At 01:35 PM 2005-05-15, Doug Rehman wrote:
>I took the concept and built a three element yagi on a 6' boom. Using double
>shielded RG-58 cable, my driven element was resonant on 50.125 at about 66"-
>about 40% shorter than a conventional wire dipole. I used the same scaling
>to cut the reflector and the director.
If you look closely at these N0KHQ-type antennas, you will see that they
are center-loaded by the inductance of two shorted sections of transmission
line. I visualize it this way: The current on each side flows down the
center conductor, is reflected at the short at the far end, flows back on
the inside of the shield, then finally flows out on the outside of the
shield to the open end of the element. If the electrical length of each
stub is less than 1/4 wavelength, the equivalent circuit is an inductance
(with a series resistance).
This might be OK, but according to TLW (v1.0), the impedance of a 33"
shorted stub of RG-58A at 50.125 MHz is 13.22 + j 206.73 ohms. This is a Q
is only 15.6 (a typical copper wire coil Q might be 200 or
more). Considering just the driven element, there is a total of 26.44 ohms
of loss resistance appearing at the feedpoint. If the total feedpoint
impedance at resonance is 50 ohms, the loss due to the driven element is
3.2 dB. There is also loss in the director and reflector. This could
easily explain the marginal performance of your mini-yagi.
For scaling, a small change in the length of the shorted stub makes a large
change in its reactance. For example, a -3% change in length (33" to 32")
gives a -15% change in inductance (and a -26% change in loss resistance,
too!). (Stub impedance changes rapidly near 1/4 wave resonance.)
N0KHQ uses the velocity factor of the coax to determine the length of the
elements, but it seems fortuitous that this gets reasonably close to the
resonant length for an element.
Keep clam,
Terry N6RY
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