Discone Antenna: A Wideband VHF and UHF Antenna for Hams

Most antennas are built for one band. A dipole, a Yagi or a vertical is cut to a length that suits a frequency, and it works well there and poorly elsewhere. The discone antenna breaks that rule. It is a strange-looking assembly of a flat disc above a cone, and it works over a frequency range of about ten to one, so a single antenna and a single feed line can serve the 2 metre band, the 70 centimetre band and a wide range of frequencies in between and above. This guide explains how the discone works, what gain and SWR to expect, where it is a good choice, where it is a poor one, and how to mount it and protect your receiver.

How the Discone Is Built

The antenna has two parts. The disc is a flat circle or a ring of rods on top, and the cone below it points downward, with its narrow end near the disc. The coaxial feed line runs up through the center of the cone. The center conductor connects to the disc, and the shield connects to the cone. The two parts are kept apart by a small insulator at the feed point.

The design is described in the article on the discone antenna, which also notes that it was developed in the 1940s. The principle is that the cone acts as a gradual transition from the feed line to free space, similar to a horn. Because there is no sharp resonance, the antenna presents a fairly constant impedance over a wide range of frequencies. The lowest usable frequency is set by the size of the cone. The slant length of the cone should be about a quarter of a wavelength at that lowest frequency, and the disc diameter is a little smaller than that. Above the low frequency limit, the antenna simply keeps working until the dimensions become large compared with the wavelength.

Because the elements are made from rods in many commercial models, the antenna is light and has little wind resistance, which is a benefit on a mast.

What Performance to Expect

The discone is a compromise antenna, and it is best to know the numbers before you rely on it.

• Pattern. Omnidirectional in the horizontal plane, with vertical polarization. There is no beam and no rotor.
• Gain. Roughly in the range of 0 to 3 dBi, depending on the frequency and the design. At the highest frequencies the pattern tends to tilt upward, which reduces the signal toward the horizon.
• SWR. Typically below 2 to 1 over most of the rated range, but often higher near the low end and at special frequencies. Always check the actual SWR on your own bands with an analyzer.
• Bandwidth. Ten to one is a common figure, for example from about 100 MHz to 1 GHz or more, depending on the model.
• Efficiency. Good, because there are few losses in the structure, but it cannot give the gain of a tuned antenna.

If you want to compare the number with other antennas, our explanation of dBi and dBd shows how to convert and what the figures mean in practice. A gain of 0 to 3 dBi is similar to a basic dipole, so you should not expect a big signal advantage over a simple vertical. The benefit is coverage.

Where a Discone Is a Good Choice

The antenna shines in situations in which you need many bands from one place.

Software Defined Radio and Scanners

A wideband receiver covers a huge range, and a wideband antenna matches it. For a receiver of the type described in our guide to SDR for ham radio, a discone allows you to listen to the amateur bands, air band, public safety, weather stations and broadcast signals with one cable. This is the most common use, and it is where the antenna is particularly good value.

Monitoring and Testing

A discone is useful for monitoring repeater and simplex activity across VHF and UHF, for testing handheld radios and for checking a signal at several frequencies without changing antennas. It also helps in a club shack, where several people want a general-purpose antenna for demonstrations.

Transmitting on More Than One Band

Many operators use a discone for 2 metre and 70 centimetre FM and simplex. The SWR is usually acceptable on both bands, and the antenna is much simpler than a pair of separate verticals with a diplexer. For the 2 metre bands, our overview of the J-pole, collinear and ground plane antennas gives the alternatives, and the 70 centimetre guide covers the higher band. A discone gives less gain than a collinear, but it avoids a second antenna.

Where It Is a Poor Choice

The discone is not a universal solution, and the limits are worth knowing.

• Weak-signal work. For meteor scatter, tropospheric ducting or satellite use, you want gain and, for some modes, horizontal polarization. A discone gives neither.
• Heavy interference. A wideband antenna delivers all the signals in its range to your receiver, including powerful broadcast and paging stations. Overload and intermodulation are common with simple SDRs unless you use filters.
• HF. A discone for VHF and UHF is far too small for HF. Large discones for lower frequencies exist, but they are impractical for most stations.
• Directional work. There is no beam, so you cannot reject a nearby interferer by turning the antenna.
• Horizontal polarization. Weak-signal SSB and CW on VHF is usually horizontal, and a vertical discone suffers a polarization loss against such stations, as our guide to vertical and horizontal polarization explains.

Checking the SWR Yourself

Published figures describe a typical sample, and your installation will differ. The mast, the nearby roof, the cable and the connector all change the result. After installation, sweep the antenna with an analyzer across the range that you plan to use, and note the standing wave ratio at the centre of each band that matters to you. Our guide to the NanoVNA and calibration sweeps describes how to do this with a low-cost instrument. Expect the curve to show some peaks and dips, because a discone is never perfectly flat. What you need is an acceptable value on your own bands, and a stable trace that does not change when you move the cable, which would point to common-mode current on the shield.

If the SWR is high on one band, try a few simple changes before you blame the antenna. Check the connector and the center pin for corrosion, replace a damaged jumper, move the antenna a little away from the mast or a nearby metal object, and add a ferrite choke to the cable below the antenna. Many reported problems turn out to be a bad connection, which is a much cheaper fix than a new antenna.

Building a Simple Discone

Because the geometry is forgiving, a discone is a popular construction project. A common approach uses a disc cut from aluminium or copper sheet, or a ring of eight to sixteen rods, and a cone made from the same material or from rods. The size is chosen for the lowest frequency of interest: the slant length of the cone is about a quarter of a wavelength at that frequency, and the disc diameter is about 70 percent of that length. For example, a design for 100 MHz would have a slant length of roughly 75 centimetres. The feed point is an SO-239 or N connector at the narrow end of the cone, with the center pin joined to the disc through a short, rigid connection.

Rods are easier to make than solid surfaces and weigh much less, and they work almost as well. Use stainless or plated hardware, avoid mixing metals that corrode against each other, and seal the feed point well. Test the antenna with an analyzer before you put it on a mast, and adjust the spacing between the disc and the cone by a few millimetres if the match is poor. A homemade discone will not match a professional model in all respects, but it is a pleasant weekend project that teaches a lot about wideband antennas.

Choosing a Discone

When you shop, compare a few details. The table summarises the main choices.

Cheap models sometimes have poor connectors or thin parts that corrode quickly, so inspect the feed point and the quality of the weather seal before you install it. A discone that is made for receiving only may use a thin center insulator that cannot take transmitter power.

Mounting and Feed Line

A discone should be mounted as high as practical, with a clear view in all directions, because VHF and UHF signals travel by line of sight. Keep it away from other antennas and metal structures, at least a wavelength at the lowest frequency if possible, because nearby metal distorts the pattern. Mount it on top of a mast, so that nothing is above the cone.

The feed line matters more than usual, because the loss at VHF and UHF is significant. A long run of thin cable can waste half of the signal at 70 centimetres. Use a low-loss type, and keep the run short. Our comparison of coax for ham radio gives typical loss figures. Seal the connections with self-amalgamating tape and mastic, and add a drip loop where the cable goes into the building. Finally, install a lightning arrestor or a proper ground at the entry point.

Protecting the Receiver

Because a discone is a wideband antenna, it also hears strong signals that a tuned antenna would reject. This is the main source of complaints from SDR users. Four measures help.

1. Add a broadcast FM notch filter. The very strong FM broadcast signals around 100 MHz overload many wideband receivers.
2. Use a band-pass filter for the band you are using, for example 144 to 148 MHz, when you need the best sensitivity on one band.
3. Reduce the gain. Lowering the gain in the SDR often removes ghost signals caused by overload.
4. Do not add a preamplifier automatically. Most strong-signal problems get worse with more gain, and a short, low-loss feed line is a better start.

If you transmit on a band, make sure the receiver has a protected input or a separate antenna, because a transmitter nearby can damage an unprotected SDR.

Common Mistakes

• Expecting beam-like performance. A discone has about the gain of a dipole and no more.
• Mounting it too low. Obstacles at VHF and UHF cost a lot of signal, so height pays.
• Using thin cable on a long run. Cable loss can cancel any benefit of the antenna.
• Neglecting filters. Without a notch for FM broadcast, a simple SDR often shows false signals.
• Skipping the weatherproofing. Water in a connector ruins the antenna, so use tape and mastic.

The Bottom Line

A discone antenna is a simple, rugged way to cover VHF and UHF with one feed line. It is not a high-gain antenna, but it is an excellent partner for an SDR or a scanner and a good general-purpose choice for FM on 2 metres and 70 centimetres. Mount it high and clear, use low-loss cable, protect the receiver from strong broadcast signals and choose a tuned or directional antenna when you need weak-signal performance. For monitoring, testing and general listening, few antennas offer so much coverage for so little effort.

Frequently Asked Questions

What frequency range does a discone cover?

Typically about ten to one, for example from around 100 MHz to 1 GHz or more. The exact limits depend on the size of the cone and the quality of the design.

Can I transmit with a discone antenna?

Yes, if the antenna is rated for transmitting and the SWR is acceptable on your band. Check the power rating and the connector, and measure the SWR before use.

How much gain does a discone have?

Roughly 0 to 3 dBi, similar to a simple dipole. It is chosen for bandwidth and not for gain.

Is a discone good for SDR reception?

Yes, it is one of the most popular choices. Use a notch for FM broadcast and reduce the gain if you see false signals.

Does a discone work on HF?

No. A VHF and UHF discone is far too small for HF, and an HF discone would be very large.

What polarization does a discone use?

Vertical, when mounted upright. This suits FM and repeater work but costs some signal against horizontally polarized weak-signal stations.