------------------------ From: williams <[email protected]> Subject: Balun for a reference dipole Date: Mon, 07 Sep 1998 09:29:21 +0200 To: "[email protected]" <[email protected]>
> Hello All, > Could someone tell me what kind of balun is used on a typical reference > dipole? > Thanks, > 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 > Hi Kevin! 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! The upper limit for practical dipoles is about 1 GHz. First, the dipole is starting to get pretty small, and manufacturing and usage variations are tough to control. Second, pyramidal horn antennas begin to get reasonably small at this frequency. Why a horn? Well, they are physically more robust, don't need baluns, are inherently directional, are more efficient, and have a gain (thus AF) which is reasonably predicted from their physical dimensions alone. (But not ever well enough to be able to avoid proper calibration on a range.) I use an EMCO 3115 ridged horn antenna from 1 GHz up (to way beyond 2 GHz), but for this range, I think it's a pretty good choice. Older horn antenna sets, such as the Polarad CA series, have a horn (the CA-L) which covers 1 GHz to 2.3 GHz, with an AF of about 23 dB. The CA-L has an aperture of about 12", and is quite portable. Another old horn series, the Singer EMA 910, has a 1010 horn which covers 1 GHz to 2.5 GHz, with an AF of about 15 dB, and is only a bit bigger than the Polarad model. A 1 GHz to 2 GHz horn can be used down to 900 MHz; just have your antenna cal lab get you the data when they do the periodic testing. Now, back to dipoles. Mark talked about his dipoles being labeled T1, T2 and T3. This raises a bit of a flag in my mind. The T series dipoles originated as accessories to the very old (mid 50's) Empire Devices NF-105 field strength meter. The NF-105 had 5 plug-in tuning heads, and, you guessed it, the antennas (almost) matched the tuning head designations. The DM-105-T1 dipole covered 20 MHz to 140 MHz, the DM-105-T2 dipole covered 140 MHz to 400 MHz, and the DM-105-T3 dipole covered 400 MHz to 1000 MHz. All of these antennas used a length of coax wound around a wood dowell in a box at the rear of the antenna boom to create a matching transformer. The problem is, three antennas are not enough to cover this frequency span; you really should use four. Right now, I'm looking at some antenna calibration measurements I did on a T series dipole set in 1980. (I knew that data would be of some use, someday.) It shows the antenna factor for the T3 zooming after about 925 MHz. It's almost 10 dB above predicted at 1000 MHz. The balun just isn't broadband enough. (Granted, this is data on an old antenna. The rights to these antennas have passed through several companies since then. Maybe somewhere along the way, the dipoles were redesigned. Maybe.) The Roberts' antennas are more conservative, using four dipoles for the same frequency range. Another contemporary dipole antenna set was made by Stoddart Aircraft Radio, for their NM series field strength meters. The Stoddart NM-52A had an accessory dipole, the 91598-2, which covered 400 MHz to 1000 MHz. Interestingly, this antenna did not use a coax balun. Instead, it had a pair of tubular stainless steel stalks, with a sliding shorting bar that created a matching transformer. (So you had to adjust both of the dipole arms and the shorting bar for each frequency.) BTW, remember that you have to actually calibrate every antenna that you use. The manufacturer provided only a theoretical calibration curve, NOT a calibration curve for YOUR antenna. Manufacturer's data for these older antennas is not to be trusted. Cortland provided a wealth of things to think about. His mention of the German Schwarzbeck dipoles made me recall that I had evaluated buying a set of those antennas around 1980. The Schwarzbeck design departed from typical US design in that it used a passive resistor matching network to couple the 72 ohm dipole arms to the 50 ohm coax. There was a small, thick film (!) resistance element located between the dipole arms, with a balanced resistor network coming away from that element. IIRC, the AF was almost 15 dB higher than a typical dipole, since the resistive elements made the antenna very inefficient, but the gain flatness was excellent. Needless to say, you had to be very careful about using this as an illuminating antenna, since you could fry that thick film element quite easily. (Maybe the input power should be limited to no more than 10 dBm.) -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 09/08/1998 Time: 11:03:07 -------------------------- --------- 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).
