AJ, Yes, you should measure close to zero ohms DC resistance at each port of a ferrite isolator. That's because the components inside of the unit are metallic composites that are part of the tuned circuit. However, if you had used an impedance bridge instead of a DC ohmmeter, you would have seen close to 50 ohms. So, the answer to your opening question is 50 ohms nominal impedance, but zero ohms DC resistance. You will also measure close to zero ohms DC resistance at each port of a bandpass cavity, since the internal coupling loop is a DC short but is resonant at 50 ohms. You will see an open when measuring each port of a pass/notch cavity, since the notch-adjust capacitor blocks DC.
BTW, each load on your isolator should measure very close to 50 ohms, not 52 ohms. Each load resistor on a new Telewave T-1560SP isolator I just checked measures within 0.2 ohm of 50 ohms. Try using the "relative" function on your meter to cancel out the resistance of your test leads. In order to function properly, a ferrite circulator must see 50 ohms impedance at each port. Needless to say, a properly-designed lead resistor has both a DC resistance of 50 ohms and an impedance of 50 ohms when operated within its intended frequency range. Most commercial dipole antennas are DC grounded for protection against lightning surges. Such antennas will always measure zero ohms DC resistance, plus the small resistance of the feedline. However, they will present a 50 ohm impedance when operated at the design frequency and fed with a 50 ohm feedline. It's a good idea to measure the DC resistance of a newly-installed antenna with a known-accurate DVM, and record that value in the station record book, so that it can be checked when a problem with the antenna or feedline is suspected at some future date. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of AJ Sent: Tuesday, February 03, 2009 9:49 PM To: [email protected] Subject: [Repeater-Builder] Nominal impedance of a Telewave T-1560 Dual Isolator??? What exactly should the nominal impedance of a VHF Telewave T-1560 Dual Isolator be? The reason I ask... I broke out the DMM this evening trying to make some sense of a repeater issue we've been having... 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). 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 147.080+ 444.750+
