Originally Posted by jd8000
And a bit more of a general antenna theory question here, but can anyone point to something that explains how the 1/4 wave stubs on the ANT751 work to knock out the UHF signals from the VHF elements? I like to think I understand antenna stuff better than the average Joe, but that part is just FM to me, especially how it gets rid of the UHF signal from the VHF elements, but apparently has no effect on the UHF signal from the UHF element. I understand enough to know that being 1/4 wavelength (for two different wavelengths) is important, but beyond that it's just
Winegard UHF Tetrapole Stub Function
from Winegard Patent US3,518,693
The dipole 12 differs from the conventional folded dipole in that the upper dipole arms 12a are not continuous but split to form separate inboard ends. However, with the feeder stubs 20 mounted at the inboard ends of the upper dipole arms 12a as previously described, a low impedance, or R-F short, is presented across these ends at frequencies within the UHF frequency band. As a result, driven element 12 serves as a folded dipole in the conventional sense for this frequency range with signals being fed across the gap at the inboard ends of dipole arms 12a without any substantial effect.
At frequencies below the UHF range, however, the R-F short between dipole arms 12a is no longer present and a high impedance (open circuit) is effected. With this result it will be understood that the antenna constructed in accordance with the present invention will operate effectively on frequencies within the UHF band and at the same time permit signals within the VHF band or other frequencies from a separate antenna structure to be coupled to the inboard ends of dipole arms 12a, and therethrough to the terminal connection points 12d to the down lead T.
No adverse effect is encountered for either the signals in the VHF or UHF bands and signals in both bands are coupled to but one down line without an intervening coupling apparatus being required.
End of Patent Description
Shorting stubs are used in place of a UVSJ in a combo antenna for convenience and low cost. They are not quite as effective as a real UVSJ filter, because a stub is only a short for one frequency. Two pairs of wide stubs cover UHF fairly well.
The GE 34792 contains a real UVSJ filter:
The GE 29884 is the outdoor version:
Attachment 2 shows what happens to the UHF Polar Pattern if you do not use some kind of isolation network like a UVSJ or shorting stubs to remove the UHF signals received by the VHF section of a combo antenna.