Date: Thu, 3 Mar 94 04:30:25 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 V94 #53 To: Ham-Ant Ham-Ant Digest Thu, 3 Mar 94 Volume 94 : Issue 53 Today's Topics: 2m Groundplane Antenna Question Can I supplement antenna on a pager? ELNEC USERS?? Ham-Ant Digest V94 #52 mechanical analogue of radiation resistance? MFJ SWR Analyzers (3 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: Tue, 1 Mar 1994 16:37:33 GMT From: ihnp4.ucsd.edu!swrinde!gatech!howland.reston.ans.net!news.ans.net!malgudi.oar.net!utnetw.utoledo.edu!uoft02.utoledo.edu!cscon0151@network.ucsd.edu Subject: 2m Groundplane Antenna Question To: ham-ant@ucsd.edu ebs@csparc046.cirrus.com (eric smith) writes: > Here is some background info. I built a 2m groundplane antenna similar to > those described in the ARRL Antenna Handbook. I used #6 solid copper wire > for the monopole and the radial elements. I inserted the monopole element > directly into a S0239 connector. I hammered the ends of the radials flat, > bent them down at a 45 deg angle, drilled holes in them, screwed them in > place and soldered them. I mounted the antenna on a pvc mast that kept > the radials about 4 1/2 ft off of the ground. I started out with each element > 24" long. 4 1/2 ft off the ground.... I remember reading about a principle of improving ground plane antenna abilities, I believe it was important to keep the antenna at a height above the ground that was 1/2, 1/4, etc. wave. Could anyone quickly explain that rule? I think I might get an antenna theory book soon, antennae design fascinates me! 73s Brad Steinman, N8ZRP The University of Toledo Internet: cscon0151@uoft02.utoledo.edu (131.183.1.4) stu0105@uoft01.utoledo.edu (131.183.1.2) Packet : N8ZRP@w2xo.#swpa.pa.usa.noam (Amateur Radio Packet Network) * Member of the All-Ohio Scanner Club (OH-48-1859) * yah... ------------------------------ Date: Tue, 1 Mar 1994 21:26:38 CST From: ihnp4.ucsd.edu!swrinde!gatech!howland.reston.ans.net!vixen.cso.uiuc.edu!news.eecs.uic.edu!uicvm.uic.edu!u29255@network.ucsd.edu Subject: Can I supplement antenna on a pager? To: ham-ant@ucsd.edu My good friend, N9SCU, (who does not yet have internet access) asked me if I could help him. He says that in his new desk at work, his pager does not receive; he has to put it in someone else's desk and check it every so often. What kind of antenna, maybe a nice little wire (what length) could be led to the pager, and then the signal "induced" (how?) by a coil (diameter? turns?) to get the signal into the pager. Or, maybe you know how to crack open a Motorola digital number-only pager to get at the antenna terminal & ground to maybe hard-wire an external antenna (what kind/size/length?). As you can see, I have a lot of questions, but your comments are welcome on any of them. Thanks in advance! 73 8) AA9IF ------------------------------ Date: Tue, 1 Mar 1994 17:07:19 GMT From: ihnp4.ucsd.edu!swrinde!cs.utexas.edu!howland.reston.ans.net!gatech!paladin.american.edu!constellation!osuunx.ucc.okstate.edu!olesun!gcouger@network.ucsd.edu Subject: ELNEC USERS?? To: ham-ant@ucsd.edu Gordon AB5DG ------------------------------ Date: 2 Mar 94 15:48:21 GMT From: news-mail-gateway@ucsd.edu Subject: Ham-Ant Digest V94 #52 To: ham-ant@ucsd.edu Bob, KD1GG writes: >I have a 50' tower that I would like to use somehow >to help construct an inverted L for 160. Is an >inverted L with a 50' height and a flat top section >of about 100' usable on 160? Anyone know where to >find some design guide, like radial placement etc.. I'll tell you of my experience with a similar configuration. I use a 48' tower w/15' of mast above it that hold 2 monobanders for 20M & 15M. At the 48' point I attached a coax feedline with the braid going to the tower & a horizontal (sometimes sloping) wire about 112' long connected to the center conductor. This antenna works amazingly well - I've worked over 105 countries with it. Its not a resonant antenna, so I use a homebrew tuner to bring the SWR down to a managable level. The antenna is fed with RG-11 (75 ohm) coax. Having modelled it on ELNEC has shown that it is (almost) unidirectional with a good low angle. Main lobe is at 35 degrees, but the lower -3dB point (in the vertical plane) is at 9 degrees. It has plenty of horizontal component to allow working locals and the vertical component (the tower's contribution) is substantial enough to have allowed me to work 3Y0PI on 160 SSB. I attached 2 radials 140' long to the base of the tower and connected these to my swimming pool. I don't think this is efficient from a ground resistance point of view, but it sure was easy to do. If you place more radials at the base of the tower the low angle performamce should improve considerably. My models have shown that 120, 140' radials bring the main lobe down to 24 degrees. I hope that gives you enough encouragement to try something similar at your location. Good luck & enjoy top band. 73 de Walt - K2WK ------------------------------ Date: Wed, 2 Mar 1994 10:05:47 GMT From: ihnp4.ucsd.edu!library.ucla.edu!europa.eng.gtefsd.com!howland.reston.ans.net!gatech!wa4mei.ping.com!ke4zv!gary@network.ucsd.edu Subject: mechanical analogue of radiation resistance? To: ham-ant@ucsd.edu In article alanb@sr.hp.com (Alan Bloom) writes: >Alan M. Horowitz (horowitz@nosc.mil) wrote: >: Mostly, we can find mechanical analogues to electrical phenomena. >: What is the mechanical analogue of radiation resistance? > >In an automobile, horsepower is like the transmitted power into an >antenna. > >The rolling resistance and wind resistance (i.e. power required to >make the car go) are like the radiation resistance of an antenna. I don't think that's a very good analogy. If we decrease rolling and wind resistance, our car can go faster. If we decrease radiation resistance, I don't think our signal goes further. :-) Radiation resistance is a "fictitious" resistance in the same sense as centrifugal force is a "fictitious" force, or as a load line is the output "resistance" of an amplifier. It's just a mathematical intermediary for purposes of calculation. Think of an antenna as an automotive transmission. It's job is to match the motor torque to the load torque requirement. An antenna's purpose is to act as a coupling between the feed point impedance and the impedance of free space. Antenna resistance can be thought of as an intermediate planetary gear in the transmission. If it's grossly too big or too small, the efficiency of the unit will suffer. In the same way, if the radiation resistance of an antenna is too big or too small, the efficiency of the antenna suffers in coupling feedline RF to free space. Too big and too small are, of course, relative terms, which is why different intermediate gear sizes can be to some extent compensated by using different input or output gear sizes, or in the case of antennas, different matching networks. But when values for either get too extreme, efficiency suffers. Gary -- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | | ------------------------------ Date: Tue, 1 Mar 1994 16:57:34 GMT From: ihnp4.ucsd.edu!swrinde!gatech!darwin.sura.net!gatekeeper.es.dupont.com!esds01.es.dupont.com!MEHDIZM%esvx19.es.dupont.com@network.ucsd.edu Subject: MFJ SWR Analyzers To: ham-ant@ucsd.edu In article , henrys@netcom.com (Henry B. Smith) writes: >Is anybody familiar with either the MFJ-249 or MFJ-259 SWR analyzers? >Can the MFJ-259 really measure feed-point resistance when it is >inserted at the equipment end of the coax? > >A general question: Can you dependably determine the resonance of an >antenna by looking for the lowest SWR? > 1- Strictly speaking, no. SWR is a quantity which some of the vector nature of impedances is lost in it. Resistance is only the real part of an impedance. Reflection coeficient (Greek row) is the ratio of complex impedance of the load to the characteristic impedance of the line. Then SWR is (1+Abs(Row))/(1-Abs(Row)), where Abs(Row) means the absolute value of Row. This is where the pure resistance information gets dropped. Therefore SWR only a measure of how much power goes in, and how much is reflected. In order to arrive at resistance, you need some phase information. 2- Yes, the lowest SWR an exact indicative of resonance of an antenna, as it is connected to the line. However, if the coupling of the antenna to the line changes, there is a small shift in the resonance. ------------------------------ Date: Tue, 1 Mar 1994 16:23:50 GMT From: ihnp4.ucsd.edu!swrinde!gatech!wa4mei.ping.com!ke4zv!gary@network.ucsd.edu Subject: MFJ SWR Analyzers To: ham-ant@ucsd.edu In article henrys@netcom.com (Henry B. Smith) writes: >Is anybody familiar with either the MFJ-249 or MFJ-259 SWR analyzers? >Can the MFJ-259 really measure feed-point resistance when it is >inserted at the equipment end of the coax? > >A general question: Can you dependably determine the resonance of an >antenna by looking for the lowest SWR? No. This only works if the antenna feed point impedance approaches the characteristic impedance of the coax most closely at resonance. That's roughly true for dipoles, but not for some other types of antennas. For example, a 1/4-wave monopole has a feed point impedance at resonance of about 36 ohms. At either side of resonance, the impedance (complex) increases. So there are two points where the impedance will be closer to 50 ohms than the resonant point. So if you see 1:1 SWR on your 1/4-wave monopole, there's something wrong. (In fact as a general rule of thumb, if you have a SWR of 1:1 on any simple antenna which has no special matching network at it's feed point, there's something wrong.) Gary -- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | | ------------------------------ Date: 1 Mar 1994 21:23:39 GMT From: ihnp4.ucsd.edu!swrinde!elroy.jpl.nasa.gov!ncar!elmore@network.ucsd.edu Subject: MFJ SWR Analyzers To: ham-ant@ucsd.edu In article <1994Mar1.162350.22173@ke4zv.atl.ga.us> gary@ke4zv.atl.ga.us (Gary Coffman) writes: ... snip ... >antennas. For example, a 1/4-wave monopole has a feed point impedance >at resonance of about 36 ohms. At either side of resonance, the >impedance (complex) increases. So there are two points where the >impedance will be closer to 50 ohms than the resonant point. So >if you see 1:1 SWR on your 1/4-wave monopole, there's something >wrong. (In fact as a general rule of thumb, if you have a SWR of >1:1 on any simple antenna which has no special matching network >at it's feed point, there's something wrong.) Gary, are you *sure* of this? An extrapolation of what you wrote says that if you terminate a 50 ohm line with a pure 50 ohm reactance, the SWR is 1:1. I don't think that's right. When I work the problem out on a Smith chart, terminating a line with a reactance equal to it's characteristic impedance yields an infinite SWR. Taking this to a more reasonable point, the lowest SWR should occur at resonance, when the antenna impedance is purely resistive; any deviation from that point will yield a rise in SWR regardless of the impedance value due to the reactive components. Have I missed something? Kim Elmore, [N5OP, PP ASEL/Glider 2232456] * _._. __._ _.. _.._ _.. . _. ..... ___ .__. _. ..... ___ .__. _.. _.._ _._ * * Said by NQ0I while working on his shack: * * "All these *wires*! Why do they call it `wireless'!?" * * _._. __._ _.. _.._ _.. . _. ..... ___ .__. _. ..... ___ .__. _.. _.._ _._ * ------------------------------ Date: Wed, 2 Mar 1994 09:41:14 GMT From: ihnp4.ucsd.edu!agate!howland.reston.ans.net!gatech!news-feed-2.peachnet.edu!news-feed-1.peachnet.edu!emory!wa4mei.ping.com!ke4zv!gary@network.ucsd.edu To: ham-ant@ucsd.edu References <1994Mar1.162350.22173@ke4zv.atl.ga.us>, <2l0bor$g9m@ncar.ucar.edu>, <2l0j7q$g5c@news.acns.nwu.edu>mei.pin Reply-To : gary@ke4zv.atl.ga.us (Gary Coffman) Subject : Re: MFJ SWR Analyzers In article <2l0j7q$g5c@news.acns.nwu.edu> rdewan@casbah.acns.nwu.edu (Rajiv Dewan) writes: >In article <2l0bor$g9m@ncar.ucar.edu>, Kim Elmore wrote: > >>Taking resonance, when the antenna impedance is purely resistive; any >>deviation from that point will yield a rise in SWR regardless of the >>impedance value due to the reactive components. Have I missed >>something? >> > >I am afraid so. If what you say were true then it would not be possible >to match, let us say 10 ohm resistive, with a 50 ohm system using >only reactive components. But this is not correct. > >Reductio ad absurdum implies... No. You're neglecting the phase shift across the matching network. That's the same as rotation around the constant SWR circle and allows the transform from one purely resistive impedance to another. When the reactance is beyond the termination point, that phase shift doesn't apply. Looking at it another way, the matching network is in some sense a transformer, while a reactive antenna is not. That was my original mistake, I neglected to allow for the vectorization of the impedance. Gary -- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | | ------------------------------ Date: Wed, 2 Mar 1994 09:28:51 GMT From: ihnp4.ucsd.edu!swrinde!gatech!wa4mei.ping.com!ke4zv!gary@network.ucsd.edu To: ham-ant@ucsd.edu References , <1994Mar1.162350.22173@ke4zv.atl.ga.us>, <2l0bor$g9m@ncar.ucar.edu> Reply-To : gary@ke4zv.atl.ga.us (Gary Coffman) Subject : Re: MFJ SWR Analyzers In article <2l0bor$g9m@ncar.ucar.edu> elmore@rap.ucar.edu (Kim Elmore) writes: >In article <1994Mar1.162350.22173@ke4zv.atl.ga.us> gary@ke4zv.atl.ga.us (Gary Coffman) writes: > ... snip ... >>antennas. For example, a 1/4-wave monopole has a feed point impedance >>at resonance of about 36 ohms. At either side of resonance, the >>impedance (complex) increases. So there are two points where the >>impedance will be closer to 50 ohms than the resonant point. So >>if you see 1:1 SWR on your 1/4-wave monopole, there's something >>wrong. (In fact as a general rule of thumb, if you have a SWR of >>1:1 on any simple antenna which has no special matching network >>at it's feed point, there's something wrong.) > > Gary, are you *sure* of this? An extrapolation of what you >wrote says that if you terminate a 50 ohm line with a pure 50 ohm >reactance, the SWR is 1:1. I don't think that's right. When I work >the problem out on a Smith chart, terminating a line with a reactance >equal to it's characteristic impedance yields an infinite SWR. Taking >this to a more reasonable point, the lowest SWR should occur at >resonance, when the antenna impedance is purely resistive; any >deviation from that point will yield a rise in SWR regardless of the >impedance value due to the reactive components. Have I missed >something? No, you haven't missed anything, but I did. Comes from posting before the caffeine kicks in. :-( You're correct that the reflection coefficient will be minimum, but not necessarily zero, when the antenna is at resonance. What I said about the impedance being 50 ohms either side of resonance is correct too, but complex impedances are *vector* quantities while the SWR equation resolves them to scalars. Arrgh! Gary -- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | | ------------------------------ End of Ham-Ant Digest V94 #53 ****************************** ******************************