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December 27th 04, 05:12 PM
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On Mon, 27 Dec 2004 15:55:14 GMT, (Robert
Lay) wrote: On 27 Dec 2004 14:09:32 GMT, (PDRUNEN) wrote: Hi Group, Have a MFJ-269 here (new toy) and did a bit of expermienting with the TV antenna over the Christmast weekend, I have one of those 13 element antenna from radio shack, I don't know the exact part number. I tied the MFJ-269 into the 300 ohm to 75 ohm balun at the antenna. The antenna being about 20 feet up on the house peak. Looking at the impedances I got, nothing really looked good at the TV frequencies as I would have expected. If the match is this bad, I would expect alot of loss from the fact that I run 75 ohm cable down from this antenna to the TV set. Could my reading be mis-leading since the balum is a 4:1 going from 300 ohm to 75 ohm? Has anyone done this before and posted the data? I would be interested to see. I don't have my data in front of me at this time so I don't have any measurements to post. I really find the MFJ-269 an exceptional device for the antenna experimenter. Tnx de KJ4UO Dear KJ4UO, You might look through the user's guide and check me on this, but is it not the case that the MFJ-269 is designed and calibrated for 50 ohms? The 269 has an "Advanced 3" menu (page 30) that allows you to set the SWR reference impedance to values other than 50 ohms, and measure line loss and SWR in systems other than 50 ohms. bob k5qwg If that is the case, as I think it is, then anything other than 50 ohms is going to look like it has high SWR. If you TV antenna is what it is supposed to be, then looking into the 300 ohm Xfrm would look like 300 + j0 (ideally) and if you look into the 75 ohm cable, instead, it would look like 75 + j0 (ideally). The SWR in those two cases would be 6 and 1.5, respectively and that's the BEST you would get. Bob, W9DMK, Dahlgren, VA http://www.qsl.net/w9dmk |
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