Jeff, Speaking of the RLB, did you ever get one of the newer RLB's from Amtronix? I still interested in someone measuring the parameters of that unit against one of the more expensive RLB's, such as the Eagle brand.
If the measurements are fairly close to each other, then the Amtronix RLB would be a good unit to have, especially for the price that he's asking. Also, your post below was really good information to have. Thanks! 73, Don, KD9PT ----- Original Message ----- From: Jeff DePolo To: [email protected] Sent: Tuesday, April 20, 2010 5:46 PM Subject: RE: [Repeater-Builder] Sinclair C-Series cable lengths > I adjusted the loop positions, trying to maintain symmetry of > the curve, aiming for 1 db > on the analyzer. I didn't adjust the loops while looking at > the RL. How would I translate RL > into IL? You can't directly translate from RL to IL or vice-versa. Here's how to tune a pass cavity: 1. Ballpark the insertion loss using the stickers on the loops and/or by measuring the insertion loss at whatever frequency the cavity is presently tuned to. 2. Rough-tune the cavity to something near your desired frequency. Don't bother being too critical here - the resonant frequency is going to wander a bit as you adjust the loops in the following steps. 3. Terminate one cavity port with a high-quality 50 ohm load (high quality: >= 30 dB return loss). Connect your RLB to your SA/TG, with the DUT port connected to the other port on the cavity. You *must* use a cable between the DUT port and the cavity that is known to have excellent return loss! The cables between the SA/TG and RLB should be good quality, but are nowhere near as critical as the cable between the RLB and the device under test. 4. While measuring the return loss, make minor adjustments to one of the loops to maximize the return loss. Again, ignore the frequency of the return loss "dip", it's going to vary slightly as you adjust the loop, just go for maximum return loss at whatever frequency the dip happens to fall at. Keep the screws snugged down well on the loop assembly; if it's not sitting tight and flush in the top of the cavity the tuning will change when you go to tighten the screws later. There's a little chicken-and-egg here; you have to loosen the screws to adjust the loop, but when you tighten them it's going to change it a bit, so you have to emperically find the sweet spot. With most cavities, you should have no problem getting well in excess of 20 dB return loss - shoot for 30 dB if you can, even though at that point uncertainty due to the test equipment's limitations will be dominating the measurement accuracy. 5. Reverse the connections you set up in #2 above. Check to make sure the return loss is still high looking into the other port (it should be). 6. NOW, adjust the resonant frequency using the rod to put the return loss maxima it where you want it (i.e. at your pass frequency). Assuming the cavity was rough-tuned in step #2 above, the return loss should not change as you fine-tune the resonant frequency. 7. THEN, check the insertion loss through the cavity using the SA/TG. It should be fairly close to what you set it to in #1 above; if it's more/less than what you'd like, adjust ONE loop for more/less insertion loss, and then repeat again from step #3. DO NOT adjust the resonant frequency via the tuning rod during this step!!! Unless the cavity was poorly designed, tuned, or handled, the return loss maximum should align very closely with the insertion loss minimum. Once you've properly tuned the cavities individually, then cable them together and re-check return loss and insertion loss. Report back how it goes and what numbers you come up with. --- Jeff WN3A
