I think cables and cavities are being mixed up a littler here. When using hard line cable to build the "cavities" for a 6 meter duplexer then that hard line is not operating as a typical coax line. It is operating as a quarter wave resonant cavity. It has a high impedance on one end and a short on the other end. Very different from the way a typical coax cable is operated. Yes Q does matter in a cavity.
The only reason to increase insertion loss in a cavity is when more selectivity or a deeper or narrower notch is needed. Most often cables within combiners / duplexers work in harmony with the associated cavity. Replacing one of those cables with a poorer quality cable will only reduce (if any difference is noted) the amount of rejection that was obtained from that cable and cavity combination. Those cables are usually operated as quarter wave shorts in these circuits at some specific frequency. At other frequencies they operate as regular non resonant coax cable in the circuit. Duplexers, combiners, coax cables are all passive devices. By their nature they do not produce intermod or any other signals, unless of course if they are defective. Some transmitters (and some receivers) do not like reactive loads off frequency. Even though a duplexer or transmitter combiner may present a flat or nearly flat load at the wanted frequency, that same duplexer may present enough of a reactance off frequency to cause unwanted oscillations in the transmitter. In a case like that changing ANYTHING in the circuit can have an effect on the transmitter's ability to remain stable. That is why sometimes changing cable lengths between the transmitter and duplexer can cure some problems. (normally this cable length is non critical) (changing this cable length can also have an effect on power output of the transmitter at the wanted frequency because of the load that it sees on the wanted frequency) Yes that interconnect cable can have Q associated with it if it is transforming an impedance from one value to another at some unwanted frequency. But when the cable is operating as a regular coax cable transferring power at its nominal impedance Q is not a factor to be considered. In the case of parasitic suppressors in an amplifier circuit Q is reduced by the suppressor at some specific unwanted frequency. The circuit Q of the wanted frequency is not changed. This reduces oscillation tendency at the unwanted frequency by reducing the gain of the amplifier circuit at that frequency. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of skipp025 > Sent: Friday, September 05, 2008 1:47 PM > To: [email protected] > Subject: [Repeater-Builder] Re: Cable Q-- Please Define > > Higher Q parts/paths MAY sometimes support unwanted/parasitic > action/energies otherwise not normally sustainable when > losses in lower Q circuits overcome those paths/sources. > > > An example... > > The six meter duplexer made from 1-1/4 and 1-5/8 inch hard > line can also be made of common coax. The Q of the common coax > is relatively low enough so the flexible coax version won't work > very well, the higher Q 1-5/8 inch line being the better choice. > > Both the 1-1/4 and 1-5/8 inch home-brew rigid line duplexers > are considered usable... graphs of both rigid line version are > on the various web pages and clearly show the performance numbers. > > If you experience a grunge/imd problem with/through using the > better 1-5/8 inch hard line duplexer... the same mix/grunge/intermod > problem might not be sustained(able) through the 1-1/4 duplexer > because of it's higher internal loss (lower Q). Keep in mind the > 1-1/4 inch diameter hard line 6 meter duplexer is still quite usable. > > In common land mobile antenna combiners... we can and do increase > the cavity, coax and network insertion loss to reduce problems > in some specialized cases. > > A lot of this is just about trying to describe how sometimes > a reduction in an antenna/duplexer hardware and feed-line > Cable Q (quality) can attenuate unwanted energies. > > In my opinion the South American Telewave VHF Transmit Combiner > story we saw here on the group a while back was very much about > having high-Q cavities and very, very small amounts of unwanted > energies fairly possibly solved with a number of modest changes > including increasing the loss numbers on some of the combiner > channels. > > The combiner was engineered by Telewave and the potential mix > numbers looked pretty darn good. But the as-built hardware had > mix problems no-one seemed to be able to source using the off > the shelf tricks. Reducing the Q of a circuit was probably not > an "off the shelf method" used or even thought about by most > people. > > When working on/with high powered tube rf amplifiers we often > use parasitic suppressors to reduce Q and make the amplifier > ultra stable. Reducing the circuit Q a slight amount is enough > to prevent unwanted parasitic oscillations and potential > spurious energy mix generation. Fractional reduction in > gain/performance traded for grunge free operation. > > Tis something to consider... > > cheers, > s.
