AM BROADCAST TECHNICAL DETAILS
Broadcast Engineers want to know and understand how the EH Antenna works and what it can do for their radio station. The theory of the antenna will be found in the theory section of this web site. It is the intent of this document to extol the virtues of the antenna in a perspective directly applicable to AM Broadcast.
Basically, the EH Antenna is a short fat dipole. What makes it unique is that it is voltage fed between the elements of the dipole, which is the basic concept of the EH Antenna. This allows the E and H fields to be developed in time phase at the antenna, thus enabling a large radiation resistance to develop. Combined with a large capacity between the elements, this antenna has a very large instantaneous bandwidth in spite of the fact that the antenna is small. A tuning coil is used to resonate the antenna. The following provides additional information on each of the major attributes of the antenna:
Small Size: The antenna is only a small fraction of a wavelength. Our standard low frequency antenna has a diameter of 3 feet and a total length of 39 feet. At 500 KHz this would be 2% of a wavelength. The same antenna may be used up to 1200 KHz simply by changing the tuning coil. The length would then be 4.8% of a wav elength. For frequencies above 1200 KHz our antenna has a diameter of 8 inches and a length of 8 feet and 8 inches plus the tuning coil.
Broad Bandwidth: In spite of the small size the instantaneous bandwidth is very large. For example, at 700 KHz the +/- 3 dB bandwidth is 105 KHz. There are no networks used other than the tuning coil, which is tapped to provide an excellent match to the transmission line. Typically the line is 50 ohms, but the tap may be set for any desired real impedance.
High Efficiency: The cylinders are relatively short and large in diameter, thus the loss in the antenna elements in negligible. The only significant loss is the RF resistance of the tuning coil. This is typically only a couple of ohms. When this is compared to 170 ohms radiation resistance for the antenna it is obvious that the efficiency approaches 100%.
Low Height : The EH Antenna is a miniature dipole and produces the same pattern as a conventional large dipole. However, when a dipole is mounted close to ground the radiation along the ground is reduced compared to what it would be at a height of ¼ wav elengths. For example at a height of 1/8 wav elength the radiation is down about 2 dB. However, the loss in ground radials of a conventional broadcast antenna is about 3 dB. Therefore the radiation along the ground of the EH Antenna at a height of 1/8 wav elength is a little more than from a conventional ¼ wavelength broadcast tower.
No Radials Required: Because the EH Antenna is a complete dipole, no ground radials are required. This allows the antenna to be mounted on self supporting towers which could be located in the center of a populated area.
Low EMI: A significant concern is proximity of a broadcast antenna to persons. The E and H fields of an EH Antenna are constrained to the immediate area of the antenna. Because the antenna is mounted at a significant height there is no concern of radiation affecting persons close to the tower. There is also virtually no concern of interference to other services including telephones, computers, etc. Reduction in interference is further enhanced by the fact that there is virtually no radiation for the antenna at harmonic frequencies.
Reliability : The components of the antenna include the cylinders, the tuning coil, and a capacitor used to isolate the feed line. There are no external tuning units or other components, therefore only a single capacitor might have a limited life span. However, high quality capacitors that are appropriately rated should have virtually infinite life.
Economics : Although the technical details suggest a great antenna, the economics are just as impressive. The overall antenna system cost is reduced because:
Implementation savings :
- No buried radials are required
- Tower only needs to be 1/2 height
- No matching networks are required
Long term savings:
- High reliability means less down time and less maintenance cost.
The prototype AM broadcast antenna is in Eatonton, Georgia. Performance testing was done by Stu Graham of Graham Brock, Inc. You may contact him at www.grahambrock.com if you desire further information relative to the tests. That antenna was located only 1/10 wav elength above ground and suffered because of currents on a steel tower section inside the cylinders, yet it was only 0.84 dB below the standard ¼ wav elength tower used for comparison.
These issues were corrected when the EH Antenna was installed in El Salvador.
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