Eric, Impedance matchers don't give you "a matched system in a minute or so". They just match one side to the other. Example, a radio connected to a duplexer that doesn't present a 50 ohm load to the transmitter, for what ever reason. The "matcher" makes the radio see a 50 load or what ever, from the missmatched duplexer, coax, or antenna connected to it. The "system" is still missmatched and losses occur in the rest of the system, along with the other problems caused by a missmatch in the "system". The proper thing is, to fix the missmatch rather than cover it up. Now lets assume, that the "system" does present a proper load and the problem is the transmitter(PA)isn't the same impedance as the rest of the "system". That's the time to use a device to "match" the transmitter(PA) to the remainder of the "system". Solid state PAs often do not present the same impedance that we normaly expect. 50 ohms. 73 AC0Y
--- In [email protected], Doug <[EMAIL PROTECTED]> wrote: > At 09:36 PM 1/8/04, you wrote: > >Doug, > > > >I have looked inside impedance matchers made by Telewave and by EMR, and > >both have nearly identical construction. There is an inductor made by a > >turn or two in a #14 wire that runs between the two N connectors, with a > >Johansen ceramic variable-piston capacitor connected at each end. It's > >just a pi network filter, with values appropriate to the band for which > >it is specified. Sorry, but I don't have values for the components. > > > >Considering the amount of time and effort it takes to experiment with > >random lengths of cable, trying to find or fabricate one with exactly > >the "magic" length to match a transmitter to a duplexer, the Z- matcher > >is the only way to fly. For less than $100 and an insertion loss of > >0.15 dB, you have a matched system in a minute or so. > > > >Here's a case history: My radio club has a Packet Node with both VHF > >and UHF ports, and a Yaesu FT-1500M 2m transceiver is used for the VHF > >side. It feeds a Decibel Products DB-224E 4-bay dipole antenna that has > >a very good match at 50 ohms and a 1.05 VSWR at 145.030 MHz. We set the > >power to about 10 watts, thinking that the radio would "loaf" at that > >setting, and the power was appropriate for the hilltop location. One > >day, the radio stopped transmitting, although its receiver was fine. A > >bench check proved that the M67781L PA module had blown, so another > >module (80 bucks!) was purchased and installed. While I had the > >repaired radio on the bench, I tested the radio through a Bird 43 meter > >into a dummy load. Hello! At the "low" power setting, the radio was > >very inefficient, the module was very hot, and the source impedance of > >the M67781L module was far from 50 ohms. In other words, the old module > >was toast because we ran it at low power! I then connected an impedance > >matcher and tinkered with the tuning and the drive level to find a happy > >medium. I was able to find a sweet spot where the ratio of power output > >versus DC current was greatest, and the PA module was running cool. In > >this particular application, cutting and experimenting with cable > >lengths to find a match is not very practical. > > > >73, Eric Lemmon WB6FLY > -------- > Thanks Eric. Well it is pretty much like I thought. Shouldn't be to hard > to duplicate. When I get some time I will. I know what it is like to blow > a brick. I have blown two in my 741a which is mounted under the passenger > seat in my van. Th diplexer went open at the common connector and I had the > swr meter in the common lead. > > Take care Eric. It is now a warm -22C here at the present time. I just > hope it keeps warming up. > > 73 > Doug ve5da Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ To unsubscribe from this group, send an email to: [EMAIL PROTECTED] Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
