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

Subject: Re: [TowerTalk] Resonant frequency, up or down?
From: Jim Lux <>
Date: Mon, 20 Apr 2015 15:28:54 -0700
List-post: <">>
On 4/20/15 2:53 PM, Mike Smith VE9AA wrote:
Remind me (please) 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.


Mike VE9AA

Mike, Coreen & Corey

Keswick Ridge, NB


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