At 10:02 PM 20061204, K4SAV wrote:
>When you do calculate a real matching network, something other than 60
>uH may be used. A realistic number for feedpoint impedance with a good
>radial system should be something around 12 ohms. This isn't going to be
>easy to match with a single LC network so the network is likely to be a
>coil plus a couple of caps, or maybe a wideband transformer. The SWR
>curve will also be very sharp. You may want to estimate this to see if
>it will be acceptable, or otherwise plan a method of adjusting the
>frequency.
This doesn't relate to the original question about inductor forms
(see link at end), but matching 12 ohms is quite easy with a simple
"hairpin" match. It just requires that the antenna (including any
loading coil) be tuned slightly capacitive, then apply a shunt
inductor across the feedpoint. This forms the equivalent of a
highpass Lnetwork that steps up the 12 ohms to 50 ohms.
It's easy to calculate, too:
Calculate the required network Q = sqrt (50/12  1) = 1.78
Calculate the capacitive reactance Xc = 12 * Q = 21.35 ohms
(This is how much the loading inductor can be reduced at the
desired operating frequency).
Calculate the shunt inductive reactance Xl = 50 / Q = 28.1 ohms
Calculate the shunt inductor value L = Xl / (2 * pi * F) = 28.1 /
( 6.28 * 1.83) = 2.44 uH (for 160m)
Note that with a network Q of 1.78, the bandwidth of the matching
network on 160m will be about 1 MHz, far more than the antenna
itself. In fact, the combination of the hairpin and antenna actually
show a very slight increase in system bandwidth (compared to using a
perfect broadband transformer, for example).
The loss in the shunt inductor is relatively low. At 1500 watts,
using a shunt L with an unloaded Q of 200, it will dissipate 13.3
watts for a 0.04 dB loss. And in a loaded system, this almost
exactly balances the reduction in loss due to shrinking the size of
the main loading coil. Who says there is no such thing as a free lunch?
I use this approach to match my 26 foot flagpole on 80m with good
results. Works great on mobile antennas, too.
If you want to see how to build this type of capacitorless matching
network, check out the inductor mounting at
http://www.arraysolutions.com/Products/as80tuner.htm
73, Terry N6RY
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