> AJ <aj.grant...@...> wrote: > What exactly should the nominal impedance of a VHF > Telewave T-1560 Dual Isolator be?
The desired characteristic is to be close or at 50 ohm impedance at a desired frequency or within a range. > The reason I ask... > I broke out the DMM this evening trying to make some > sense of a repeater issue we've been having... A basic Digital Multi-meter is normally used to measure Volts, Amps and resistance, none of which are impedance. > Removed each one of the 35 watt dummies off of the load > port of the isolator, one at a time, and verified they > sit right at 52 Ohms from pin to shield (male N-type > connector). Because they are simple 50 ohm resistors inside the heat sink. For the sake of this conversation... 50 ohms resistive and 50 ohms impedance are the same result using a plain non inductive resistor (inside the load). > Then I check the load ports on the isolator itself where > the 35 watt dummies were connected - 0 Ohms (direct short) > from pin to shield. > > I then checked the in and out ports of the isolator (tx > and antenna ports). These both showed a direct short from > pin to shield. > > I also verified that there is a direct short from pin to > pin on all 4 connectors (tx, ant, load 1, load 2), along > with shield (chassis/ground) to shield all around. > > So... My question is... > > Should there be a direct short from center pin to chassis > on this isolator? > 73s, > AJ, K6LOR/R In your example... yes there probably should be a DC ohms short from the center pin to ground. Keeping things simple... Impedance is a combination of DC and AC resistance. And the AC and DC resistance can combine in various combinations to equal X value of impedance (and a gazillion other possible values can be possible by changing values). It is possible to have one and/or pretty much none of the other (resistance and/or impedance). In the case of a plain resistor and a common ohm-meter... you normally have no AC resistance (call "reactance") so the end result of a combination impedance is considered all DC resistive, which you can measure on your multi meter ohms scale. When you have steady DC voltages, things live pretty much in the resistive world. If things change in the case of AC (moving) voltage, then reactance (AC resistance) can (and most often does) happen... and the combination of both DC Resistive and AC Reactive combine to result in some total combination impedance value. Your multi-meter doesn't measure impedance. Power from your radio transmitter and RF in general is considered AC voltage (energy). RF radio energy from your transmitter just happens to be at a much higher frequency than the AC voltage coming out of the typical house wall socket. If you can find someone with an MFJ-259 or MFJ-269, you can place it on various AC devices like a classic base loaded coil mobile antenna and see it indicates (hopefully) about 50 ohms AC impedance. Remove the MFJ box and measure the antenna with your multi-meter on ohms resistance and it will often indicate a dead short. The MFJ-259, MFJ-269 type boxes are considered basic consumer ham/cb impedance meters. Hope that helps a bit... cheers, s.
