jbn@glacier.STANFORD.EDU (John B. Nagle) (10/07/88)
Is it possible to construct a UHF antenna which is both circularly polarized and omnidirectional in the horizontal plane? Or are these requirements mutually exclusive? I'm looking for an antenna for 1.2GHz with these properties. Any ideas? I'm trying to reduce the effect of reflection nulls on my robot TV link. John Nagle KA2XXF (part 5 experimental, not ham)
dleigh@hplabsz.HPL.HP.COM (Darren Leigh) (10/07/88)
In article <17770@glacier.STANFORD.EDU> jbn@glacier.STANFORD.EDU (John B. Nagle) writes: > > Is it possible to construct a UHF antenna which is both circularly >polarized and omnidirectional in the horizontal plane? Or are these >requirements mutually exclusive? I'm looking for an antenna for 1.2GHz >with these properties. Any ideas? I'm trying to reduce the effect of >reflection nulls on my robot TV link. I'm not an antenna expert, but I'll dig deep into my electrodynamics and see what I can come up with. My guess would be to have a vertical dipole and a circular loop like this: | | <- Vertical dipole | | ____________|____________ (_________________________) | ^ | | | Circular loop | (seen edge on) | For circular polarization, drive these so that the magnetic field from the vertical is 90 degrees out of phase with the magnetic field of the loop when it reaches the loop, and so that the magnetic field from the loop is in phase with the magnetic field of the other side of the loop when it gets there. In other words, make the radius of the loop an even multiple of quarter wavelengths and drive the two parts 90 degrees out of phase. This antenna should be omni-directional in the plane of the loop. Directly above and below you're going to get nulls so this design will be no good if your robot flies. ;-) Real RF types please correct me if I'm wrong. > John Nagle > KA2XXF (part 5 experimental, not ham) Sounds like a neat project, John. Good luck with it! ======== Darren Leigh -- (not part 5 experimental; not ham until I memorize the code instead of the code tapes) Internet: dleigh@hplabs.hp.com UUCP: hplabs!dleigh
haynes@ucscc.UCSC.EDU (99700000) (10/07/88)
I have an antenna on VHF that might have the properties you're looking
for. It is from FAA surplus and is labeled circularly polarized, so I
guess it is. It's rather hard to describe, but here goes.
The elements are swastika shaped. Imagine a swastika made out of solid
metal. Now take the outer, hooked part of the swastika arms and bend
them up from the open ends so they make an angle of about 30 deg. to
the plane of the swastika. This is one "pole" of the antenna. The
other is another swastika with the arms hooked in the opposite direction
and bent down from the plane. The plane of the swastikas (inner arms) is
horizontal. I don't know if there's anything (like a matching device)
between the coax connector and the elements. The outer conductor of
the coax is clearly connected directly to the lower swastika, but there
may be something between the inner conductor and the upper swastika.
In case that description is totally confusing I'll try a different one.
Imagine two thick disks, horizontal, one above the other like an Oreo
cookie. (Must be time for my bedtime snack.) Sticking out from the
edges of the disks at four points 90 degrees apart are four parallel
conductor transmission lines (made of about 3/4" diameter rods at VHF).
At the end of each of these transmission lines is a dipole of the
same diameter. The dipoles, rather than being horizontal are tilted
(rotated) at about 30 degrees to the plane of the disks and transmission
lines. So if you look horizontally at the antenna the near dipole
is tilted one way and the far one is tilted the other way.
I assume the FAA wants this kind of antenna to receive equally well
from airplanes in all directions; and circularly polarized so it
doesn't care about the orientation of the (linearly polarized) antenna
in the plane.
haynes@ucscc.ucsc.edu
haynes@ucscc.bitnet
..ucbvax!ucscc!haynes
"Any clod can have the facts, but having opinions is an Art."
Charles McCabe, San Francisco Chronicledya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (10/07/88)
In article <17770@glacier.STANFORD.EDU>, jbn@glacier.STANFORD.EDU (John B. Nagle) writes: > Is it possible to construct a UHF antenna which is both circularly > polarized and omnidirectional in the horizontal plane? Or are these > requirements mutually exclusive? I'm looking for an antenna for 1.2GHz The requirements are not mutually exclusive, as many TV stations, even UHF, utilise circular polarisation. Scala Electronics (Medford, Oregon), Bogner (Long Island) and Dielectric Systems (?) manufacture these antennas, but they are quite large and very expensive. If you don't care about efficiency (much) try a normal mode helical antenna. This is one where would orient the helix normal to the floor, and obtain the dreaded sin(theta) radiation characteristic for both electrical modes. They are very easy to construct and you should be able to get an axial ratio in the .8 to 1.2 range very easily (in which you'll have elliptical polarisation). Computing the helix dimensions will be easy, but determining the operating impedance will be more difficult. Any good antenna text should have this information; you'll be able to get up and running with this probably in one day or so. York David Anthony DataSpan, Inc
wjc@XN.LL.MIT.EDU (Bill Chiarchiaro) (10/08/88)
The ARRL Satellite Experimenter's Handbook shows some omni-az/circular-pol antenna designs. The most interesting is the quadrifliar helix, which is fashioned from wire in the form of four intermingled helices. It has been used on amateur satellites and with GPS receivers. Its radiation pattern does, however, favor the long axis of the antenna. Also, it is supposedly quite sensitive to its environment. Perhaps a more workable design is the Lindenblad antenna, which comprises a number of dipoles (at least four) spaced around the periphery of an imaginary, horizontal circle. The dipoles are tilted about 30 degrees out of the horizontal plane. I believe the "swastika" antenna mentioned earlier is of this design. Also, I remember once seeing such an array spaced around an observation deck on the Empire State Building -- I think it was for FM radio broadcasting. The Lindenblad concentrates its radiation at low angles; it's truly omniazimuthal. There is also the turnstile (crossed dipoles over a ground plane), but its circularity is not good at low angles. Some references from the Satellite Experimenter's Handbook are: Lindenblad: Brown & Woodward: "Circularly-Polarized Omnidirectional Antenna," RCA Review, Vol. 8, June 1947, pp. 259-269 Quadrifilar heilx: Kilgus, "Resonant Quadrifilar Heilx Design," Microwave Journal, Dec. 1970, pp. 49-54 Have fun, Bill N1CPK
dleigh@hplabsz.HPL.HP.COM (Darren Leigh) (10/08/88)
In article <2419@hplabsz.HPL.HP.COM> dleigh@hplabsz.UUCP (Darren Leigh) writes: >In article <17770@glacier.STANFORD.EDU> jbn@glacier.STANFORD.EDU (John B. Nagle) writes: [John wanted an omni-directional, circularly polarized antenna. I offered the following design but got it wrong: see below] | | | | | | <- Vertical dipole | | | | | ____________|____________ | (_________________________) | | ^ | | | | | Circular loop | | (seen edge on) | | | | |For circular polarization, drive these so that the magnetic field from |the vertical is 90 degrees out of phase with the magnetic field of the |loop when it reaches the loop, and so that the magnetic field from the |loop is in phase with the magnetic field of the other side of the loop |when it gets there. In other words, make the radius of the loop an |even multiple of quarter wavelengths and drive the two parts 90 |degrees out of phase. I forgot that the current on the far side of the loop is going the opposite direction as that on the near side; therefore the diameter should be an odd multiple of half wavelengths. To do this antenna right then, the radius of the loop should be an odd multiple of quarter wavelengths and the two elements should be driven in phase. ======== Darren Leigh Internet: dleigh@hplabs.hp.com UUCP: hplabs!dleigh
leadfoot%leftlane@Sun.COM (Mark Curtis) (10/08/88)
In article <1122@xn.LL.MIT.EDU> wjc@XN.LL.MIT.EDU (Bill Chiarchiaro) writes: > > Also, I >remember once seeing such an array spaced around an observation deck >on the Empire State Building -- I think it was for FM radio >broadcasting. The Lindenblad concentrates its radiation at low >angles; it's truly omniazimuthal. > >Have fun, >Bill >N1CPK I saw that antenna last year sometime at a distance of about 4 feet. I wonder what the field strength was where I was standing?
prager@Data-IO.COM (David Prager) (10/08/88)
First a question. Is there a difference between a circularly polarized antenna and an antenna whose radiated enerty has equal vertical and horizontal components? If not, I'd suggest that anyone looking for an antenna with these characteristics check into broadcast antennas for the commercial FM band. These antennas are well designed (usually), omnidirectional, and have equal H and V components. The major manufacturers maintain test ranges and do considerable research on their equipment. While omnidirectional, effective gain is achieved through phased stacking of the radiating elements. Arrays of antennas can them be center or end fed depending on preference and mounting set up. There is a company called Shively Labortory in Maine who publishes a wonderful catalog. The designs and specs for most of their line is publicly availible. I don't think that many hams need antennas that are built this beefy, but the design issues have already been worked out. David once upon WA3KQN ------------------------------------------------------------------- David Prager (w)206-881-6444 (h)206-485-4397 Data I/O Corp 10524 Willows Rd. NE prager@DATA-IO.COM Redmond, WA. ...uw-beaver!entropy!dataio!prager 98073-9746 or whatever works.