[Elecraft] Small QRP antenna

[Jim Allen]

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 <jim at audiosystemsgroup.com>
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] Small QRP antenna
> Message-ID:
>    <b7aeba7c-1bed-abbf-4557-e9bf657486cd at audiosystemsgroup.com>
> 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.
> 
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