Ham Radio Forum

The characteristics of slot antennas in large conducting sheets are perfectly complementary to ordinary linear-dipole antennas made from wire or metal tubing. The shapes of their radiation patterns are identical, except the polarizations of the E and H fields in the radiated energy are interchanged and the E component normal to the plane of the conducting sheet is discontinuous.

Frequency response characteristics are also complementary in that slots (the absence of conducting material) that are extremely narrow compared to a wavelength have narrow frequency bandwidths, just as linear-dipoles made from wire (conducting material) that is extremely thin compared to a wavelength have narrow bandwidths; and conversely, in that wide slots have wide bandwidths, just as linear-dipoles made from thick conductors have wide bandwidths.

If a slot dipole in a conducting sheet of infinite size is fed directly across its center points, the input impedance is related to the input impedance of a complementary linear-dipole in free-space with both antennas at resonance as follows:

Z(s)Z(d) = Z(0)Z(0) / 4 = 35,476 ohms

Where:
Z(s) is the input impedance of the slot antenna
Z(d) is the input impedance of the linear-dipole antenna, and
Z(0) is the intrinsic impedance of free space = 120pi ohms = 377 ohms

Therefore, if the input impedance of a complementary linear-dipole antenna (one that has the same conductor thickness and length as the width and length of a slot antenna) made of wire or tubing is known, the relationship above can be used to calculate the center-slot input impedance of the slot antenna at resonance.

That is a precise mathematical way to determine the exact input impedance of a center-fed slot antenna at resonance for the special case of a conducting sheet with infinite size and a linear dipole in free-space. Even though neither conducting-sheets of infinite size nor perfect free-space conditions ever exist with practical antennas, the relationship above is still useful to estimate the approximate input impedances of center-fed slot antennas in reasonably large sheets.

Precise mathematical methods also exist to calculate input impedances of ordinary linear-dipoles made from wire or tubing, but few amateurs use those methods to calculate the impedances of antennas they plan to install. Instead, most everyone knows that the center feed-impedance of a wire dipole antenna in free-space is approximately 72 ohms. If a wire dipole antenna is to be installed high above ground and away from other objects it is generally assumed without mathematical calculation that its center feed-impedance will be about 72 ohms. If the antenna is to be installed closer to ground most amateurs simply assume without calculation that its feed-impedance will be lower.

Similar practical "rules-of-thumb" can be used to estimate the feed-impedance of slot antennas. Slots in large conducting sheets that are open on both sides generally have center feed-impedances near 500 ohms. However, if a slot antenna was cut into one side of a perfectly-conduction metal box, so that zero susceptance was shunted across the input terminals, the input impedance of the slot would double. Note that this is opposite to what happens when a tuned reflector is placed behind a linear-dipole. In that case the linear-dipole feed impedance drops, instead of rises, because the two antennas are complementary.

Just as a linear-dipole antenna can be off-center-fed to raise its feed impedance, a complementary slot antenna can be off-center-fed to lower its feed impedance. At resonance the feed impedance of a slot in a large sheet varies from approximately 500 ohms across its center to near zero across either end, so the feed connecting point can moved along the length of a slot as necessary to match impedances between those approximate limits.

It should already be clear from what I have written above, but in case it isn't, the length of a slot antenna for resonance is precisely the same as the length of wire antenna for resonance at the same frequency if the sheet the slot is cut in is large compared to a wavelength, if the sheet is a near-perfect conductor, if the wire is in near free-space conditions, if the diameter of the wire equals the width of the slot, and if the wire is supported by near-perfect insulators.

I had never heard of a slot antenna until reading this. They are very interesting. Why hasn't there been more usage by hams? What have they been used for?

Thanks for providing such detailed information about them. I knew I should have gone to engineering school!

Ashley

Back to Corey's mobile whip idea near the top of this topic, could you get away with mounting a mobile whip antenna on a fence pole?

Human exposure to high levels of electromagnetic radiation or RF burns if some touched the antenna could be problems with a fence post mounted mobile whip. Those problems usually aren't of much concern with a moving vehicle, but they could be serious risks with a mobile whip mounted on a fence post if operating at high power levels.

Ken

If a mobile whip antenna is mounted on a fence post, be sure to run a large ground lead down the fence post to ground. A 4 to 6 inch wide flat copper strap to ground will be better than just a wire. The base impedance of a short mobile whip antenna is so low on 75 meters that it will be necessary to have an extraordinarily low ground circuit impedance to minimize losses.

Matthew

Of course it won't do any good to run either a heavy ground wire or a wide strip of sheet copper down the fence post unless it connects to a good buried ground radial system at the bottom. Whip antennas work reasonably well on vehicles because of the capacitance large vehicle bodies have to earth. There will be large ground losses with a mobile whip antenna mounted on a fence if the ground system consists only of a ground rod or a few short buried ground radial wires.

Jeff

K3MT has an interesting stealth antenna solution that is basically a long-wire antenna buried in the grass. He claims it works. See http://users.erols.com/k3mt/graswire/graswire.htm for more information.

Bruce

Though it hasn't been identified yet the photo that is currently on the home page obviously shows a mobile loop antenna. A loop like that could be placed in a wooden shed and used as a stealth HF base-station antenna.

Ken

The slot antenna (the Alford Slot), is shown in some detail in the RSGB Handbooks.
Don't forget that if the slot is Vertical, the radiation is horizontal. It's this which makes it so useful for 13cm TV and repeaters.
They are a real Brute to match properly without some decent test kit. A field Strength Meter is most valuable.

years ago i read about a cia stleth antenna. it was made to look like a tv antenna, might do for you, but who knows. good luck joe ki6jxg