Jim, The 8447D seems a fine amplifier of its type. I've used another make that was MUCH more susceptible to intermodulation and distortion. But it is broadband, and it does amplify the mega-signals as well as the low level ones you want to see. So, even if the _amplifier_ does not crunch, you may find the analyzer front-end being overloaded by strong (and add 25 dB!) signals. Anything you do that raises the signal level at the analyzer may have this effect. I once bought -- out of my own pocket -- a 9913 type coax to replace RG214 on a ten-meter site. The resulting increase in signal raised IM so much that mixing products actually obscured the signals we wished to see.
Was THAT a surprise (ever pulled cable on a 10 meter site?). The solution was a 10 dB attenuator on the preamp. However, if you are willing to use the HP PREselector, which can give 20 dB of gain on its own, you can eliminate the 8447D, and when you do this, the odds are much in favor of being able to see low level signals you could not before. Of course, you need a low-loss coax OR enough gain at the antenna to overcome the coax loss. This is pretty much a requirement anyway because given the loss in 30 meters or so of cable, the noise floor at an analyzer is approaching the limit around a GHz unless you can reduce coax loss. That's why I changed coax. It's worse at a 30 meter site. Some folks go rigid coax all the way to the antenna pedestal, and it is a good investment. If you did use an amplifier at the antenna, you would have to take its thermal response into account, provide power, and keep an eye on the gain. Low loss coax, on the other hand. is much simpler to deal with. Now, as to your particular setup... if you mean to do quick, close pretests, then you will probably be fairly happy with a 1 meter test distance (given a small EUT). It may happen that a fixed antenna height will do for you. At this distance, you could probably use RG-58 and not notice a problem, and the 8447D preamplifier might not even be needed except with low-gain loops and probes -- it is handy for those. A 1 meter scan can be very useful. But if you need to sweep the antenna height -- the FCC wants 1 meter to 4 meter heights -- you NEED a mast with remote elevation control. You CAN build these, and you can buy them. It's a lot easier to buy them. If you plan on sometime doing automated testing, make sure it reports height in a form you will be able to use. However, if you have the chance to get a real open air test site or a real anechoic chamber, I recommend them, the latter most of all. Getting rid of ambient signals for radiated testing is a major step forward and makes automating testing later much, much easier. Having readings you can measure up against those of your supplier (or customer!) is a Good Thing. (Don't expect an _exact_ match, of course.) Cheers, Cortland ====================== Original Message Follows ==================== >> Date: 28-Sep-98 12:25:25 MsgID: 1064-12413 ToID: 72146,373 From: Jim Eichner >INTERNET:[email protected] Subj: RE: Doubt on Measuring with Spectrum Analyser Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 You wrote: Although it is preached everywhere, a low-gain amplifier at the antenna end of a site is a good way to begin,but rarely done. Most folks, it seems, opt for inexpensive coax and a high-gain, broadband preamplifier, which invites such problems. However, that is properly another thread.) We are in the process of figuring out what is involved in setting up a basic radiated emissions measurement capability for ourselves, having gone as far as we can using near-field probes, RF clamp-on current probes, etc. We intend to buy a pair of antennae, a tripod, a pre-amp, and some cable, and hook it up to our existing SA. Your comments re pre-amp placement are familiar to me, but I'm not sure about the low-gain part of it. What we have had recommended to us is the HP 8447D, which has >20dB gain across the band (30M-1GHz) we're interested in. Is this too high for use at the antenna end of the system? If so, why? Also, I thought putting the pre-amp at the antenna end would let us use cheap co-ax, since the S-N issue would be dealt with by having a stronger signal from the antenna. Am I missing something? Also, it sounds from your e-mail as if there is an option to the use of a broad-band pre-amp. What would that be? Thanks in advance for your help. Regards, Jim Eichner Statpower Technologies Corporation [email protected] http://www.statpower.com Any opinions expressed are those of my invisible friend, who really exists. Honest. --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
