Why a Vertical That Was Perfectly Tuned at Dawn Drifts Off Match by Noon

A ground-mounted vertical is supposed to be a set-and-forget antenna. You tune it once, note the dip in the standing wave ratio, and trust it. Then one morning an operator checks the meter at first light and finds a clean match, returns at midday and finds it crept upward, checks again after a rain and finds it shifted yet again. Nothing was touched. The antenna stands exactly where it stood. Yet its standing wave ratio breathes on a daily rhythm and lurches after weather, and the cause lies not in the aluminum overhead but in the dirt beneath it. The ground under a vertical is not inert backdrop; it is half the antenna, and its electrical character changes with the sun, the dew, and the rain. The diurnal drift of a vertical's standing wave ratio is the ground itself reporting its changing conductivity through the only instrument the operator is watching.

The insight that unlocks this is that a quarter-wave vertical is only half an antenna. The radiator is one half; the earth beneath it is the other, the missing half that completes the radiating system. A vertical works against the ground the way a dipole works against its other leg, and if the ground changes, the antenna changes. Because soil conductivity depends on moisture, and moisture follows a daily and seasonal cycle, the antenna's match follows that cycle too, and the standing wave ratio meter becomes an unintended soil-moisture gauge.

The three resistances that add up to the feedpoint impedance

To see why the ground matters to the match, break the feedpoint impedance into its parts. The impedance a transmitter sees at the base of a resonant vertical is the sum of three resistances in series: the radiation resistance, which represents power actually radiated; the conductor loss resistance, the ohmic loss in the aluminum and connections; and the ground loss resistance, the power dissipated heating the soil. Written as a sum,

R_feed = R_radiation + R_conductor + R_ground

For an unloaded quarter-wave vertical the radiation resistance is the textbook figure of about 35 ohms, and the conductor loss is usually negligible. The ground loss is the wild card. It can be small over excellent soil with a dense radial field, or it can be enormous over poor soil, in some cases even exceeding the radiation resistance itself.

The numbers make the match almost an accident of the ground. The classic illustration takes a radiation resistance of 35 ohms, conductor loss of zero, and a ground loss of 15 ohms, summing to

R_feed = 35 + 0 + 15 = 50 ohms

a perfect match to 50 ohm coax. But notice what produced that beautiful match: 15 ohms of ground loss, a number set entirely by the soil and the radials. Change the ground loss and the feedpoint impedance changes directly. If the soil dries and the ground loss climbs from 15 to 25 ohms, the feedpoint becomes

R_feed = 35 + 0 + 25 = 60 ohms

and the standing wave ratio rises from a perfect 1.0 to

SWR = 60 / 50 = 1.2

purely from the soil drying out. The same antenna, the same radials, the same everything except the dampness of the dirt, now shows a different match.

Moisture as the electrolyte that carries the ground current

Why should moisture change the ground loss so much? Because soil conducts electricity not through the mineral grains but through the water films between them, and that water carries dissolved salts that make it an electrolyte. The conductivity of soil is very much better when moist, because the moisture acts as an electrolyte enabling current to flow, while dry soil, with its conductive water films broken, conducts poorly. A marsh is one of the best radio grounds precisely because it stays saturated; sandy soil over dry rock is among the worst.

The ground current of a vertical flows out into the soil and must return to the feedpoint, and the resistance it meets along that path is the ground loss. When the soil is moist and conductive, the return current flows easily and the ground loss is low. When the soil dries, the current struggles through poorly conducting dirt and the ground loss climbs. Since the moisture in the top layer of soil rises and falls daily, with dew settling overnight and the sun drying the surface through the day, the conductivity of the very layer the ground current uses changes on that same daily clock. The standing wave ratio, tied through the feedpoint impedance to the ground loss, follows along.

A numerical walk through one daily cycle

Trace a single day with numbers to feel the rhythm. At dawn the soil surface is damp with dew, its conductivity high, and suppose the ground loss is at its minimum of 13 ohms. The feedpoint impedance is

R_feed = 35 + 13 = 48 ohms, SWR = 50 / 48 = 1.04

a near-perfect match, which is why the antenna checked at dawn looks ideal. Through the morning the sun dries the surface layer, the water films thin, conductivity falls, and the ground loss rises. By noon suppose it has climbed to 22 ohms:

R_feed = 35 + 22 = 57 ohms, SWR = 57 / 50 = 1.14

The match has drifted upward, exactly the midday creep the operator noticed. As evening brings cooling and the dew returns, the ground rehydrates and the loss falls back toward its dawn value, the standing wave ratio settling again. The daily swing in this example, from 1.04 to 1.14, is modest but plainly visible on a meter, and over poor soil with a sparse radial field the swing is far larger, because the antenna leans more heavily on the ground when the radials cannot collect the return current themselves.

Rain and snow add their own lurches on top of the daily cycle. A downpour saturates the soil far beyond ordinary dew, dropping the ground loss sharply and shifting the match, sometimes improving it as the soil briefly becomes an excellent conductor. Snow cover and freezing complicate matters further, since water becoming ice changes its effective conductivity, and operators routinely report that snow shifts their tuning point, often requiring the antenna to be tuned longer and sometimes yielding a lower standing wave ratio than bare ground gave.

Why a good radial field both cures and masks the effect

The radial field is the operator's main lever over all this, and understanding its dual role clarifies what the standing wave ratio is really telling. Radials are wires laid out from the base of the vertical that give the return current a low-resistance metal path instead of forcing it through the soil. A dense radial field collects up to nearly all the return current in the metal before it ever enters the dirt, slashing the ground loss and making the antenna far less dependent on soil conductivity. Studies of elevated resonant radial systems show that a well-designed radial field can capture up to 95 percent of the RF current that would otherwise dissipate in the ground.

This produces a subtle effect on the diurnal drift. With a sparse radial field the antenna leans heavily on the soil, so the standing wave ratio swings widely as the soil moisture cycles. With a dense radial field the soil barely participates, the ground loss is low and stable, and the daily swing nearly vanishes. So a vertical whose match holds steady through the day is reporting not just good radials but a system that has been made independent of the soil. The diurnal drift, then, is partly a diagnostic: a large daily swing in the standing wave ratio is a sign the radial field is inadequate and the antenna is relying too much on the dirt.

There is a trap hidden here that the standing wave ratio meter sets for the unwary. A poorly efficient antenna can show an excellent standing wave ratio, because ground loss resistance, which dissipates power as heat, counts toward the feedpoint impedance just as radiation resistance does. An antenna whose 50 ohms is made of 35 ohms radiation and 15 ohms ground loss radiates only 35/50 or 70 percent of the power, dumping the rest into the soil, yet shows a perfect match. A dummy load, all loss and no radiation, shows the best match of all. So a low standing wave ratio is not proof of a good antenna, and the diurnal drift is a reminder that part of that comfortable match may be loss masquerading as a match.

How deep the ground current reaches and why the surface layer rules

A question lurks beneath all this: if only the top few centimeters of soil dry out each day, why does that thin surface layer dominate the ground loss? The answer is that the return current of a vertical concentrates in a shallow zone near the surface, and the depth of that zone depends on the soil conductivity and the frequency through the skin effect, just as it does in a metal. The depth at which the ground current falls to about a third of its surface value is the skin depth in soil:

delta = sqrt( 2 / (omega mu sigma) )

or in practical form for earth,

delta = 503 * sqrt( rho / f ) meters

where rho is the soil resistivity in ohm-meters and f the frequency in hertz. Put numbers to it. Take moderately conductive soil with a resistivity of 100 ohm-meters at 3.5 MHz:

delta = 503 sqrt( 100 / 3.5e6 ) = 503 sqrt( 2.86e-5 ) = 503 * 0.00535 = 2.7 meters

So at 80 meters the current penetrates a few meters into the earth. But the current density is highest right at the surface and falls off with depth, so the topmost layer, the very layer that dries and rewets daily, carries the largest share of the return current and contributes the most to the loss. When that surface layer dries, its resistivity can rise by a large factor, and because it carries the densest current, its drying dominates the change in ground loss even though deeper, moister soil is unaffected.

The frequency dependence adds a band-by-band twist. On higher bands the skin depth shrinks, concentrating the current even closer to the surface where the daily moisture swing is largest, so the diurnal drift in standing wave ratio tends to be more pronounced on the higher HF bands than on the lower ones for the same antenna and soil. An operator who notices the daily swing is worse on 20 meters than on 80 is seeing the skin depth pull the current up into the most weather-sensitive layer.

Measuring the soil instead of guessing at it

An operator who wants to move from noticing the drift to quantifying it can measure the soil's contribution directly, and the methods are within reach of a home station. The simplest is the substitution check: measure the feedpoint resistance at resonance with an antenna analyzer, subtract the known radiation resistance of about 35 ohms and the small conductor loss, and what remains is the ground loss. Tracking that residual across a day and across weather turns the analyzer into a soil-conductivity monitor:

R_ground = R_feed_measured - R_radiation - R_conductor

A reading of 48 ohms at dawn implies a ground loss near 13 ohms; a reading of 57 at noon implies 22 ohms; the difference, 9 ohms, is the soil drying expressed in ohms. Logging this over weeks reveals the local soil's signature, its dry-season baseline and its rain response, in numbers rather than impressions.

A second, more rigorous approach compares the antenna's behavior against added radials. Each time radials are added, the ground loss falls and the feedpoint resistance drops toward the pure radiation resistance. Plotting feedpoint resistance against the number of radials and extrapolating to where it stops falling estimates the irreducible radiation resistance, and the gap between that floor and the measured resistance at any moment is the ground loss at that moment. This separates the fixed part of the impedance from the soil-dependent part cleanly, letting the operator see exactly how much of the comfortable match is radiation and how much is loss that happens to land at 50 ohms. The exercise often delivers a sobering verdict, revealing that a vertical with a flattering standing wave ratio over thin radials is radiating a good deal less than its match suggests, with the difference vanishing into warming the dirt.

Reading the ground through the meter

The practical wisdom is to treat a vertical's standing wave ratio as a composite reading that includes a soil-condition term, and to interpret its changes accordingly. A daily rhythm in the match is normal over real soil and signals how much the antenna depends on the ground; minimizing it with more radials both improves efficiency and stabilizes the match. Tuning the antenna is best done at a representative moisture condition rather than at an extreme, since a match perfected on a freshly rained-on soil will drift once the ground dries to its usual state. An operator who logs the standing wave ratio against time of day and weather builds, without intending to, a record of the local soil's electrical behavior.

The deeper lesson is that an antenna system extends into the earth it stands on, and the earth is a living, changing part of the circuit. The aluminum is the visible, stable half; the soil is the invisible, restless half, breathing moisture in and out on daily and seasonal cycles, and carrying the antenna's match along with it. The operator who understands that a quarter-wave vertical is only half an antenna, and that the other half is dirt whose conductivity rides on the weather, stops being puzzled by a drifting standing wave ratio and starts reading it for what it is, the ground itself reporting its condition through the one meter that happens to be watching.