Guys, I appreciate the thoughtful responses. I've been taught the same stuff about impredance transformation vs. electrical length, but I've also seen these traps work.
Gary wrote: "This measurement that you made indicates that the cable is a quarter wave (not ½ wave) at 146.15 if indeed you do have the far end open." Gary, a half-wave in free space at 146 MHz, with no accounting for velocity factor, is 40.4207 inches. a 1/4-wave stub would be 20.2104 inches if the cable had a velocity facotr of 1.00, and much shorter with any real-world cable. Jim wrote: "Paul, I think where the problem with your measurements comes up is that your 41 inches of RG-8 is close to 3/4 wave at 2 meters, not 1/2 wave. Taking a rough estimate of 19 inches for a quarter wave on 2 meters...The velocity factor of RG-8 is probably near .66, so your half wave of RG-8 coax would be close to 24 inches, not the 41 inches you quote." Jim, a 19-inch 1/4-wave whip is already cut for a ~95% velocity factor, to compensate for typical materials. You won't get the right electrical length if you apply a second velocity factor to the already-shortened 19 inches. My coax is RG-8M, similar to RG-8X. I couldn't find a published velocity factor for M, but thought I remembered .87. RG-8X is .84 (not .66). I cut a piece off an old scrap cable (with connector) to 41 inches, measured from the tip of the PL-259's center pin to the cut point, as a starting point for trimming. When I stripped and shorted the end, (now about 40.75",) and got it on the T-connector, I found it was already showing a "center" of 146.15 MHz. Even with a .84 VF, this is much closer to a half-wave than a 1/4- or 3/4-wave. Since my original post, I have actually stuck the shorted stub on a 2m rig, and verified that incoming signal strengths from distant repeaters remain consistent with or without the stub. If the VF of my cable is .84, that's still 214 degrees of electrical length at 146 MHz, very close to 180 degrees, and nowhere near either 1/4- or 3/4-wavelength. By the way, I'm not necessarily proposing using a shorted piece of RG-8M as a permanent filter at a repeater site, but it sure can be a handy troubleshooting item coiled up in a toolbox. Jim wrote: "If you have access to a wide frequency range antenna analyzer, hook your coax (shorted on the far end) to the analyzer and find the lowest frequency that shows a minimum resistance and minimum reactance. That is the frequency that your coax is a half wavelength. This takes in account the connector you would connect to your Tee and the velocity factor of the coax." I just tried it...99 MHz, R=1, X=0. So, the coax and PL259 together have a velocity factor of .66? Wow...if that's the case, it is indeed a 3/4-wave stub, and I should be able to cut the stub waaaaay back, until it's 1/4-wave in electrical length, and find a length which yields similar results. Hmmm... Jim, I may owe you the beverage of your choice at a future hamfest. I'll pay for it with the savings from a lifetime of buying shorter pieces of coax. 73, Paul, AE4KR > ...so I put my MFJ 259B analyzer, a 50-ohm dummy load, and a 41-inch piece of > RG-8M (1/2-wave cut for 2m) on a T connector and look at SWR and impedance. > > Here are the resulting measurements of resistance, reactance, and SWR: > > 146.15 MHz (Shorted 1/2-wave): R=47, X=2, SWR=1.0... > 73.08 MHz (Shorted 1/4-wave): R=23, X=20, SWR=2.4 (Z= about 31 ohms) > 73.08 MHz (Open 1/4-wave): R=25, X=26, SWR=2.7 (Z= about 38 ohms) > 146.15 MHz (Open 1/2-wave): R=3, X=1, SWR=12.0 (Z= about 3 ohms)
