Thread: My messed up db4 View Single Post
25-Apr-2011, 2:06 AM   #8
GroundUrMast
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Join Date: Oct 2010
Location: Greater Seattle Area
Posts: 4,773
Wavelength = Speed of Light / Frequency. UHF channel 30 is near the center of the UHF band has a center frequency of 569 MHz. Therefor the wavelength of UHF channel 30 is about .527 Meters or 20 3/4". This same basic formula is at the heart of calculating any antenna element lengths.

You will notice that there is about 1/2 a wavelength between each of the dipole (bow-tie) elements of common UHF panel antenna. The phasing line that connects the top element to the next element below will therefor be about 1/2 a wavelength. A UHF signal traveling the length of that 1/2 wave long line will be 180° out of phase with the signal entering the line. By twisting the phasing line one half turn (180°), the signal is returned to a phase angle of zero degrees and will be in phase with the signal present at the center-point of the second element from the top. This provides a means to combine signals from each of the two elements, in phase, so they add together to provide twice the signal power to the feed line.

In the common panel antenna designs, the phasing lines also serve to transform the low impedance of paralleled elements. Two 300 ohm dipole elements in parallel have an impedance of 150 ohms. 1/{(1/300)+(1/300)} By using a 1/4 wave length section of transmission line with a characteristic impedance of 450 ohms (actually the math works out to 424.264 ohms but 450 ohm twin-lead is the closest manufactured product), one is able to transform a 300 ohm impedance to match a 600 ohm load. This is convenient because two 600 ohm impedances in parallel form a 300 ohm impedance. Hopefully you see the progression toward forming a four element panel antenna out of two two-panel sections.

The characteristic impedance of parallel lines at RF frequencies can be calculated - http://en.wikipedia.org/wiki/Twin-lead . You can find calculators online that will do the math for you... In the case of two #12 gauge wires spaced 1.25", the characteristic impedance is about 416 ohms. That's fairly close to the ideal 450 (424.264) ohms I mentioned above. Once you understand the math, you will see that moving the phasing line conductors together will lower the impedance, making the wire diameter smaller will increase the impedance.

It's not realistic for me to think I can evaluate your antenna... But you can read and learn for your self.

Why your crude antenna works as well as it does...? Some errors combined to cancel one another out I suspect.
Attached Files
 Air Insulated Open Wire Balanced Line.pdf (5.4 KB, 966 views)
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