------------------------ From: williams <[email protected]> Subject: Re: Balun for a reference dipole Date: Wed, 09 Sep 1998 09:42:23 +0200 To: "[email protected]" <[email protected]>
> Greetings All! > [email protected] wrote: > > You need to measure 900 Mhz now, and later up to about 2 GHz? You want to > > > be able to see 60 dBuV/m to 80 dBuV/m field strength signals. And you want > > > to do it with a dipole? Maybe you shouldn't do it with a dipole! > Well, only in the GSM band around 900MHz, and later in the 1.8GHz band > as well. It just would have been convenient to re-use a good balun. A good (used) balun is hard to find. I would rather avoid using a balun by choosing an antenna that doesn't need a balun. > The reason why I have been thinking about the dipole and not an LP or a > horn, is that I do not want to use too directional an antenna, as I will > be measuring fields in a room where the RF source is a transmitter > located some tens of metres outside. I don't have some source in > the > room which I could aim a directional antenna at. I am therefore > not sure of > the angle of arrival of radiation when I'm in the room, > and I can't be sure > I'm measuring the peak field strength in this > situation? Put your spectrum analyzer in Peak Hold, take the horn antenna in your hand, and spend several minutes sweeping the antenna methodically around the equipment location in the room, varying the polarization on successive sweeps. Now go read your spectrum analyzer; I think you will have a very high probability of having found the peak signal strength, regardless of the angle. > I had done a quick calculation of the power I would expect Now wait; did you want to measure field strength, in V/M, or power density, in W/M^2? Your assumed signal levels are too low to measure with a typical 3 axis bolometer power density meter. And you can't relate field strength to power density unless you know the wave impedance, which will be anything but 377 ohms if you are not in far field conditions. > with a > typical AF of 28dB at 900MHz for a dipole in the HP11966H set, and at > 10mV/m 80dBuV/m I should be looking at around -57dBm into a shielded > spectrum analyser. I was planning to use a bandpass filter and > preamplifier. Lessee... 80dBuV/m - 28dB AF = 52dBuV at the antenna output. Then, give about 4dB for cable loss, and it's 48dBuV into the spectrum analyzer. That's also -59dBm, but who cares? That's a very usable signal level. Why would you need a pre-amp? Also, why would you need a BP filter? Is there another strong signal in the room which must be discriminated against? > Kevin Williams, MIEEE > Electromagnetic Software and Systems (EMSS) [http://www.emss.co.za] > Technopark, Stellenbosch, South Africa > email: [email protected] > tel/fax: +27 21 880-1880 +27 21 880-1727 -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 09/09/1998 Time: 09:55:11 -------------------------- --------- 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], or [email protected] (the list administrators).
