Thread: Antenna design
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Old 26-Mar-2017, 2:52 AM   #16
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,353
Optimum reflector for UHF antenna is 3 inches to add close to 3 db of gain according to what I can ascertain from this forum and HDTV primer.
I assume you mean distance between the reflector and bowtie elements. The distance is a function of frequency. The original Channel Master 4221 had the reflector 4.25 inches behind the elements. For the present 14-51 UHF band, optimum spacing is about 4.5 to 4.75 inches. As the FCC takes away more of the UHF band and gives it to cellular interests, the optimum spacing will increase as will the element length. Think in terms of 0.2 to 0.25 wavelength.
Is it safe to assume that the additional 6 inches the wave travels to and from the reflector to the active element is negligible in relation to the distance from the transmitter?
At what reflector distance does out of phase signal occur? To me, at least intuitively, the shorter the reflector distance, the less likely the reflected signal would be out of phase.
There is an optimum distance that is just the right distance away.

You want the energy from the reflector to arrive at the driven element to add to the energy in the driven element in phase. Your intuition is correct, the wave front does take a little longer to arrive at the reflector and then travel back to the driven element, but it doesn't seem to matter as long as the two energy sources are in phase. In a reflector antenna, the adjustment is made by changing the distance. In a yagi the adjustment is made by changing the distance and the reflector length.

In Bill Orr's Beam Antenna Handbook, he gives an interesting explanation of how a yagi parasitic element works. The incoming wavefront induces a current in the driven element, the reflector, and the director/directors. Since energy can not be created or destroyed, the energy in the parasitic elements has to go somewhere. It is re-radiated by the parasitic elements and coupled into the driven element to add to its energy, giving the desired increase in gain.
I have a antennacaft Y7-13 and the active elements on the antenna are 12 inches apart and linked together with the balun.
IIRC, the Y10-7-13 and Y5-7-13 are a log periodic design that is different than a yagi to cover the whole VHF-High band. Look in your ARRL Antenna Book for LPDAs.
If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 26-Mar-2017 at 3:03 AM.
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