On Mon, 18 May 2015 12:37:14 -0700, "Sal M. O'Nella"
wrote:
They were also used for HF transmit on some ships.
Here's a link to an early Navy research paper on the discone/cage
antenna sometimes called a "discage." Unfortunately, only the text
is clear; the pictures are not good.
http://www.navy-radio.com/ant/discage-661464.pdf
Some photos:
https://www.google.com/search?q=discage+antenna&tbm=isch
I'm not sure, but some of the discage antennas in the photos are more
like fat "cage dipole" or "cage monopole" antennas. The wire cage
increases the effective diameter of the monopole driven element to
increase it's bandwidth.
I couldn't find a better copy of the 1967 Discage paper. Actually,
I've never heard of a discage antenna. My idea of a cage antenna is
the Viet Nam war era "Elephant Cage" direction finder antenna:
https://www.google.com/search?tbm=isch&q=elephant+cage+antenna
Note that a single structure was fed from the top and the bottom by
two separate coax cables.
Not so weird. Visualize a biconical antenna fed as two monopoles from
the narrow pointed ends, instead of being fed as a dipole. Now,
reverse the positions of the biconical elements and we have something
that looks like the discage.
I had hundreds of inspection and test visits to Navy ships in the 1980s
and 1990s. In practice, I never saw that antenna installed for use
at the extreme low end, as described in the linked paper. There was nearly
always a fan antenna for 2 -6 MHz.
I don't have any Navy experience but I can see why a fan antenna was
popular. It's simple, fairly easy to install, and you can cut
elements randomly and still have a working antenna. Best of all, it
doesn't eat deck or turret space.
The discone/cage freq ranges were nominally 4 - 12 MHz (cage) and
10- 30 MHz (discone). Both sections were invariably fed through a
multi-transmitter coupler, either four or eight transmitters, depending
on the ship.
I know the fan antennas all had custom matching boxes at the
feed point to bring the SWR within the 3:1 circle. I can't recall if the
sections of the discage also had such matching.
Despite having a fairly broad bandwidth, it's not a perfect or
constant 50 ohms. Unless the radios can tolerate a moderate VSWR (3:1
perhaps?), some kind of tuner might be useful. However, at 3:1 and
with some kind of VSWR controlled tx power reduction, probably the
impedance matching boxes would be sufficient.
Thanks.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558