[TowerTalk] AM Broadcast Vertical Length

[Paul Christensen]

> There were (and may still be) a few stations that used the Franklin
> antenna, or two half-waves in phase.

Here's the Brittish patent by C.S. Franklin:

http://www.aktuellum.com/circuits/antenna-patent/patents/242342.pdf

There's aren't many commercial Franklins around due to the serious amount of 
steel and property required.  As in the case of WHO, some are "modified 
Franklins."  KFBK, KSTP, WOAI, KNBR have, or at one time had been using 
Franklins.  I believe WOAI, San Antonio has a 180/120 modified Franklin.

Stacking two half-waves results in a full-wave vertical dipole, but the two 
elements are phase-cancelled.  The Franklin employs a 90-degree network to 
bring the top and lower elements in phase.

Here's a very simplistic diagram that shows the perils of using a 5/8-wave 
vertical in the AM broadcast service.

http://www.radiomagonline.com/media/ar/0701/107br0703lg.jpg

Paul, W9AC