Would it be possible to make a AM rotortiller antenna

[Lil'Yeshua]

Scaled to any frequency, this design still has negative 3 db and the dipole has twice the effective radiated power. These antennas are among the very worst in terms of gain and signal. They are desirable because they have been designed to produce very little downward radiation in a circular design. Something that is important on VHF for minimizing RF exposure. Because they have a negative gain you rarely see a single one installed. They are almost always phased and stacked with multiple bays to provide the needed gain.

Like a quadradecker sandwich!

 

[vo1ks]

You clearly have little idea what you're talking about. CP and the Rototiller design are very common for FM broadcasting. There is a reason to use it in favor of strictly vertical or horizontal polarization. Signal penetration and overall range is usually much better with a CP antenna compared to HP or VP.

-3dB is not a signal loss. The energy is split between vertical and horizontal planes. You will need very expensive test equipment to detect someone dropping to half power.

For HF use, on paper circular polarization would show -3dB gain compared to a dipole. However, during skywave propogation it would exhibit considerably less signal fade. As the receive signal polarity changed the CP antenna would respond equally to vertical, horizontal, or anything in between. Similarly on the other end, the linear polarized receive antenna will respond better to the CP signal than one changing from vertical to horizontal polarity.

If both stations use CP, one would need to use right hand circular and the other left hand circular. Otherwise it would be comparable to receiving HP with a vertical only antenna. The signal loss would be quite high.

Scaled to any frequency, this design still has negative 3 db and the dipole has twice the effective radiated power. These antennas are among the very worst in terms of gain and signal. They are desirable because they have been designed to produce very little downward radiation in a circular design. Something that is important on VHF for minimizing RF exposure. Because they have a negative gain you rarely see a single one installed. They are almost always phased and stacked with multiple bays to provide the needed gain.

 

[Lil'Yeshua]

Just wonder if for 10/11 meters an dual or quad CP beam antenna array could be utilized except that the elements shortened and have load coils in order to keep the size of the array down to something more manageable. It would be a neat project to experiment with.

 

[Captain Kilowatt]

You clearly have little idea what you're talking about. CP and the Rototiller design are very common for FM broadcasting. There is a reason to use it in favor of strictly vertical or horizontal polarization. Signal penetration and overall range is usually much better with a CP antenna compared to HP or VP.

-3dB is not a signal loss. The energy is split between vertical and horizontal planes. You will need very expensive test equipment to detect someone dropping to half power.

For HF use, on paper circular polarization would show -3dB gain compared to a dipole. However, during skywave propogation it would exhibit considerably less signal fade. As the receive signal polarity changed the CP antenna would respond equally to vertical, horizontal, or anything in between. Similarly on the other end, the linear polarized receive antenna will respond better to the CP signal than one changing from vertical to horizontal polarity.

If both stations use CP, one would need to use right hand circular and the other left hand circular. Otherwise it would be comparable to receiving HP with a vertical only antenna. The signal loss would be quite high.

Warren FYI the CKDY-1FM site in Weymouth uses such an antenna. It's a three bay circular from Shively. 2200 watts and 180 feet of 1 5/8 heliax yields 3000 watts ERP. The fading or rather lack of while in a moving vehicle was quite impressive especially on the fringes.

 

[Lil'Yeshua]

Warren FYI the CKDY-1FM site in Weymouth uses such an antenna. It's a three bay circular from Shively. 2200 watts and 180 feet of 1 5/8 heliax yields 3000 watts ERP. The fading or rather lack of while in a moving vehicle was quite impressive especially on the fringes.

Got a pic of it you can show us?

 

[Shockwave]

Vo1ks, you wouldn't want to bet the farm on that one would you? I happen to be very well versed in FM broadcast antennas and the ERI Rototiller was specifically designed to produce the lowest level of downward radiation in the CP line of antennas. The positioning and unique curve of the elements cancels the hazardous downward radiation.

CP only benefits an FM station if its signal is experiencing a high degree of mulitpath distortion. In the absence of multipath problems, placing all the power in linear vertical polarization will provide better coverage and efficency. If you want to debate this fact, stop and read this BBC report first: http://downloads.bbc.co.uk/rd/pubs/reports/1986-13.pdf

CP antennas are popular in North America for two reasons. First, most commercial stations are still mandated to operate with horizontal component to remain compatible with older systems. Second, licensing regulations allow them to run their full ERP in both horizontal and again in vertical if they add the vertical component.

That allows them to bypass the 3db loss by using more transmitter power output or additional antenna bays when using CP. You cannot apply this scenario to any other band where you would not automatically compensate for loss in gain with more power or phasing extra antennas together.

Captain, just out of curiosity what type of antenna was CKDY using prior to installing the Shively?

 

[Captain Kilowatt]

Captain, just out of curiosity what type of antenna was CKDY using prior to installing the Shively?

None. It was a new installation from the beginning. It was an FM rebroadcaster of CKDY-AM which has an extremely deep null on the back. The FM signal allowed expanded coverage.

 

[Shockwave]

Just to follow up, CP can have advantages when working DX on this band because it's responsive to signals in any polarization. However, using the Rototiller design would not be an effective option here. The omni directional pattern combined with negative gain would place it well behind many other options that would be easier to install.

 

[DualAntennas]

I just wanted to know how big a rotortiller would have to be for 11-meter CB?

 

[Captain Kilowatt]

Take a pair of 1/2 wave dipoles, bend them into the right shape and then measure it. Get back to us on the size.


The short answer to the above solution is .........they are 1/2 wave dipoles. You figure out the rest.

 

[ghz24]

I haven't modeled a rototiller yet but I suspect it will have the same problem as Circularly Polarized beams have (at CB frequency).
The ground increases the gain of the horizontal component dramatically more than the vertical component making the signal more elliptical than circular..
The axial ratio quoted was 3 dB in free space

To my old eyes those half circle shaped elements are not flat, that is they are really sections of a helix.

I just wanted to know how big a rotortiller would have to be for 11-meter CB?

Approximate diameter of the helix 135 inches if it is a 1/2 wave long half circle
That looks like a hard antenna to model let alone build, then try making identical ones to stack or phase.

Is this antenna reversible ? Left handed to right handed.
It sounds like it's not.

They are desirable because they have been designed to produce very little downward radiation in a circular design

Don't you think that is caused/controlled by proper spacing and phasing of the stacked elements rather than the geometry of the elements?
Circularly Polarized antennas don't always scale well especially omnidirectional ones.

CP only benefits an FM station if its signal is experiencing a high degree of mulitpath distortion. In the absence of multipath problems, placing all the power in linear vertical polarization will provide better coverage and efficency. If you want to debate this fact, stop and read this BBC report first: http://downloads.bbc.co.uk/rd/pubs/reports/1986-13.pdf

Wow! that's from 1986 they don't separate slanted linear from circular.polarization they lump it all together as mixed. They also accept as fact that they are unable to produce a true CP omni. 27 years is a long time in technology. Computers had barely impacted antenna design in 1986.
I'm sure it was relevant to 1986 technology but it's a bit dated to just trust verbatim 1986 minus another 27 years was 1959 just saying
If the transmitter and the receiver both use the same directional rotation of CP then multipath is almost non-existent because reflected signals are reversed in direction and attenuated about -20 dB. But could you explain how the receiver having a linear polarized antenna helps multipath?

DualAntennas: I'd agree with shockwave rescaling this to 11 meter and making clones to phase together is a very ambitious I would look to another design. If you're really wanting an omni CP antenna that you can try in a phased array I'd suggest instead the cycloid dipole at least you can correct the axial ratio problem.
It has simple straight sections and 90 degree angles so it's easier to model and build/clone.
I've already scaled one to 27.1 MHz like this one

in free space. Notice as shock stated total gain is less than a dipole and the vertical and horizontal component gain is really low at -
I've been interested in a four element square array like a super scanner +1 with 4 cycloid dipole elements instead of 3 standard dipoles and steered by phasing instead of reflection or direction, all 4 elements hot.

The south and east elements are fed 90 degrees delayed separation is ~ 10 ft.
left hand vs right handed is intrinsic in the geometry.

 

[Shockwave]

Don't you think that is caused/controlled by proper spacing and phasing of the stacked elements rather than the geometry of the elements?

Spacing is a big factor in "end fire" downward radiation produced in CP arrays but this spacing effects all CP bays the same way. Closer spacing reduces end fire at the expense of gain. In the case of the Rototiller, the antenna itself has been carefully designed to produce minimum downward radiation.

Unlike all other CP antennas, this one doesn't use a pair of straight crossed elements or a vertical dipole with a helical twist in the horizontal plane at the feedpoint. This one curves the element ends back inwards at a specific angle that reduces this problem combined with the phasing between the two crossed elements.

I don't remember the specifics as it has been several years since I examined this antenna but I'm sure the ERI site has information regarding this exclusive design feature. That's why it's popular in the highest power applications where CP is required along with attention to RF exposure levels reaching the ground around the site.

Wow! that's from 1986 they don't separate slanted linear from circular.polarization they lump it all together as mixed. They also accept as fact that they are unable to produce a true CP omni. 27 years is a long time in technology. Computers had barely impacted antenna design in 1986.

I'm sure it was relevant to 1986 technology but it's a bit dated to just trust verbatim 1986 minus another 27 years was 1959 just saying
If the transmitter and the receiver both use the same directional rotation of CP then multipath is almost non-existent because reflected signals are reversed in direction and attenuated about -20 dB.

While that report is 27 years old the finding are confirmed by another study conducted 15 years later on the same topic here: VERTICAL POLARIZATION AND FM TRANSMITTER ANTENNA PERFORMANCE OPTIMIZATION Surprisingly the basic design of FM broadcast antennas has not evolved like the rest of the electronics industry. The biggest changes have been in areas of reducing downward radiation and methods of combining digital signals to feed the antenna system.

But could you explain how the receiver having a linear polarized antenna helps multipath?

There is no explanation to describe how linear polarization would help with multipath distortion. This is the one area in FM broadcast where I support the use and added expense of CP arrays. In just about all other cases vertical only polarization will provide the same coverage area at half the TPO or number of antenna bays.

I've been interested in a four element square array like a super scanner +1 with 4 cycloid dipole elements instead of 3 standard dipoles and steered by phasing instead of reflection or direction, all 4 elements hot.

The 4 element Super Scanner is also an Idea I've had interest in. The first time I seen something like this was a Polaris VHF Marine radio system that had this "4 pole" array mounted in a square configuration. It was designed for RDF use and not for gain but I'm sure this just had to do with how the elements were wired and selected by the radio.