Date: Thu, 16 Dec 93 04:30:18 PST From: Ham-Ant Mailing List and Newsgroup Errors-To: Ham-Ant-Errors@UCSD.Edu Reply-To: Ham-Ant@UCSD.Edu Precedence: Bulk Subject: Ham-Ant Digest V93 #144 To: Ham-Ant Ham-Ant Digest Thu, 16 Dec 93 Volume 93 : Issue 144 Today's Topics: Antenna Tuner Questions (2 msgs) Send Replies or notes for publication to: Send subscription requests to: Problems you can't solve otherwise to brian@ucsd.edu. Archives of past issues of the Ham-Ant Digest are available (by FTP only) from UCSD.Edu in directory "mailarchives/ham-ant". We trust that readers are intelligent enough to realize that all text herein consists of personal comments and does not represent the official policies or positions of any party. Your mileage may vary. So there. ---------------------------------------------------------------------- Date: Wed, 15 Dec 1993 16:17:16 GMT From: news.cerf.net!pagesat!direct!herald.indirect.com!kg7bk@network.ucsd.edu Subject: Antenna Tuner Questions To: ham-ant@ucsd.edu I am teaching a ham class and have been asked questions that I can't answer (not unusual) but I can't find the answer in the Handbook or Antenna Book... Here they are: 1. The Handbook says that an antenna tuner's loss is normally less than 0.5 dB. Assuming a 10/1 SWR, is the loss greater when one is matching a load of 5 + j0, 500 + j0, 10 + j50,...etc.? 2. Which is the best antenna tuner, a T, Pi, Z,...etc.? 3. What is the transfer function of a transmission-line transformer type Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately 250 + j250 or is it like the coax series-section transformer equations? thanks, Cecil, kg7bk@indirect.com ------------------------------ Date: 15 Dec 93 21:08:47 GMT From: ogicse!hp-cv!sdd.hp.com!col.hp.com!news.dtc.hp.com!srgenprp!alanb@network.ucsd.edu Subject: Antenna Tuner Questions To: ham-ant@ucsd.edu Cecil Moore (kg7bk@indirect.com) wrote: : 1. The Handbook says that an antenna tuner's loss is normally less than : 0.5 dB. Assuming a 10/1 SWR, is the loss greater when one is matching : a load of 5 + j0, 500 + j0, 10 + j50,...etc.? I don't know what "normally" means, but I can tell you that there are plenty of poorly-designed antenna tuners being manufactured with more than 5 dB insertion loss. With most designs, the loss is a strong function of the antenna load impedance. If you measure the loss when "matching" a 50-ohm load, you probably don't get worst-case results. : 2. Which is the best antenna tuner, a T, Pi, Z,...etc.? An L-network is the simplest. However, since any antenna tuner requires at least two adjustments (to match the real and imaginary part of the load impedance), an L-network requires that both the capacitor and inductor be adjustable. Since adjustable inductors are harder to make, a Pi or T match is usually preferred. When I was at Drake, I designed the MN-2700 antenna tuner. Unlike most amateur tuners, it actually has an insertion loss spec (.5 dB) which applies to any load impedance within the tuner's matching range. Matching range is specified at up to 5:1 SWR at any phase angle, although it will typically do much better than that with high (>250 ohm) loads. It uses a type of Pi/T network: Fixed Variable 50 ohm -------*---Inductor---*-----Capacitor----- Antenna source | | Fixed Variable Capacitor Capacitor | | Gnd Gnd The fixed capacitor and inductor are band-switched. Since there are no variable components between the 50-ohm source and the inductor, the current through it (and thus the loss) are independent of the load impedance when the tuner is adjusted for perfect match. Since the capacitors are nearly lossless, the tuner loss is independent of load impedance. All the Drake tuners (MN-4, MN-2000, MN-7, MN-2700) use this same circuit topology. : 3. What is the transfer function of a transmission-line transformer type : Balun? i.e. Does a 4:1 Balun transform 1000 +j1000 to approximately : 250 + j250 or is it like the coax series-section transformer equations? Theoretically, both the real and imaginary parts are scaled by 4:1. So yes, 1000 + j1000 becomes 250 +j250. However, be aware that most baluns have stray reactances that become more important with high SWR. Also, high SWR causes increased current or voltage in the balun for a given power level: With a 10:1 SWR, for example, current can be as high as 3.16 times the 50-ohm value, so your 1 kw balun is now a 100 watt balun! AL N1AL ------------------------------ End of Ham-Ant Digest V93 #144 ****************************** ******************************