Tom Donnelly wrote: >> When making measurements to CFR 47, Part 15, and using >> ANSI C63.4-1992 there are 2 requirements that conflict >> for measurements at lower frequencies. The first >> allows broadband antenna use as long as the results >> can be correlated to a 1/2 wave tuned dipole. The >> second requires a 1 to 4 meter height scan. At >> lower frequencies where the tuned length of the dipole >> exceeds that which would allow use of a 1 meter lower >> limit a conflict occurs as the dipole cannot go down >> to 1 meter. When broadband antennas are used at these >> lower frequencies, and a 1 meter lower limit is utilized, >> the result cannot be correlated to the dipole. The only >> way to establish the required correlation is to adjust >> the lower limit of the height scan range to that which >> would be utilized by a dipole. The key here is that >> both antenna must be electrically centered near the >> same elevation.
There is no requirement for changing antenna factors with polarization, either, so a correlation good enough for horizontal polarization seems quite good enough to meet the requirement specified. Ground problems with dipoles are well known, and should render the dipole less desirable as its impedance (horizontal) and balance (vertical) vary with proximity to ground. Regardless of the ability of a dipole to fit into the space available, *we still need to know the field strength* at that location. Victim devices are rarely as large as the dipoles we're talking about, after all. The objection to using a biconical or other small antenna at low heights seems based on a misconception that since the standard antenna can't fit into the situation, even if we have an established correlation (free space or horizontal) we have to settle for what the standard would measure if it were there. But standards are often not applicable to field measurements. >> This will typically produce a lower measured level >> as you noted, however I do not believe this will >> cause the regulatory evaluation problems as you >> pointed out. That does depend on whether a regulatory agency measures field strength at the low elevation or ignores it, doesn't it? >> Since the date which the original message was posted I >> had a detailed conversation with Joe McNulte at the >> FCC's Office of Engineering and Technology. He concurred >> with the need to establish correlation and restrict the >> height scan of broadband antennas accordingly. This is NOT a good approach. It does create a blind spot for fields in some circumstances, which cannot be what the Commission really wants. It will also hinder acceptance of mutual agreements with other authorities, who do not observe the same restriction. >> Keep in mind there is an underlying premise to the >> FCC/ANSI standards that everything should correlate >> back to a predictable set of conditions (measurements >> on an OATS with dipole antennas). You are allowed to test >> in an anechoic chamber of GTEM as long as you can >> correlate your results to an OATS, you can use >> broadband antennas as long as you can correlate your >> results to 1/2 wave tuned dipoles. The nature of that correlation is what we are talking about. The implications of the way correlation is obtained go well beyond Part 15 OATS testing. I'd not want to be the person who tried to convince anyone that RADHAZ potential could only be measured at a position where a vertical dipole could fit. But physics being what it is, there seems no way to defend one measurement method for one thing, and another for a different. Volts per meter should be the same regardless. >> When testing to the CISPR family of documents (including >> the harmonized versions) an 80 MHz tuned dipole is >> called out for testing at frequencies 80 MHz of below. >> Again a broadband antenna is allowed but now the results >> must be correlated to the 80 MHz tuned dipole at >> these frequencies. Using a 80 MHz tuned dipole the need >> to restrict the lower limit of the height scan is not >> required. As with the 1/2 wave tuned dipole I have found >> the best correlation occurs when the electrical centers >> of the antennas are near the same elevation. Yes, this seems a much more logical approach. It allows measuring fields pretty much where they are. However, it does differ greatly from the FCC's approach to a standard antenna, and (as I mentioned) means items tested to the FCC approach may well exhibit emissions higher than Part 15 methods restricted (for vertical polarization) to 3.5 to 4 meters elevation. This is not a new discussion -- as you know! -- and if I seem to confuse this one with OATS attenuation measurements, it's because that's where I've had it in the past. We need to keep our eyes on the target, which is to know what fields exist at locations where victim devices might be --and this includes near the surface, and at lower frequencies, and at places where a full-sized, vertically polarized dipole could not fit. Let the games begin! (Te morituri salutamus) Cortland
