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## Re: [TowerTalk] Resonant frequency, up or down?

 To: towertalk@contesting.com Re: [TowerTalk] Resonant frequency, up or down? Jim Lux Mon, 20 Apr 2015 15:28:54 -0700 mailto:towertalk@contesting.com>
 ```On 4/20/15 2:53 PM, Mike Smith VE9AA wrote: ``````Remind me (please)..as you raise a dipole up off the ground, does the resonant frequency go up or down? `````` ```In general, putting a dipole in or near something that is not free space will lower the resonant frequency (and also, often, increase the loss as well). ``` You can look at it in several conceptual models: ```1) the effective permittivity in which the dipole is operating is a sort of combination of air, wire insulation, and the soil. The farther you are from the soil, the smaller the fraction. Air/Free space has a relative epsilon of 1, pretty much everything else is >1 (water is 82, etc.) ``` ```To a first order, (and the ghost of J.R. Wait will rise up and smite me because what I'm about to say is technically wrong), you can treat a wire laying on the ground as having an effective epsilon of (1+soil epsilon)/2. (that is, half way between air and soil) ``` ```2) the propagation speed is slower in a medium with relative epsilon >1, so a "half wavelength" is physically shorter. Or, another way, for a given physical length, the resonant frequency is lower. ``` ```3) another way to look at it is to imagine the antenna wire as being a bunch of inductors in series and capacitors to ground (like a transmission line). Changing the "capacitance to ground" changes the Z (=sqrt(L/C)) and propagation velocity (=1/sqrt(LC)) of the transmission line. ``` More C makes the speed slower and the impedance lower. ```More L (using a coil instead of a straight wire) makes the speed slower and the impedance higher. ``` ```Soil doesn't do much for L, but it sure affects C: not only is there the permittivity (epsilon) aspect, but there's also the "big capacitor plate) effect. ``` `````` I had an all band dipole perfectly set a week ago and now have changed the height on a temporary mast which I have reconfigured and it seems like the frequencies are all off, but I will change the height of it yet again before I go out in the field. Easier for me to trim or add length now before I leave and wish to do a little math and get it a bit closer now, rather than later. I am already in the ballpark. `````` ```Now you're getting into "rule of thumb" territory... and I'm terrible on this, so I just wind up hauling the darn thing up and down a couple times to make a couple measurements, and assume it's linear for small scale changes. ``` ```That is, if you measure a 20m dipole (5 meter legs) at full height and it's, say, resonant at 13.9 MHz; then you bring it down and shorten each leg by 0.1 meters, and haul it back up, and now it's resonant at 13.95; you'd assume that every 10 cm is worth about 50khz, so to get it to 14.150, you'd knock off 40 more cm. ``` ```(length examples illustrative.. in reality a 2% change in length would result in roughly a 2% change in frequency, or about 280 kHz. ) ``` ```I have never been successful at adjusting resonance to a specific value near the ground and then having it scale nicely up high. ``` `````` TNX! Mike VE9AA Mike, Coreen & Corey Keswick Ridge, NB _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk `````` _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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