Discreet Antennas

I have had excellent luck with full-wave loop antennas in discrete situations, often installing them horizontally. A loop for 40 meters will fit on the roof of most houses or apartments. The usual application is to tuck it under the shingles, or staple it under the eaves. In one case we used black insulated wire on a house with black asphalt shingles, and the ham’s wife didn’t even notice it. I’ve found that the conventional formulas are typically 10% too long for roof mounting and, with the smaller size, the impedance gives a good match to coax without a matching section. These antennas are usable on all harmonics of the design frequency, though the SWR will higher (maybe 2:1).

Installing

There is, of course, a lot of variation with the individual installation. Repeated trips out to prune an antenna to resonance are not always practical if one doesn’t want to draw attention to it, or if it is in an inconvenient place like the roof. I recommend simply installing a big loop of whatever length fits the available space and feeding it with balanced line (twinlead, twisted pair, etc.) through a tuner.

In one installation, the two ends of the loop came into the house through the windows on different sides of my upstairs bedroom, with the feed point tacked to the ceiling over the bed. A matching system can be installed at the point where the feedline enters the building, if desired, and coax run from there to the rig. Maximum radiation is from the point in the loop opposite the feedpoint, and every 0.5 wavelenght along the wire from there. This allows the radiating portion to be high in the air without having to get the feedline up there too.

Another advantage is that one can use a loop smaller than a full wave and still get reasonable results. With low-loss feedline and a husky matchbox you can use a loop on frequencies where it is only a half wave in circumference, or less. I have used 40 meter loops on 80, and I know of a 250-foot loop which puts out an outstanding signal on 160 meters. (Or did, anyway, until the tuner melted.)

Dual Band Tuning Networks for Verticals

I have used similar vertical antennas to what W51NU describes and they work great. Mine have been two-band designs, for 40/80 or 20/ 40 using a matching system that requires no band switching. In the drawing, the antenna is quarter wave resonant on the lower band, where L and C₁ are series resonant to allow straight through operation. On the higher band the combination of L and C₁, appears inductive, and this inductance along with C₂ forms an “L” network to match the feed-point impedance. The higher frequency need not be twice the lower one, but the antenna should be no more than 5/8 wavelength at the highest frequency.

Actually the antenna need not be exactly resonant on the lower band, either, as the series L and C₁ can be adjusted to provide some series reactance for matching. A good approach would be to make the antenna a little longer than a quarter wave to raise the feedpoint impedance closer to 50 ohms, instead of 30 ohms, with L and C₁ adjusted to cancel the inductive reactance of the antenna. The biggest problem is determining the values of L and C₁. One could calculate the feedpoint impedance from formulae, but I use the pragmatic approach.

I build a small “L” network tuner (typically C = 100 pF variable and L 10µh with a clip to tap it every turn) and connect it to the antenna, then adjust L and C for 1:1 SWR on the higher band. The required inductive reactance is determined by measuring the coil and counting the turns used:

(1) L (actual) = R²N²/(9R + 10L)

Where:
R is the coil radius in inches
N is the number of turns up to the clip
L is the length of the coil up to the clip

Then calculate the reactance of this inductance using the formula given below, and call the result X₂ If the antenna is not resonant on the lower band, insert a fixed capacitor (perhaps 100pF for 40/80) in series with a coil, and adjust the coil tap for lowest SWR.

Again, measure the coil and count turns, compute the inductance and the reactance, then add the reactance of the capacitor (which will be negative, so you will end up subtracting) to determine the desired reactance, which may be capacitive or inductive, so be sure to preserve the proper sign. Call this reactance X₁. (If the antenna is resonant on the lower frequency, use X₁ = 0.)

(2) The reactance of a coil is given by:

X = 2piFL

(3) and, the reactance of a capacitor is:

X = IO⁶/2piFC

where:
X is the reactance in ohms
F is the frequency in MHz
pi is 3.14
L is the inductance in µh, and
C is the capacitance in pF

Once you know the desired reactance on each of the frequencies, you can calculate the values of L and C₁ by solving the simultaneous equations:

(4) X₁ = 2piF₁L – I0⁶/2pi F₁C₁

(5) X₁ = 2piF₁L – I0⁶/2pi F₂C₁

This reduces to:

C = I0⁶/2pi ((F₂ – F₁) / (F₂F₁(X2 – X1))

Then:

L = X₁ / pi F₁ + I0⁶/4pi² F₁C₁

where:
L is µh
C₁ is in pF
F₁ is the lower frequency in MHz
F₂ is the upper frequency in MHz
X₁ is the required reactance at F₁
X₂ is the required reactance at F₂
pi is 3.14

Although you might hit it right on the first time, allow a few extra turns on the coil and some tuning range in the capacitance for final adjustment.

Tune the rig to the lower frequency and check the SWR. If necessary adjust C₁ for best match. Then switch to the higher frequency and adjust C_2. If this doesn’t give you a good match on both bands, change the tap on L and try again. The radiator need not be vertical, either. I usually end up with some top loading, either an inverted “L“, a “T“, or just a sloping wire up and over a handy tree. Make the vertical part whatever is mechanically convenient, and add one or more horizontal wires to make up the missing length.

My current vertical for 20 and 40 is about 18 feet high, top loaded with three wires each about 14 feet long that also serve as guys. The top loading should be no longer than a quarter wave at the highest frequency for best results. The base of my antenna is level with the edge of the roof, and with five radials, gives good DX performance with 50 watts output (Including Africa on 40 meters from the West Coast.)

Originally posted on the AntennaX Online Magazine by Dale Hunt, WD6BYU

Last Updated : 22nd April 2024

Discreet Antennas