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## RE: [TowerTalk] antenna FS measurements

 To: "Tom Rauch" ,"Jim Lux" , ,, RE: [TowerTalk] antenna FS measurements "Jim Jarvis" jimjarvis@ieee.org Wed, 30 Jun 2004 01:33:58 -0000
 ```I'd suspect the short term stability of the MJF attenuators is reasonably good. Most things ARE, for short intervals. The big source of error is likely switches in the attenuator. I agree with Tom, however, that the best method, given what we typically have to work with, is a variable attenuator, and driving to ONE S-meter reading. That removes the receiver variables, as far as we can see them. Our discussion, I would submit, is taking place within the 10%-1% window. Or perhaps the 20% to 5% window. I've limited what I've done to a select few directions.... Dead front, +/- 35 degrees of front, dead rear, +/- 15 degrees of rear.. and +/- 90 degrees. Usually good enough to know if it's close to reasonable. Having greater resolution of a 10-20% number is not meaningful, except to get a view of the overall pattern, given that absolute amplitude isn't right. N2EA jimjarvis@ieee.org -----Original Message----- From: Tom Rauch [mailto:w8ji@contesting.com] Sent: Wednesday, June 30, 2004 1:13 AM To: Jim Lux; TexasRF@aol.com; jimjarvis@ieee.org; towertalk@contesting.com Subject: Re: [TowerTalk] antenna FS measurements > Why not? Presumably the gain of the radio will be stable (in the short > run). The sound card gain likewise. The sampling rate of the sound card The word "presuming" about sums it all up. It's a HUGE chain of gain stages including everything from RF through IF and audio stages and right up to the A/D conversion. This method would depend on that entire system to be LINEAR, not just gain stable. Gain stable is probably one of the less glaring problems. An expensive spectrum analyzer intentionally designed to be accurate is working pretty well if it is within 2dB over a wide range of input levels. We can safely bet a receiver designed to operate with AGC won't be near that good. > > MFJ sells a surface mount step attenuator that is accurate > > within a small fraction of a dB per step. Of course I'd > > check it first. I have three or four, and they are within > > .05dB per step. > > That makes it hard to do an automated measurement. I doubt anyone will do an automated measurement anyway. It would be tough to obtain a stable source (it has to have a pattern to the RX antenna direction that is steady, and that includes polarization). Worse yet, it is subject to ground effects on it's pattern. > procedure which depends on lots of manual operations is going to tend to > reduce the total number of measurements made, so you lose the good > statistics. Off hand, I'd trust the measurements from a sound card, or from > a DVM measuring the audio or IF output more than manually entered switch > flipping. Having worked with receivers and even DVM's most of my life, I wouldn't. (If you're measuring the IF level, I would have a question about > the linearity of the detector). Sure, manual methods can make good If you are measuring the very same IF level through an unknown detector and several additional AF stages including a sound card, I'd have a question about linearity also. > measurements, but over the long run, for instance, I'd trust that automatic > network analyzer more than the slotted line and voltmeter. Likewise for The typical amateur receiver is not designed, constructed, or corrected as well as a \$40,000 network analyzer. It's more like the slotted line and voltmeter. > power measurements. Carefully done attenuator substitution measurements to > the same detected level are metrologically good, but tedious, and probably That about sums it up also. Reseting level to the same point removes all errors except attenuator calibration errors and gain drift errors. It's a good method. > Out of curiosity: How stable are those MFJ attenuators (over aging and > temperature.. I assume that the connector repeatability is in the 0.5 dB I've never seen cheap HF connectors vary 0.5dB no matter how many times they are disconnected and reconnected. Geeze, if you transmitted through the thing at 1500 watts it would probably glow red. Are you sure you don't mean .05dB? Or did someone forget to solder something? As for temperature, I don't worry about temperature. My room stays around 78 degrees as long as I pay my electric bill on time. Flying something around the antenna may be interesting and challenging, an interesting hobby even unto itself, but you really need to look at the entire system. With the thousands of dollars and hundreds of hours required to set up the system, I'd budget a little more time and money to calibrate with a known good attenuator. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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