Eric, I was referring to commercial inline shorted-stub arrestors, such as those made Huber+Suhner, that typically are intended for use at 800 MHz and higher frequencies. I did not mean to imply that home-made quarter-wave stubs cannot be used. In fact, instructions for fabricating just such a device are found here: <www.repeater-builder.com/ge/datafile-bulletin/df-10002-01.pdf>
73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Eric Grabowski Sent: Monday, July 27, 2009 10:30 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein series) Eric, Why is a quarter-wave shorted stub impractical at 2m? I have been told that a quarter-wave shorted stub would serve two purposes: a) provide a dc path to ground for static caused by precip or wind, and also b) substantially reduce the strength of a transmitter's second harmonic. 73 and aloha, Eric KH6CQ --- On Mon, 7/27/09, Eric Lemmon <wb6...@verizon.net> wrote: From: Eric Lemmon <wb6...@verizon.net> Subject: RE: [Repeater-Builder] Sinclair Q202 duplexers (frankenstein series) To: Repeater-Builder@yahoogroups.com Date: Monday, July 27, 2009, 4:51 PM Norm, A Polyphaser does not put a DC ground on the center conductor of the feedline- nor does any other inline arrestor of any brand, except a quarter-wave shorted stub. But that is impractical at 2m. My point was simply that a single bandpass cavity on either the TX or the RX side, between the duplexer and the antenna tee, will put a DC ground on the feedline at the transmitter end. Most antennas are DC grounded, but a lot can happen to that feedline between the antenna and the duplexer. One 2m repeater I have on a hilltop suffered a lot from wind-caused static discharges (aka triboelectric charging) until I put a single bandpass cavity on the receive side. My intent was to prevent desense from the adjacent FM broadcast station, but the static elimination was a bonus. Regarding the determination of high pass versus low pass, this is usually determined by the design of the duplexer. Some designs are symmetrical, while others are asymmetrical. In most cases, the loop coupling will be different between the high side and the low side, so it is convenient to simply follow the manufacturer' s settings, and their tuning instructions. In the case of the Sinclair Q202-G, the loop assemblies are all identical, and the notch tuning capacitors are the same as well, regardless of which pass side they're on. You're correct about bandpass duplexers being unsuitable for the 600 kHz split at 2m. However, I have a 8" bandpass duplexer on a commercial repeater that is using a 5.26 MHz split on VHF, and it works perfectly. I specified it because of the antenna being the high point on the tower, and I wanted DC ground at the duplexer for repeater protection. 73, Eric Lemmon WB6FLY <snip all following>
