiple of 1/4 wl electrical 75 ohm line
length. For other electrical line lengths, you should be able to
adjust the L-network experimentally for an impedance with the
appropriate reactance to get 50 ohms at the TX.
73, Terry N6RY
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