Here is a semi what-if question:

Why were directional broadcasting antennas not deployed on U-boats?

U-boats tended to chat on the radio quite a bit, and we now know that allied Huff-Duff was a lot more effective than Germany thought, and a major contributor to allied victory.

But still, intuitively, the vulnerability of long hand keyed morse dispatches back to Germany was obvious. It is evident that the Bdu realized this with the late war development of burst tranmission devices.

Did anyone ever think of making a directional boradcasting antenna instead of the omnidirectional antennas used? Either the antenna could be directional itself, or it could be surrounded by a grounded shield to stop broadcasting in undesired directuions. A directional antenna would broadcast full strength in the intended direction, but with little or no radio signal leaking out in to the sides to be picked up by enemy listeners. An enemy would have to be between the U-boat and Germany to intercept the signal.

The basic principles were well known, and directional antennas were in widespread use in radars etc.

Why not for broadcasting shortwave radio traffic?

(To answer my own post, here are some theories I have:

1) The vulnerability, like so many others, was not fully appreciated till too late, if at all. Look at all the easy, common sense changes to Enigma that would have rendered Bletchley Park deaf, but were not implemented.

2) The low frequency of over the horizon shortwave of those days (below 1 Mhz?) would make any shielding technically infeasible or physically unwieldly in size. As I remember, radio waves will curve around objects smaller than their wavelength, so for a 1Mhz shortwave set, a wavelength of ~ 1500 ft would make a parabolic type dish reflector like a radar uses totally impractical.

Does this reasoning (and my recollection of WWII military shortwave frequencies) sound correct?

If directional broadcast antennas are too large to be practical at shortwave frequencies, how could Huff Duff antennas, listening on those same frequencies, be directinoally sensitive and be small enough to mount on a ship?