Kent, For maximum protection and longevity, the first or only load should be capable of handling the full output of the transmitter. If the worst-case situation occurs, due to an open or completely shorted feedline, the isolator's load would theoretically have to absorb the full output of the transmitter. That seldom occurs in the real world, because there is loss in the feedline going up to the fault, and more loss coming back down. Therefore, an isolator with a 35 watt load can probably handle a 50 watt transmitter. However, most antenna faults are not all-or-nothing faults, but something in-between. An antenna with ice buildup or wind damage may suddenly become such a poor match that it reflects 30% of the forward power back to the source. In this case, a 35 watt load might suffice for a 100 watt transmitter. On a dual isolator, the second load can be half the rating of the first load.
An isolator is an excellent investment for the protection of inexpensive RF power amplifiers that have absolutely no high-SWR protection, such as some Hamtronics, RF Engineering, Maggiore, or Mirage devices. In this case, the isolator's primary function is to present a uniform 50-ohm load to the output stage. An isolator is also a good idea if your antenna and feedline is less than heavy-duty commercial-grade stuff, meaning that they were bought because they were cheap, rather than to maximize longevity and resistance to the elements. Nevertheless, an isolator that costs over $400 with a barely-adequate load deserves a larger-rated load, for perhaps 10% more outlay. In the end, it's worth the extra money. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Nevada Amateur Radio Repeaters, Inc. Sent: Wednesday, August 20, 2008 5:24 PM To: [EMAIL PROTECTED] com Subject: [Repeater-Builder] Celwave Circulator Ref: <www.telewave.com/pdf/TWDS-4015.pdf <http://www.telewave.com/pdf/TWDS-4015.pdf> > I looked at the Celwave dual stages circulator rated at 150 watts a dual stages with a 35 watt dummy load on each stage. I don't understand the design since I have seen and used two stage circulator with a 100 watt dummy load on the first stage and a 25 watt dummy load on stage two. What make two 35 watt dummy loads in series ok? What am I missing? Kent W7AOR www.narri.org
