Re[2]: frequency scanners low cost equipment
I recently reviewed the antenna factors for a commercially made overlapping bow tie style antenna and find the factors to be rather high (for example, over 30 dB at many frequencies over 300 MHz). However, it is still a very simple antenna to build, and maybe your scanner will be sensitive enough to compensate for such high losses. Good receivers can detect signals as weak as -15 dBuV. To improve the antenna factors, I would suggest that the overlapping bow ties should be combined with an ordinary corner reflector. I would expect the gain to improve (up to 10dB best case) at frequencies over 50 to 100 MHz for a reasonable size reflector, plus you would benefit from attenuation of unwanted ambient signals from the rear and sides of the antenna. The design requirements are: S = the spacing from the apex of the reflector to the antenna f = frequency (lowest for this case) in MHz. Rule is: minimum S = 0.25(300/f) maximum S = 0.7(300/f) / / -- reflector / / O -- S -- @ -- antenna (in this case, our bow tie antennas) \ \ \ \ The reflector is expected to be a full wavelength (300/f) long, but I suspect you can shorten this to some degree. For 50 MHz, minimum value of S is 150 cm, length is 600 cm. For 100 MHz, minimum value of S is 75 cm, length is 300 cm. For 200 MHz, minimum value of S is 38 cm, length is 150 cm. For 300 MHz, minimum value of S is 25 cm, length is 100 cm. This antenna is simple to build, but it is too large to be built for frequencies approaching 30 MHz. You could still incorporate a large bow tie to capture such frequencies, however. I believe the angle of the reflector should be about 90 degrees. A simple BALUN for this antenna would be just a few common ferrites over the coax near the antenna end of the cable. Still, be careful how the cable moves when moving the antenna. Try to use very low loss coaxial cable such as Belden 9913 to improve overall antenna factors. Other literature may be more helpful, such as the ARRL publications on this topic. Good luck. Eric Lifsey National Instruments
Re: low cost equipment
Hi Moshe, In addition to my previous scanner specification data, I like to add: You find scanners of all kind of sizes from handheld to topdesc models. ALL scanners do not have the same features, of course, but most do have following: The electronic SMD is smal and low power due to the actual technologie. Its able to run on internal batteries, from a external 12VDC or from AC mostly with a simple AC/DC cord adapter. Typical power consumption in normal opperation is 160mA, 110mA in standby and 20Ma in power saving mode (9 till 16 V DC External or 4.8 internal). I mentioned in my previous mail usenet and mail lists. You will find several vendors on the www, search on yahoo http://www.yahoo.com/Business_and_Economy/Marketing/ (or any other web search server) with the keyword scanners in order to find them. A block diagram is rather difficult to draw, hope my previous mail and data specification helps. Drawings and allignment documentation can also be purshased at a very reasonable price from vendor. In this documentaion you can also find the spurious freq. spots, bandwith spec., sensivity , .. They have a good sensitivity around the 1 uV (180 kHz bandwith). Hope this helps you Moshe. moshe_vald...@isr-rhv-p1.ccmail.compuserve.com wrote: 1. How these scanners are built, any info like block diagrams preferably on the net. How big are they? Do they run on batteries? 2. Where I can buy the ones you recommend. Do the vendors have an internet page? Or at least an address or some way to make contact with them.-- Paul Rampelbergh Wezembeek-Oppem (Belgium) -
Re: frequency scanners low cost equipment
Thanks for the post, Paul. I'm not trying to mis-state your position, believe me! It's interesting to see what you have to do in order to make your scanner do the job for you. Yes, I found the frequency accuracy of even handheld scanners superior to the HP spectrum analyzers I was using. This is a concern only when dealing with someone who doesn't know they can be expected to differ. On antennas: Almost any antenna you can build will do the job for you. Those concerned with predicting antenna factor from construction need precisely built baluns and elements - but for ordinary measurement, all you need is to calibrate whatever you've built -- assuming it's efficient enough to detect signals below the limit, and that's not hard. Have you seen the antennas now being marketed as 30-1000 MHz antennas? they consist of multiple (three, I think) bow-tie antennas of sheet metal, assembled onto a common feed point. The longest one is actually resonant about 70 MHz, the next appears to be resonant about 100, and the smallest perhaps at 200, but the shape of the plates insures a gently changing impedance versus frequency. As for calibration, I think the most elegant (and acceptable, BTW) way is the three-antenna method, where one measures the site attenuation between three antennas, one pair at a time. This produces a data set contacting all antenna factors, which may be extracted from three simultaneous equations. This _does_ call for three antennas, but you can borrow two of them. Cheers, Cortland == Original Message Follows Date: 12-Feb-97 01:19:09 MsgID: 1044-129130 ToID: 72146,373 From: Paul Rampelbergh INTERNET:rampelberg...@infoboard.be Subj: frequency scanners low cost equipment Chrg: $0.00 Imp: Norm Sens: StdReceipt: NoPart 1 of 1 Hello, Sorry Cortland, I do not allow you to state a position witch I didn't mention subject purpose of utilization range of scanners. The purpose of the use of a scanner was and is for PRE-COMPLIANCE tests in order to find out where the problem areas are before an expensive and complete compliance test. I talk only about the AR8000 from AOR (non commercial advertising). I don't want to open the discussion on the different point of views we had before, but I like to state that the new scanners have good performances as long as you don't want to measure the exact amounts you are below the rule. Linearity for instance has no importance as long as you know the indication below which you have to stay. Anyhow, I think the first step to do in order to find out if the equipment is susceptible to reach compliance, is to have measurements done in not ideal conditions (small shielded enclosure for instance). NOTE: The people who can afford a anechoic chamber don't have to bother about scanners. Its a poor mans solution anyhow! 1. So as you are not, for the first instance, in ideal conditions you gather the disturbing signal frequencies. For this application, the stability and accuracy of a scanner SUPERIOR to older well know expensive equipment's like the Rohde Schwatz generator SMDA - BN 41314, due to the actual technology (phase locked loops crystal). NOTE: Its also, in most of the cases, possible to deduct those freq. from the design but its good to measure it with the scanner just to make sure you get them programmed in for field tests. Modern scanners can memorize the frequencies of interest manually or automatically. 2. Now once you have those frequencies you have to calibrate your receiver (scanner) and there there are some problems. 2.1. Shielding of the receiver is insufficient. Solution: metallic shielding spray or other mechanical means. 2.2. the front end of the receiver do not need pre-tuning, if you have a strong signal close to the frequency you need to measure, you MAY need an external additional input filter to avoid intermodulation. 2.3. You need calibration of the interesting frequencies, the other ones are of NO interest. This is the most difficult part, you need a tests generator, I have the one mentioned above but it needs probably recalibration. 3. The antenna's. Who can tell me how and where I can get the required data to build a Bi-connical antenna for 30 - 200 Mhz frequencies. How much can the calibration deviate from one design to another? The log-periodical antenna is no problem to build, you can find data in the ARRL handbook. Its low cost, use of copper water pipes should do it. Inputs for more data? antenna 4. Now you are able to go to the free air, smell some good cold whether, an measure the previously obtained disturbing frequencies. I was unable to do this due to the stubborn position of our local authorities, not to mention the IBPT (=FCC). To come back on the position of our local authorities
frequency scanners low cost equipment
, dictatorial, monopolistic bureaucrats and irresponsible authority attacks. I Quit on the subject for now. The BIPT is proposing a modifications to Belgium laws for telecom, I'm wondering how good they will recover on the EMC laws and try to impose once again their monopolistic position. STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP Now once for ALL Cortland Richmond, STOP. STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP, STOP The dangerous people is not the expensive nor the low price test equipment owners, but the outlaws as they do NOT respect any Law. Using comments of telephone industry representatives concern is pure intentional discrediting talk that doesn't make any sense. You can use some expensive test equipment as well in outlaw conditions. In fact, you are in a professional context of experienced people who can design telephone decoders. So quit please and let other people share their experience on EMC low cost equipment. Cortland Richmond you never described your job function, are you involved with similar authorities, rules making people or do you have any financial advantage by discrediting possible competition low cost equipment's? re- Aaargh. === I like to continue the discussion on low cost equipment without ALL the nothing to do with it considerations. Lets find out how good, bad or limiting this kind of low cost equipment is, lets look in which context it can be used or lets find out its useful area. I don't like to limit mail exchange to scanner type equipment, but rather to ALL kinds of low cost devices. I'm thinking for instance on a gas lighter for ESD, etc.. What are the results? That would be at least a constructive input. Lets look at home made antenna's, test cell's and other EMC test equipment. For sure it will cost less than old equipment which anyhow will require outside certification. I don't say that we don't need outdoor certification with more sophisticated equipment, but at least lets try to find a possible way around or at least reduce cost due to a one time certification pass. Don't mention publications, I purshased an promissing book Handbook of Antennas for EMC by Thereza Macnamara. Nice expensive and purelly of no use. Its a theoritical consideration not covering the actual used antennas. So I sugest to remove book recomandations from those discutions and have instead more practical designs and considerations. I'm sure I'm not the only one who like to limit cost for EMC. -- Paul Rampelbergh Wezembeek-Oppem (Belgium) -
Re: frequency scanners low cost equipment
Hello all, thanks for the responses. Still I don't have a clue re: 1. How these scanners are built, any info like block diagrams preferably on the net. How big are they? Do they run on batteries? 2. Where I can buy the ones you recommend. Do the vendors have an internet page? Or at least an address or some way to make contact with them. thanks again moshe valdman
Re: LOW COST equipment for EMC.
Hi, Gabriel Roy/HNS wrote: For REALLY low cost test equipment for use during the developmental stage, refer to Dr. Keenan's book Digital Design for Interference Specification Section 6.3. ...snip... Dr. Keenan's book is published by TKC in florida, (813) 544-2594 Also on http:/www.tkcemi.com/publicat.html Thanks for info Gabriel, -- Paul Rampelbergh Wezembeek-Oppem (Belgium) -
