Sorry, Jim, you are misquoting the wrong guy. Tweren't me, McGee. 73 Jim Allen W6OGC > Message: 24 > Date: Fri, 30 Dec 2016 13:25:28 -0800 > From: Jim Brown <[email protected]> > To: [email protected] > Subject: Re: [Elecraft] Small QRP antenna > Message-ID: > <[email protected]> > Content-Type: text/plain; charset=utf-8; format=flowed > > Sorry Jim, you're mistaken on many counts. > > An antenna does not have VF, but the wire we use for it does. VF of an > infinitely thin bare wire in free space is 1. Surrounding it with a > dielectric (insulation) makes it longer electrically, usually by a few > percent. So does making the wire fatter. These differences are usually > described using VF. > > Placing a conductor close to earth (within a few inches) causes it to > couple to the earth, which also makes it electrically longer. We could > also describe this using VF. > > VF is NOT constant with frequency, whether in a transmission line or in > wire. VF of all transmission lines starts out quite small at audio > frequencies, rises quickly through the audio spectrum, eventually > reaching a near constant value at mid-VHF. It is this near-constant > value that is computed by the simple equation that doesn't include > frequency. For most lines, VF has reached about 98% of its final value > at 2 MHz. > > Likewise, Zo is not constant with frequency, nor is it a pure > resistance. It starts out quite high at low audio frequencies and is > dominated by capacitance. Zo falls rapidly in the audio spectrum, and is > quite close to its final value at 2 MHz, but is still capacitive, > typically 1-2 ohms. > > This can be clearly seen in N6BV's TLW software that comes on the CD > with the ARRL Antenna Book. Choose your favorite coax, set the frequency > to 2 MHz, make the line 300-400 ft long, and terminate it in a pure > resistance. TLW will tell you Zo. Now select Volt/Current next to the > Graph button (lower right corner) and you'll see that there are > standing waves on the line (the graphs are not a straight line). Now set > the Load to the R and X values for Zo and hit Graph again. Now the V and > I lines are nearly straight, indicating quite low SWR. They would lay > exactly on top of each other, but the R and X values for Zo are rounded > off. > > The fact that Zo, VF, and attenuation vary with frequency is clearly > predicted by the full transmission line equations. There's a brief > discussion of this in > http://k9yc.com/Coax-Stubs.pdf > which also shows how VF and attenuation can be computed and plotted vs > frequency by making two measurements of a sample with a vector analyzer > like the AIM, SARK, and VNWA analyzers. The two measurements are then > exported to AC6LA's excellent freeware Excel spreadsheet called ZPlots. > > There's a longer discussion of this, specifically written for audio > people, but obviously important for radio, in > http://k9yc.com/TransLines-LowFreq.pdf > > 73, Jim K9YC > >> On Fri,12/30/2016 11:12 AM, Jim Allen wrote: >> but ?velocity factor? is a characteristic of transmission lines. >> Interestingly, it is independent of frequency (up to the limit of the >> dielectric). It depends on the geometry of the line and the dielectric >> material. > > Antennas don?t have a velocity factor. The shortened elements >> are caused by capacitive loading against (RF) ground. There is a percentage >> of the free-space electrical length due to capacitive loading, but it is not >> a velocity factor. > > ***************************************** ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected]
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