Of course, you could always stretch out a half wave dipole - around 310 feet should do it at 1500 KHz or 155 feet per leg. The FCC might frown on that, as you would likely exceed the allowable field strength and you wouldn't be compliant with the alternate limit of a 3 meter (9'8") antenna :-) probably for the best, can you imagine managing a 300 foot antenna? Sheesh!
Most low power AM broadcasters stick to the 3 meter antenna and try to improve range with an antenna matching coil. This makes 'antenna design' very simple - get something conductive that is around ten feet long - done! Low power AM broadcasters use copper pipe or tubing, aluminum tubing, light or heavy gauge copper wire or even the shield of a length of coax supported by plastic pipe, old TV antenna mast - anything that conducts electricity and that can be isolated from ground. In this configuration, it's the matching coil that really makes the antenna.
The worst mistake I made was thinking matching coils were an exact science when the exact opposite is true - matching coils can be big or small, air coil or iron powder (but never ferrite) core, horizontal, vertical, wound around a toroid or any combination of these. Once a coil is wound on its core, the inductance of the coil can be changed by tapping the coil at specific windings, by stretching or compressing the coil windings on the form or by connecting several smaller coils (even of different types) in series to add together for more inductance. Your imagination is the only limit!
There is far too much information to share an a simple blog post, so please to be enjoying our many annotated links to readings about building antenna matching coils to increase range in low power AM broadcasting. As always, intellectual property and quoted content from the sites below are © the original authors of the linked works:
- A thorough pamphlet on designing and building AM transmitting antennas with matching coils
- A companion Excel worksheet to calculate coil dimensions and turns for the antennas in the above pamphlet
- Try Winding and Using Toroids, if you choose to use an iron powder core bar or toroid for your matching coil, as described in the above pamphlet
- An online calculator to perform calculations for single layer air core coils, along with construction tips
- A short handbook on Winding large, low resistance coils out of scrap materials
- Nice tutorial on coils for AM use, inside and outside of the transmitter
- Check this link for Antenna construction using a vertical air coil
- Visit this site for a simple pictorial guide for using a vertical air coil by W5CDT
- Click this link for Antenna construction using a center loaded vertical air coil from Miles Robert Audio
- See this page for Low power AM vertical antenna loading coil construction, this page for Coil construction using a pipe fittings and visit this page for variometer Loading Coils for low power AM operation, all from North Country Radio
- Kyle Drake offers some interesting info in his Low Power Broadcaster's Handbook here, in his Overview of Shortened Vertical Antennas
- Also check out Kyle's Appendix D: Loading Coil Inductance Values