[Antennas] "Com-Rad" restricted space antenna

[Barry E.Maguire]

Check out,
http://www.untenna.com/tech.html
g.l.
Barry
wb4izr
----- Original Message ----- 
From: "David Kelley" <dkelley at bucknell.edu>
To: <antennas at mailman.qth.net>
Sent: Monday, November 22, 2004 9:44 AM
Subject: Re: [Antennas] "Com-Rad" restricted space antenna


> 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
>
> ______________________________________________________________
> Antennas mailing list
> Home: http://mailman.qth.net/mailman/listinfo/antennas
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Antennas at mailman.qth.net