Vertical monopole receiving antenna for HF
A vertical monopole is easy to set up and provides good reception with a minimal need for space. If you don't have enough space for a long horizontal antenna, it's an excellent alternative to attic antennas, which typically suffer from low S/N due to their proximity to house wiring. Of course, to pick up the international flamethrowers, even an attic antenna will work--their signal is so strong, you could probably pick it up with your dental fillings. But you'll get much better sensitivity (and better sound) from an external antenna located as far from the house as possible. It's also a lot easier to make a good solder connection to an external wire than to your dental fillings.
Theory
(Not finished)
Getting it up
A convenient setup is to use a simple wire suspended vertically from
a tree. The problem is: how to get the wire up into the tree without
getting killed. If you
have an email account, you probably get dozens of emails every day giving
advice on how to get various things to go up higher, by taking something
called "v*a*ra". Unfortunately, this method can only raise objects, at
most, by two or three feet. So, we need something better.
There are also specialized slingshot devices that use a weight attached
to a fine fishing line. According to the vendor, these little items can
raise a wire as high as 70 feet. However, using a slingshot may not be as
easy as it looks. The weight can get caught in the branches, never to be
seen again; or the wire may get stuck on the way up, or it may be suspended
somewhere in some upper branch, only to fall onto someone's head a few
months later. The weight may even follow its own natural pheromones,
which cause it to be attracted to any breakable object. This will cause it
to head straight for a neighbor's window. I used a long pole which allowed
me to raise the antenna to 50 feet with no difficulty. Using a ladder or
longer pole, it should be possible to get it up to 75 feet. Try doing
that with v*a*ra!
Make sure there are no utility wires near the tree before starting. Apart from the obvious safety issue, utility wires are a major source of interference. The antenna should be as far from buildings and any electrical wiring as possible.
A weight is attached to a line using masking tape. The line is taped loosely to the pole. Masking tape is used so that if the weight should get stuck in the tree, it will eventually fall of its own accord when it gets wet. After you suspend the weight over a branch, pull the line to rip the masking tape, causing the weight to fall. The wire is pulled through and fastened to the tree trunk. Various types of slip knots can also be used. The antenna is held in place with a small anchor made of a bent steel rod or coat hanger.
Transmission line impedance matching
Since it is desirable to avoid picking up interference from the hundreds of other electronic gizmos in the house, the antenna is connected to a shielded transmission line, which is buried underground. A common misconception is that a transformer is a perfect solution to antenna-transmission line impedance matching. However, this is only true if the antenna is to be used at a single frequency. The impedance of a vertical monopole antenna can vary from about 7-10Ω to over 600 Ω depending on frequency, with the magical value of 37Ω found at λ/4. At other frequencies, the impedance (and the VSWR) can be much higher, and a step-up transformer actually makes the signal weaker.
Should the transmission line should be matched to the actual antenna impedance, which varies with frequency, or to the theoretical free space impedance (which is 73Ω for a dipole and 37Ω for a monopole at resonance)? Published sources are unambiguous about which strategy is more appropriate:
J. J. Carr, in Practical Antenna Handbook, page 438, says:
The matching network must have an impedance that is the complex conjugate of the complex load impedance. For example, if the load impedance [at frequencies away from resonance] is R+jX, then the matching network must have an impedance of R-jX.
D. F. Bowman, in Antenna Engineering Handbook, page 43-1, says:
Maximum power transfer from the source to the load is obtained only when ... (1) the generator is loaded by the conjugate of its internal impedance; and (2) the line is terminated in its characteristic impedance .... The primary use of the simple [lattice] sections is for matching at a single frequency.
In other words, one end of the impedance-matching network should match the characteristic impedance of the transmission line (typically 50Ω), and the other should be designed to cancel out the reactance of the antenna at the desired frequency. This reactance may be capacitive or inductive. A pair of inductively-coupled coils (as used here) isn't the only way to match them, or even necessarily the best way, but it's a simple compromise. Bowman provides a useful graph for estimating the mismatch using a line transformer or a quarter-wave transformer.
Unlike a dipole, a vertical monopole has very low impedance, and does not necessarily even need a transformer to couple to a transmission line with a characteristic impedance of 50Ω. However, a vertical monopole with a length of 0.15λ has an impedance of only around 10 ohms. Connecting such an antenna to a transmission line would produce a very high SWR, causing much of the signal to be lost. Although this can be a serious problem for transmitters, it is also undesirable for receivers. To prevent this, I used a transformer made from an EMI suppression bead of type 73 ferrite (JW Miller FB Series, Mouser 542-FB73-287) with 3 turns of 26 gauge enameled wire on the primary and 6 turns on the secondary. The reactance measured at 5 MHz on the primary and secondary, respectively, was 19 and 79Ω, and the inductances were 0.0625 and 6 μH, respectively. The transformer improved the signal strength by about up to 2 units on most, but not all, frequencies. Transformers that were effective for dipoles did not work for the vertical monopole.
Lightning
Since the antenna is outside, a lightning arrestor is essential. Although
a lightning arrestor can be easily made using a gas discharge tube, I used
a Model 300 HF arrestor from Industrial Communication Engineers, Ltd. The
metal case is grounded using four lengths of heavy copper antenna wire
twisted together to make a heavy cable 7 mm in diameter. This cable was
silver-soldered to a grounding rod, taking care that the total cable
length was less than one foot. It is also critical that the path from
arrestor->cable->grounding rod be a straight line. For some reason
the arrestor is not waterproof; so I sealed the transformer and arrestor
in a small watertight aluminum box. Be sure not to use the aluminum for any
of the electrical connections, whether they are to copper or to stainless
steel, or the two metals will corrode. The shield of the coaxial cable
was also grounded.
If the connection is to be buried underground, the wire should be connected
to the grounding rod by cadwelding instead of silver soldering. Single-use
kits designed explicitly for grounding rods are commercially available.
A special attaching unit is also available that can attach an ICE 300
Series Lightning arrestor directly to a ground rod.
The Model 300 HF arrestor has an LC filter that is supposed to suppress all frequencies below 0.1 MHz. The unit could easily be modified if this is a problem.
The cable was 50 feet of RG-58, which can be conveniently fitted with BNC connectors. On the other end of the cable, I used a MFJ-959C antenna tuner to match the cable to the receiver. The MFJ-959C is a tremendous convenience when building antennas because it eliminates any possible mismatch at the near end of the transmission line. In the final setup, it was removed, since only made a marginal improvement with this antenna on a Sony 2010. Contrary to popular belief, the Sony's external antenna connection already has a low impedance and is a good match for a 50-75 ohm transmission line.
Ground radials
A short monopole has a directional
gain of about 4.8 dBi. The reception pattern of a vertical monopole has the
shape of a donut that is bent slightly upwards away from the ground, with
the antenna in the center axis. This means that signals coming at very low
angles are somewhat attenuated. This can be seen in the figure below, which
shows the radiation pattern as a function of elevation of the signal above
the horizon.
Source: Antenna Engineering Handbook, 3rd ed., p. 4-30
This pattern was calculated for a monopole over a ground plane modeled as a circular disk with a diameter of 3 wavelengths. The figure shows that the signal is weakest for sources 2 degrees from the vertical (almost straight up from the antenna). Signals at about 5 degrees or less from the horizontal, coming from distant sources, are attenuated by about 5 dB from the maximum, which is at about 25 degrees elevation.
This pattern is strongly influenced by ground conductivity. With lower ground conductivity, the left-hand part of the curve will dip even lower, which means that long-distance signals will be more strongly attenuated. To get the best possible long-distance reception, you need good ground conductivity or a system of ground radials, which cause the left-hand part of the curve to become flattened out. However, even without ground radials, the vertical tree antenna is better than many people realize because the antenna is typically not perfectly vertical, but follows the shape of the tree branches. This deflects the pattern toward the ground, improving DX reception in the direction the antenna is pointing to.
Another unrecognized advantage of verticals is their non-directionality in the horizontal plane. This means you don't have to worry about missing half the signals if your horizontal antenna only goes east to west instead of north to south. Naturally, a vertical would not be a good antenna for listening to signals from outer space.
Performance
The difference between this antenna and an indoor horizontal attic antenna was astounding. Although the signal levels might have been comparable, the noise level from an antenna placed 30 feet away from the house was so much lower that some entire bands that appeared to be solid noise with an attic antenna were almost completely quiet. Surprisingly, the vertical antenna also worked well in the MW region down to about 350 kHz. A major advantage over an active antenna is that you can use a preselector and preamp to compensate for the low signal strength. In contrast, if you try to amplify the signal from an active antenna, you will get lots of intermodulation products mixed with real signals. Of course, a 50-foot wire antenna cannot compete with a loop for signal strength in the longwave region, and LF signals below 250 kHz from the vertical were mostly drowned in receiver noise, even when a preamp was used.
Here is a comparison of S/N levels:
Signal / noise levels for various antennas
Frequency |
Outdoor vertical antenna |
Indoor loop antenna |
Internal/ Whip antenna |
Outdoor active antenna |
Attic flashing antenna |
| 198 kHz (DIW beacon) | Faint * | 1 / 0.5 * | ND | Faint * | ND * |
| 332 kHz (DC beacon) | 3 / 0.5 * | 5 / 0.5 * | 0.5 / 0.5 | 1 / 0.5 * | 1 / 0.5 * |
| 1010 kHz (some AM BC station) | 6 / 0.5 * | 1.5 / 1.0 | 6 / 1 * | 1 / 0.5 * | |
| 3330 kHz (CHU) | 4 / 1 | 1 / 1 | 4 / 4 | 1 / 1 | |
| 5000 kHz (WWV) | 7 / 5 | ND | 1 / 1 | 6 / 6 | |
| 10000 kHz (WWV) | 2 / 0.5 | ND | 2.5 / 0.5 | 5 / 3 | |
| 15000 kHz (WWV) | 5 / 0.5 | ND | 5 / 0.5 | 7 / 4 |
The vertical antenna was a 52 foot monopole made of 22-gauge wire, with a minimum SWR at around 4.5 - 4.7 MHz (λ/4). The loop antenna in this table was a 2½ x 3 foot rectangular tuned longwave loop described in loopantenna.html, located in the basement. The active antenna was an LF H800 Skymatch Active Antenna mounted outside about 10 feet above the ground in a different tree. (Yes, all my trees double as electronic listening posts). The attic antenna was a 300 ohm twin-lead attached to the flashing at the top of the roof, and was roughly comparable in performance to an indoor random wire attic antenna. The preamp was turned off for all except the active antenna.
Signals were measured at 2PM local time in mid-summer (except for the 5MHz signal, which was measured at 7PM). DIW is an NDB in Dixon, North Carolina, about 350 miles away. DC is the 25-watt Oxonn beacon in Washington, DC (about 20 miles away). The units are number of LEDs on the signal strength meter. Where the S and N are equal, it means that a signal was detected but was not readable. "Faint" means no LEDs were lit but the signal was readable. ND means only noise was detected. I hope to convert these to more meaningful units in the near future. Items marked * were measured with the antenna connected to the D303 point in the receiver instead of being plugged into the external antenna jack, as described here.
The above table shows that the vertical monopole was superior to the other antennas in the AM broadcast band and lower HF, and was tied with the active antenna at higher HF frequencies. The loop antenna was far superior to the others in the LW and lower MW regions. The attic antenna produced a big signal, but it was mostly noise. The active antenna would probably have been equal or superior to the wire if they had been at the same height. With the whip / internal ferrite antenna combination supplied with the radio, only one of the signals could be read. At night, of course, these numbers would be much higher.
Variations
The bandwidth could be improved by using multiple wires connected at both ends to simulate a cylindrical antenna.
Back