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- Date: Thu, 16 Dec 93 04:30:18 PST
- From: Ham-Ant Mailing List and Newsgroup <ham-ant@ucsd.edu>
- 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: <Ham-Ant@UCSD.Edu>
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- 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
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