On more than one occasion, I have found those holes to be the cause of leakage, especially at UHF. Sometimes, RF leaks out of orifices that "conventional wisdom" might refute, but it does happen. A case in point: I once put a Hamtronics UHF exciter into a Hamtronics "RF-Tight Enclosure" to use as a stable low-power RF source for a signal coverage survey. Despite using feed-through filters for the power leads (the Morse ID unit was inside the case), there was significant RF leakage even when a dummy load was put directly on the N-Female RF output connector. Even though the Hamtronics enclosure halves were well fitted, there was a very small gap, perhaps 0.002" where the metal pieces joined. I found that I could press the gap closed with a finger, and the leakage went away. I had some 3M #1170 aluminum shielding tape available, so I sealed all of the seams with it. Voila! I now had a true RF-tight enclosure.
The dimensions of the holes and slots on the sides of the Motorola T-1500 series duplexer are considerably larger than the tiny apertures noted in my example above, so it should not be a surprise that some leakage can occur. No doubt, any leakage will be frequency-dependent, since the distribution of RF inside the cavities will vary with the tuning. As a result, some installations may benefit greatly from sealing the holes and slots, while others may see no benefit. I can suggest an offbeat application for this #1170 shielding tape: Many GE and Motorola radios that are popular for duplex modification use an "RCA Phono" plug for RF connections inside the radio. If the RCA plug is not seated firmly, leakage can possibly occur through the slots on the skirt of the plug. To prevent such leakage, tightly wrap one turn or so of shielding tape around the plug. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Tuesday, August 26, 2008 8:37 PM To: [email protected] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question At 8/26/2008 09:38, you wrote: >OK, I found the message& now the question: what aluminum tape is best for >this? Or is there a best tape? I just got a Motorola duplexer for a 462 >repeater I'm building, and it has the holes/slots in it. (And yes, it >*IS* a MICOR.) Of the various types of tape available, does the adhesive >need to be conductive? Or is it "good enough" for the foil to merely cover >the openings? I wouldn't bother with conductive tape, as the holes in question are too small to propagate any RF. Just cover them with electrical tape if you're worried about dust or moisture infiltration. Bob NO6B
