Hi-VHF Offset-Crossed Folded Dipoles (FD's) with Quarter-Wavelength offset between the FDs and Quarter-Wavelength Transmission Line (simulated by TL Card) interconnecting the two FDs. [In diagrams below, Feedline is NOT TO SCALE.] [NOTE: Included for illustration purposes only, TL=Half-Wavelength is correct length.]
Optimum TL is 600-ohm Impedance, although 450-ohm may also be used with minor degradation to Gain and SWR. [NOTE: Velocity Factor MUST be close to 1.0, otherwise electrical length may be too long.]
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: 178 MHz if 32-in, 202 MHz if 28-in, 210 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 = 23-in or 34-in was barely sufficient to avoid the GLITCHES, as shown below. The shorter 23-in FD had better RHCP Gain & LHCP suppression characteristics and better SWR.
Note that Right Hand Circular Polarization is strong both FORWARD and REVERSE, but Left Hand suppression varies with freq. Reversing one interconnection reverses Polarization, so VERY CAREFULLY check antenna connections and alignment. Note Left Hand Circular Polarized (LHCP) signals are suppressed LESS THAN 10 dB relative to Right Hand (RHCP).
Date(s): 21 Jan 2012. Album by holl_ands. 1 - 21 of 21 Total. 989 Visits.
