Gary Schafer wrote: > Quarter wave length cables are the thing to use to couple the cavities > together at the antenna connection side of them. > > The uhf cavity gets a cable that is a quarter wave length at the VHF > frequency and the VHF cavity gets a cable that is a quarter wave length at > the UHF frequency. These connect to a T connector at the antenna line. > > This is the same way that you connect TX and RX cavities of a duplexer to an > antenna. > > The UHF cavity loop provides a short circuit at the VHF frequency but the > quarter wave cable from it transforms the short to an open (high impedance) > at the T connection so you get no attenuation of the VHF signal there. The > VHF signal then passes to the VHF cavity as if the UHF cavity was not there. > > > > The same thing happens to the UHF signal going to the other cavity. >
I've never heard of that, but it makes sense.... > > Without the proper length cables between the cavities and the antenna T > connector both UHF and VHF signals will be attenuated depending on the luck > of the cable length. > > The quarter wave length cable is the electrical length. > > > > It you are not combining the UHF and VHF signals with cavities then a signal > splitter should be used. Even a TV cable type splitter will work ok for > this. Don't worry about it being 75 ohms rather than 50 ohms. Without a > splitter one receiver can load the input of the other considerably > (depending on the luck of cable lengths) if just a simple T is used to > connect the antenna to the two receivers. > I know of a system that has 2 VHF receivers tied to one antenna with a 'T' connector and random coax-deliberately. At the T junction, the receivers need *many* uV of signal...plus the squelch is all the way tight. Too many problems with out-of-town junk on the input. So it has many rx's and a big voter. It proves your point-if you just use a 'T' connector, it'll be deaf as a doorknob.
