Cortland: I think that the 10 dB attenuator would have done better service when connected directly at the antenna by improving the match. Two 6 dB attenuators should render better service, one at the antenna, the other directly at the amplifier. Ferrite clamp-on "beads" placed at irregular intervals on the cable help reduce common mode inputs to the amplifier and from my experience also do help (obviously). Regards, Bogdan.
Cortland Richmond wrote: > 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). --------- 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).
