Hi-VHF Offset-Crossed Folded Dipoles (FD's) with Quarter-Wavelength Separation fed by two Transmission Lines, each Half-Wavelength Long. This results in requisite 90-deg difference in signals arriving from each Folded Dipole for RHCP (Right Hand Circular Polarization).
Optimum TL is 600-ohm Impedance, although 450-ohm may also be used with minor degradation to Gain and SWR. [In diagrams below, Feedlines are NOT TO SCALE.]
With Quarter-Inch Copper Tubing (QICT), 2-in FD "Width" (distance between FD Elements) and Quarter-Wavelength Separation between the Crossed FD's, a large Gain and SWR GLITCH was found in the middle of the Hi-VHF Band, the placement depending on the overall FD Length: 189 MHz if 30-in, 202 MHz if 28-in, 209 MHz if 27-in, etc.
If the length of the Cross-FDs are much longer or shorter than when used alone, the GLITCHES can be moved out-of-band. FD Length = 26-in or 33-in was barely sufficient to avoid the GLITCHES, as shown below. The longer 34-in FD was better with good RHCP Gain, Excellent LHCP suppression characteristics and good SWR.
Note that Right Hand Circular Polarization is strong both FORWARD and REVERSE, but Left Hand suppression is stronger FORWARD. Reversing one interconnection reverses Polarization, so VERY CAREFULLY check antenna connections and alignment. Note 20+ dB of LHCP Suppression.
Date(s): 21 Jan 2012. Album by holl_ands. 1 - 21 of 21 Total. 1059 Visits.
