11/7/96

Ed Hare, KA1CV, ARRL Laboratory Supervisor
Mike Gruber, W1DG, ARRL Test Engineer
American Radio Relay League, Inc.
225 Main St
Newington, CT 06111
(860) 594-0318
ehare@arrl.org

Amateur stations operating in the HF range usually have a receiver noise floor of approximately -140 dBm in a 500-Hz bandwidth. Amateur HF stations connected to an antenna exhibit a system sensitivity of approximately -120 dBm, although this can vary some in either direction, depending on local and atmospheric noise sources. This system sensitivity is usually limited by external noise to the values measured.

While some amateur communication enjoys a significant signal-to-noise margin, it is not uncommon for amateurs to be working within a few dB of the noise floor. Some amateurs even do experimental work as much as 20 dB below the noise. When HF propagation is excellent, the limiting factor is usually in-band interference between stations. When propagation is marginal, as it varies over a daily, seasonal and 11-year solar cycle, much lower signal-to-noise margins are common.

Amateur antennas are usually mounted between 5 and 50 meters above ground. On the 3.5 - 4.0 MHz and 7.0 - 7.3 MHz US amateur bands, common antenna heights tend to range toward the lower end of the scale. Antennas used on 10.1 - 10.15 and 14.0 - 14.35 MHz tend to range from 15 to 30 meters in height above ground.

Amateur HF stations employ a wide range of antenna systems. While some antennas used can operate at a considerable loss, it is much more common for antenna systems to exhibit significant gain over an isotropic radiator, especially when modeled over real ground.

On the 3.5 and 7.0 MHz amateur bands, a half-wave dipole antenna is perhaps the most common antenna in use. Random-length wires or quarter-wave verticals monopoles are also not uncommon. A half-wave dipole antenna mounted over real ground can exhibit a typical gain of 6.0 dBi or so over average ground, slightly higher over excellent ground.

On the 10.1 and 14.0 MHz bands, one tends to see more directional antennas, especially on 14.0 MHz. A three-element Yagi or similar design is typical. Over real ground, this type of antenna typically exhibits approximately 12 dBi gain. Some amateur HF antennas can exhibit much more gain. A longwire or rhombic antenna, for example, can have gains in excess of 20 dBi.

US amateur stations are licensed to use up to 1500 watts PEP RF output power on most bands (250 watts on 10.1 to 10.15 MHz). This is a transmitter output power specification, not an EIRP or ERP specification. These parameters must be calculated from the transmitter power, published antenna gain and an additional gain of 4 to 6 dB for ground reflections, as appropriate.

If fed with 1500 watts , the magnetic fields from a 3-element Yagi array range from 0.627 A/m at 1 meter separation from the antenna, 0.185 A/m at 5 meters separation and 0.128 A/m at 10 meters separation. The electric fields are 45.4 V/m at 1 meter separation, 59.4 V/m at 5 meters separation and 49.6 V/m at 10 meters separation. (This is not an error; what occurs in the near field of a radiating element does not follow the "square-law" formula that applies in the far field.) The antenna used for the above example is a 3-element, 14.0 MHz Yagi array, 10 meters in height over average ground. These data were calculated at points in the main lobe of the antenna, at a height of 10 meters above ground, using NF.EXE version of MININEC. These data points do not represent a worst case, but may be considered as typical of what can be expected of a station operating toward the high end of operating capabilities. A small percentage of stations exceed these reference points by a significant amount.

Current US FCC regulations permit an amateur station operating from 1.8 to 29.7 MHz to radiate an electric field into "general-public" areas of 824/F(MHz) V/m or a magnetic field of 2.19/F(MHz) A/m. This ranges from 457.8 V/m on 1.8 MHz to 27.8 V/m on 29.7 MHz. In "controlled" areas, which include the station location itself, the permitted field strengths are 1842/F(MHz) V/m and 4.89/F(MHz) A/m. These levels are in compliance with current FCC regulations for continuous duty cycle operation. Most amateurs use intermittent operation with operating modes that have a duty cycle ranging from 15% to 45%, typical. The regulations permit RMS values of power to be used, averaged over 6- or 30-minutes, so the peak fields could be significantly higher. These maximum permitted levels are, however, significantly higher than the levels which typically cause EMC problems in consumer electronics equipment.

Several tables of data are attached. These data represent actual measurements taken at three local amateur stations.. Station #1 is W1AW, the ARRL bulletin and guest operator station. Station #2 is a more modest station operated at the home of W1DG in Somers, CT. Station #3 is W1INF, the operator's club at ARRL Headquarters in Newington CT.

The test results are discussed after the tables, but it is worthy to mention that the station (W1INF) located near the computer systems in the main building at ARRL shows about a 10-dB degradation in performance as compared to the other two stations measured even though those stations used antennas with lower gain. The far field directivity pattern of the antennas used at W1INF are such that most energy received would be from angles above the horizon, although this does not necessarily apply well in the near field.

In the US, there are 691,775 licensed Amateur Radio operators. Of these, 311,600 are licensed to use 1500 watts on HF amateur bands. In addition, there are 246,173 Novice or Technician Class licensees who may be licensed to use 250 watts on certain HF bands. (The FCC does not have data on how many of these licensees have passed the Morse Code test required for HF access.)

Specific data on the percentages of licensed hams that actually transmit on specific bands or information on the power levels and antenna configurations in use does not exist. It is the best judgment of several active hams at ARRL Headquarters that a conservative estimate might be that 40%-50% of licensed hams are active and, of that number, approximately 25%-35% sometimes use output power in the 1500-watt class.

As prototype systems are brought on line in the US, ARRL may be able to help locate local amateur volunteers to assist in some field trials and measurements. If anyone would like to work with local amateurs in field trials, contact:

American Radio Relay League, Inc
Ed Hare, KA1CV, ARRL Laboratory Supervisor
225 Main St
Newington, CT 06111
Telephone: (860) 594-0318
Internet: ehare@arrl.org

Amateur stations operate with very sensitive systems that can be degraded by local noise sources. They also can radiate some relatively strong fields at heights typical of telephone-wiring installations. These factors must be considered when evaluating the compatibility of high-speed digital systems that may be installed in residential areas. Several papers previously presented indicate that VDSL technology may be compatible with the Amateur Radio Service, but just compatible. These additional data indicate that those conclusions need to be reconsidered and additional calculations made.