... but does the collector voltage on the output devices remain constant during power changes? If not, how do you avoid significant output impedance shifts?
Yes. The final stages are kept at B+, the power set is done in the first stage in the PA; commonly known as the pre-driver or possibly pre-pre-driver stage. Using your logic, one could ask if a change in B+ voltage would skew the output impedance?? Say the difference between 11v and 15v (not ALL power supplies are created equal.) Scott Scott Zimmerman Amateur Radio Call N3XCC 612 Barnett Rd Boswell, PA 15531 ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] Sent: Friday, February 23, 2007 4:06 PM Subject: Re: [Repeater-Builder] Re: 220 repeater receiver recommendations? In a message dated 2/23/2007 12:00:13 P.M. Pacific Standard Time, [EMAIL PROTECTED] writes: It probably isn't exactly 50 ohms, but should be close. And the cables from the duplexer to the radio do NOT *REPEAT* NOT need to be an exact length. This again indicates an impedance mismatch in the duplexer and it should be retuned. (or a bad antenna/load) I never suggested that the cable between the TX and the duplexer had to be an exact length and if the TX output is a true 50 ohms then any cable length produces no consequence at the cavity input. But if the TX impedance is not 50 ohms, I think any cable length other than half wave will make the cable a line transformer affecting the impedance presented to the cavity with a quarter wave producing the greatest impedance shift. That's not necessarily a bad thing because the impedance match of a random length cable is statistically as likely to improve the TX/cavity match as it is to degrade it. Diddling around with this proved to be so frustrating that I eventually caved in and installed a Z-matcher at the TX output and tuned it for maximum cavity output at the pass frequency. It seemed (and seems) to me that this is a cleaner way to deal with the matching issue than trying to adjust BP/BR cavities with an input other than 50 ohms. You mentioned that the power adjustment levels would not affect TX output significantly on an M II or Micor. I'm not familiar with the power control network they employ but does the collector voltage on the output devices remain constant during power changes? If not, how do you avoid significant output impedance shifts? ------------------------------------------------------------------------------ AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com. ------------------------------------------------------------------------------ No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.441 / Virus Database: 268.18.3/699 - Release Date: 2/23/2007 1:26 PM
