A (very) short dipole, L<<lambda, not a resonant dipole. It should be calibrated. Line up electrically long portion of the coax with ferrites, all the way up to the dipole, and it forces the current to flow from the dipole legs into the (inside of the) coax cable, and not on the outside of the outer conductor, so it makes the simplest balun. It also reduces/minimizes the effect of the feeding coax to the distribution of the measured field. There are other ways of doing this, but it seems to me this is the simplest and perhaps the most suitable for the purpose. Neven
-------------- Original message -------------- From: "David Cuthbert" <[email protected]> Does one have to calibrate a dipole? They are considered a calculable antenna, are they not? At 1900 MHz a resonant halfwave dipole has an antenna factor of 33.9 dB. To achieve an antenna factor within 0.5 dB the of this the balun must have an impedance of 350 ohms or more. I was thinking that rather than ferrites to form a current balun a sleeve balun can be constructed. It needs to be 3.95 cm if there is an adequate air gap between the coaxial cable feedline and the sleeve balun. Dave Cuthbert Linear Technology Corporation _____ From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Tuesday, November 14, 2006 9:21 PM To: [email protected] Subject: Re: balun info So, a short dipole and calibration in front of a horn would also work. I think it may be better for your application. It would allow you to come closer to the transmitter antenna and still measure accurately. You don't have to worry too much about the sensitivity, you should have a plenty of field to measure if you hook up the spectrum analyzer on the other side of the cable. Neven -------------- Original message -------------- From: Ken Javor <[email protected]> What I meant was that if I had to build a dipole then I would have to calibrate an antenna factor, and in order to do that I would have to build two identical units, and measure site attenuation. I would only need to use 0 dBm or even less to transmit, so I don't think that would drive the balun design, but I mentioned it just in case. From: [email protected] List-Post: [email protected] Date: Tue, 14 Nov 2006 22:59:04 +0000 To: <[email protected]> Subject: Re: balun info Ken, I am not sure anymore, do you need to measure intensity or generate field (or both)? You two posts indicate you may have to do both, so my reply was obviously wrt measuring the field, but now I realize it is not going to help you to generate field. Neven -------------- Original message -------------- From: [email protected] Ken, Why don't you use calibrated short dipole instead, perhaps ~2 cm? You could run your own calibration in front of a horn antenna, but be careful about the influence of the leads. The best orientation for the leads is in-line with the Poynting vector, i.e. perpendicular to both E and H field. BTW, I spent considerable time studying it, e.g. how the field-pickup by the leads affects the readout. The simplest way would be to create a dipole at the end of coax, simply two short wires (perhaps on dielectric support so they are mechanically stable), no balun, and then line the good length of the coax with ferrites that are good for the frequency you measure (which would form the balun). If you go to near-field of the phone-antenna, you may have to search for the right polarity fo! r the m easurement. Are you really interested in the intensity (ExH) or is E**2/(120Pi) the mea! sure of intensity that you are looking for, assuming you do it for exposure evaluation and assuming that E-field is really of concern? You might consider, as an alternative, using one of the electrically small isotropic probes available on the market. ~15 years ago, I was using the small EMCO probe for doing exactly the same. We measured ~200 V/m about 1 cm from (I believe it was GSM) phone. You may have issues if the field is pulse-modulated, since these probes use detectors and are typically calibrated with CW. I am not sure what's currently on the market, I am doing different things now. Neven -------------- Original message -------------- From: Ken Javor <[email protected]> Ed, Thanks for the info on the Empire Devices dipole. I have both Empire and Stoddart/Eaton Ailtech dipole sets. I have both the Eaton equivalent of the EMCO 3115 and a Stoddart original 1-10 GHz logconical or log spiral. The problem is I need to make field intensity measurements of cell phones (850 MHz) and PCS devices (1900 MHz) transmitting up close, closer than 1 meter. I am afraid the larger apertures and distributed structures of the horn and logconical, respectively, will cause the antenna factor to change as I approach either of these antennas from the one meter antenna factor calibration distance. Whereas I know that the gain of a dipole is relatively constant once you are a half-wavelength away, which means I can come much closer. Ken! From: "Price, Ed" <[email protected]> List-Post: [email protected] Date: Tue, 14 Nov 2006 06:09:09 -0800 To: [email protected] Subject: RE: balun info From: Ken Javor [mailto:[email protected]] Sent: Monday, November 13, 2006 4:20 PM To: [email protected] Subject: balun info Experts, I need to build/buy a 1.9 GHz PCS band dipole receive antenna. Haven't found anything to buy. What is a suitable material and balun design to go from 72 Ohm balanced to 50 Ohm coax? Could I use an old T3 balun (400 - 1000 MHz tunable dipole) and just modify the elements? I will need to build two so I can calibrate an antenna factor. I will transmit 0 dBm or less. Thanks in advance! Ken Javor Ken: A dirty litt! le secr et about the old Empire T3 Dipole was that it's balun didn't work all the way to 1000 MHz. Everything was sweet until about 950 MHz, and by 975 MHz, the balun could be ca! ! ! usi ng a bout 5 dB more loss than expected. At 1000 MHz, you would be down about 8 or 9 dB. The competing Stoddart / Singer 90330-2 dipole used a sliding short in addition to adjusting the arm lengths (was that a J-match?). This worked well up to about 1300 MHz. One of the nicest and most flat matches was achieved by Schwarzbeck. They put a little film resistor network at the junction of the dipole arms, creating a purely resistive match. The only drawback was that you sacrificed some sensitivity, and you had to be careful to not use that antenna with a signal source above! a few milliwatts. At 1 GHz, you are high enough that horns are not too huge, so why not consider building / buying a rugged little horn like an EMCO 3115 or even an ancient Polarad CA-L? ! Ed Price [email protected] NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (FAX) Military & Avionics EMC ! Is Our Specialty - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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