The Mast · Entry 03.3
The Blaw-Knox tower
The diamond cantilever, widest at its middle and balanced on a single base insulator.
A mast that balances on a single point
The diamond cantilever is one of the most recognisable structures in broadcast engineering: a steel lattice tower that widens to its greatest diameter somewhere around mid-height, then tapers back to a peak, the whole thing balanced on a single base insulator and radiating as a complete element. Blaw-Knox, the Pittsburgh engineering and manufacturing firm, developed and patented the design in the 1930s, and it became the tower of choice for a generation of high-power AM stations.
The geometry is not decorative. A conventional self-supporting tower carries its greatest cross-section at the base, where bending moments are largest. The diamond form distributes the compressive and tensile loads differently: the wide middle section acts as a stiffening girder, letting the structure resist wind-induced oscillation with less steel than a comparable straight tower would require. The engineering result is a structure that is stiffer in proportion to its weight, and stiffness matters because a swaying tower changes its electrical length moment to moment.
| What | The consequence |
|---|---|
| Typical maximum cross-section | around mid-height, not at the base |
| Heights where the design was most competitive | above roughly 150 metres |
| Surviving operating examples | estimated fewer than thirty worldwide |
Because the mast is the antenna, that mechanical stability is not a secondary concern — it is the transmitting element itself. The Blaw-Knox stands on a base insulator, isolating it from ground potential so the RF drive can be applied at the base. The entire structure then carries the standing wave of current that produces radiation. Any mechanical movement that shifts the effective height shifts the radiation pattern and, in a directional array, can move a null off its licensed bearing.
WLW in Cincinnati used a Blaw-Knox tower during its era of maximum-power operation in the 1930s. The Blaw-Knox tower at WSM in Nashville is another well-documented example of the diamond cantilever form. Both sites were chosen partly because the design promised mechanical robustness at heights above 150 metres, where conventional guyed masts of the period needed multiple anchor points that complicated the base insulation.
Maintenance has always been the complication. The wide mid-section creates large surface areas for ice accumulation, and ice loads are asymmetric — one face exposed to wind, another sheltered. Because the base insulator carries the entire structure's weight, any corrosion or cracking of that single component threatens both structural integrity and electrical continuity simultaneously. Engineers routinely inspect base insulators with far greater frequency than other tower hardware.
Fewer than thirty Blaw-Knox diamond towers survive in operating condition. Their continued presence on the skyline is partly sentimental and partly practical: replacing them with equally stiff, equally efficient radiating structures turns out to be more expensive than maintaining what is already standing.