[Antennas] "Com-Rad" restricted space antenna

Mon Nov 22 09:44:47 EST 2004

Hue,
I would say your intuition is correct. From your description, this sounds
like a VERY short loaded monopole. I suspect that their design "is a little
quieter" because it's inefficient. The relatively wide bandwidth of 250 kHz
is strong evidence of this. I would bet that those "ground metal" cylinders
aren't too highly conductive, or the other pieces are lossy, or the antenna
was coupling to a nearby lossy object when they made their measurements.
That claim that the antenna is quieter because its windings "follow an
essentially horizontal path" is creative. Of course, there is a vertical
component to the windings as well that's as long as the coil itself. That
vertical component allows noise pick-up and, indeed, is partially responsible
for the radiation from/reception of the antenna for normal ham radio
communications.
I'd put this antenna in the same class as the EH and the magnetic loop.
They work, and they might be a good choice for the space-constrained ham,
but they still obey the laws of physics. All other things being equal, short
antennas will never exceed - and will rarely match - the performance of
full size antennas.
73,
Dave
NB4J
>Say, gang, in my papers i came across a brochure from Com-Rad Industries 
>with a postmark
>of 1981. This is a restricted-space antenna. The antenna consists of, at 
>bottom, one or two
>metal cylinders, maybe the size of a deskside wastebasket, metal, but with 
>no lid or bottom.
>Above, attached by jumpers to these "ground metal" cylinders, is a coil 
>made of tubing,
>looks like maybe 8 inches diameter, set vertically. The coil for 80-40 
>looks like maybe 20" long.
>These figures are just guessing from a not too large photo in the 
>brochure. Above the coil
>is a horizontal metal rod maybe 36 inches long. So basically it looks 
>like just a good
>loading coil, with a capacitive hat, and the "whip" portion of a "mobile 
>antenna" shrunk
>down to almost nothing. Oh, yes, on 80-40 the 2 ground cylinders look like 
>they're set on
>the floor about 4-5 ft separated. And also, there's a jumper to short 
>turns on the coil.
>>The claims:
>"The large helix [ large coil of tubing ] in our design is a vertical 
>antenna but is a little
>quieter
>because the windings of the helix follows an essentially horizontal path".
>>Does this sound likely? I dunno.
>>"A 250 kHz band-width exists on 75/80 meters, for example, compared to the 
>usual 15-20
>kHz found in miniature antennas much larger than the Com-Rad units........
>The bandwidths previously described are achieved by keeping the vertical 
>section,
>which does the radiating, very small compared to the capacity modules [ 
>those cylinders
>on the floor ] with which they are used."
>>I don't get how this gives wider bandwidth without readjusting the coil. 
>With a coil
>made of copper tubing this size, seems to me that is going to be a hi-Q 
>circuit, and
>that means narrow bandwidth.
>In fact, i kinda don't see how this antenna is basically any different 
>from the mobile whip
>antenna. You could just hook a mobile whip antenna, one with a good 
>quality loading coil,
>onto a metal garbage can, and get the same results- that's what i think. 
>About the noise
>reduction, i dunno, sounds good but i would have to hear that.
>As Kurt N. Sterba has said, anything will radiate, even a grocery cart.
>Any comments?
>-Hue Miller

--
David F. Kelley, Ph.D.
Assistant Professor
Electrical Engineering Dept.
Bucknell University
Lewisburg, PA 17837
(570) 577-1313
dkelley at bucknell.edu