We have a situation where a tuned vertical dipole antenna is broadcasting at 450MHz. Due to EMI, we would like to restrict its transmission to a 60 degree arc on one side. IOW make it quasi-directional.
The simple way would be to screen off the opposite side with bent sheet metal. But, for experimental purposes, we do not want any reflection to be caused back out the open side.
Essentially, we are looking for a dampener or absorber for all EMR but that aligned with the 60 degree opening.
Someone suggested placing a 300 degree "ring" over top of the dipole consisting of a number of 33cm long (half wavelength) vertical wires soldered to a circular wire base. The latter would then be connected to electrical ground.
Has anyone seen this done successfully?
Would it result in additional loading to the transmitter?
It is done in screen rooms. Basically ferrite tile and foam absorber:
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But that is going to be expensive and unlikely to survive outdoors for long. Ferrite tiles might work outdoors but frost could get them. Best to talk to a manufacturer. Chomerics is one of the big players in this field.
It'll mess up the antenna impedance and thus the SWR a bit, hard to predict by how much. You'll have to try. Don't blast lots of power into this, get the manufacturers blessing if the power is substantial. You don't want to see tiles popping or toxic smoke coming out of the foam.
The sheet metal would become a nonresonant (if you're lucky) parasitic element distorting the radiation pattern of the dipole. You'll still get some radiation in the undesired direction.
Absorbers get hot, wasting transmitter power.
Not me.
Putting any conductive material in the near field of your dipole will do so. "Semiconductive" stuff like absorbers as well.
Simpler and more reliable to just erect another dipole phased so as to get the pattern you want. See this:
Out of interest, can anyone here suggest what the actual affect of the ring of grounded wires, as described below, would be?
We are loking for a passive absorber, but not one that relies on expensive materials.
(From OP)
Essentially, we are looking for a dampener or absorber for all EMR but that aligned with the 60 degree opening. Someone suggested placing a
300 degree "ring" over top of the dipole consisting of a number of
33cm long (half wavelength) vertical wires soldered to a circular wire base. The latter would then be connected to electrical ground.
Easy. Any antenna that has a 60 degree horizontal beamwidth will work. On the shopping list is a 4 element vertical yagi, a patch antenna, and a corner reflector. There are others, but those are the most popular. Offhand, I would suggest a corner reflector:
No. Screening is reflective. You'll have the signal going off in all kinds of uncontrolled directions. If you stuff the antenna into a metal cylinder, with a slot in one side, you'll have a great demonstration of edge (knife-edge) diffraction. You can possibly do it by surrounding the vertical dipole with RF absorbent material, but that will eventually get hot if you're transmitting any manner of power.
Any other limitations, like antenna size, efficiency, mounting, cost, FCC certification, standards compliance, etc?
Retch. Edge (knife-edge) diffraction will ruin the pattern. It will also need to be higher than your existing half wave antenna, as much of the RF from a veritcal dipole goes up and down.
Actually, yes. It was a rotating antenna direction finder that some company built. The antenna was in the center and did not rotate. Only the large tin can covering the antenna rotated. The idea was to eliminate slip rings and contacts needed to use a rotating DF antenna. I'll see if Google can find a photo. I don't recommend doing it this way, but lacking anything more than the frequency and the problem, it's as good as any other plan.
Yep. Any metal or absorber near the vertical dipole is going to change the antenna impedance and the VSWR. If the transmitter is fairly low power, and the RF stages fairly robust, it probably won't make difference. If you're running high power, and you have a VSWR protection circuit in the output stage, it's going to shut down. If you don't have any VSWR protection circuit, it's going to burn up, oscillate, or both. You can sorta protect the transmitter with an isolator or circulator, but those just redirect the reflected power to a dummy load, which will eventually get hot.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
"Reflection" if properly phased reinforces the desirable radiation. It is simply a matter of spacing of the reflecting structure that determines phase. For example, spacing a simple dipole antennas
1/4-wavelength off a (conductive) tower will give a substantial directionality to the radiation pattern, and varying this spacing is a widely employed method of pattern control.
If you want better control of rear radiation, the standard solution is (google it) "corner reflector antenna". Note: the type I am referring to uses TWO planar mesh screens. In the Wikipedia page look at the TV antenna pic below the dishes at right.
Why try to absorb? That's crazy, and expensive as well as inefficient. Use a passive reflector system which reinforces the forward radiation at the expense of radiation in your unwanted direction. Even a yagi as mentioned in other responses - although sometimes they still have lobes at the rear (albeit well down on the front radiation).
I'm having difficulty picturing that, but it WON'T absorb significant radiation. It will act as a parasitic element and depending on eelement length/geometry you will get a modified radiation pattern. Whether it is the pattern you seek is another matter entirely.
It will almost undoubtedly alter the drive point impedance.
A yagi has too many side lobes and lousy front to back ratio in many cases. A sheet metal corner reflector has a "cleaner" pattern and roughly the required 60 degree beamwidth. Cheaper too.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
HA! That's one-dimensional thinking. As others have said, the 'anti-phase reflection' means reflected power, and averaged over a wide solid angle, the out-of-phase condition is just in a few locales, NOT the whole radiant field.
The 'add a reflector' or 'add absorption' scenarios really mean redesigning the antenna. Its impedance and directionality and bandwidth all change when you do that.
I do not see why, but if you really want to _absorb_ the power from going into the unwanted direction, placing a ring of vertical dipoles at same elevation around the radiating dipole, might even work. Of course, each of these extra dipoles must contain a 60-75 ohm noninductive resistor in the middle, in order to actually _dissipate_ the RF received by these passive elements.
In the ideal case, in this configuration 5/6th of ther transmitter power is dissipated in the resistors, while only 1/6 is radiated into the wanted 60 degree angle.
As long as you keep the ring of dipoles outside of the near field of the transmitting antenna, it should not affect the transmitter loading too much.
The distance between the passive vertical dipoles in the ring should be less than 1/4 wavelengths, so that the capture areas of each individual dipoles will overlap and most of the power density in the dipole capture area is directed into the loading resistor in the middle of that dipole, thus, reducing the power density that will go through the ring.
A plane reflector or a circular reflector will no doubt some of the radiated power back to the radiated element, thus changing the radiating element impedance and matching.
A corner reflector or a parabolic reflector far away(in the far field) from the radiation element will send power away from the radiating element, thus the antenna matching remains nearly constant.
An absorptive screen (in the far field), such as a ring consisting of _matched_ dipoles with actual resistors to dissipate the received power should help. Of course, the proximity of these passive dipoles have some mutual coupling, thus some experimentation is needed to find the correct length, at which the midpoint is resistive at the operating frequency and then select a resistor that matches the midpoint resistance and install it there. Thus all power received by the passive dipole is dissipated into heat in that midpoint resistor and nothing is reflected back (and reradiated).
The attenuation through this ring of passive dipoles is harder to calculate, but using the receiving antenna capture analysis should give some estimates.
At this stage loading the transmitter is not an issue. We want to first verify any absorption effect.
That presumes that the ring diameter and number of passive dipoles is sized so the spacing between each of the latter is less than the desired 60 degree opening. Part of the subsequent design process.
What about first just making a FULLY enclosing ring of passive dipoles and then checking with an RF meter to see if that actually does anything to drop the radiated signal level?
I am still wondering about the desirability of running the base of the ring to ground, maybe through a diode. Any ideas on that?
It depends on the radius of the circle and the spacing between the wires.
There's software called "femm" that can make predictions on how an antenna will work.
If they're wires they're not absorbers.
You're going to get diffraction at the edge of the opening. sso, depending on how much of "all" you really need your shield may need to be enlarged to ridiculous sizes.
In effect, making a horn? Which will have to curve with a minimum radius greater than the wavelength (or some relation to it, maybe 1/4 wave, give or take some pi and roots of nth Elliptic Bessel functions or whatever), not saving you any space over a horn proper. If that's a concern of course.
But what *is* the concern? Why not just go with a normal directional antenna? Required minimal rear lobes? Okay, go with something fancy like a horn or the already-mentioned corner antenna (possibly with the corner at a suitable angle like 60 degrees, probably with some bending and stuff to limit diffraction, again making something, well... horn-ey). Or, if you don't mind getting really big, do it optically, with a huge parabolic shield (but not quite parabolic, or not quite at the focus, so as to get the desired beam spread).
How big and fancy can this thing actually be? How close are the succeptible things that they can't be, themselves, shielded, or something else fancy with the combination of antenna, equipment, and space between them?
Assuming that the ring of dipoles is at a radius of 1 lambda from the radiator, the perimeter would be 6.28 lambdas, thus with 1/4 lambda passive radiator spacing, you would need 29 passive dipoles, Remove 5 and you would something like the radiation pattern.
Please note that a mesh parabolic reflector works quite well with 1/4 lambda holes, but the "warm" ground will add some thermal noise. For radio astronomy and satellite communication, mesh holes of 1/10 lambda are desirable. Thus, I would expect a passive dipole spacing somewhere between 1/4 and 1/10 lambda would give quite good attenuation in the unwanted directions.
But the amount of energy penetrating the ring of absorbtive dipoles will still present a problem. Absorbtion will IMHO always be at best a second choice after reflection/reinforcement strategies which are well documented and well proven in the real world.
The O/P still hasn't detailed why the pattern control is required/desired, nor the limiting F/B (or F/S for that matter) ratio needed to be achieved. And some idea of how many horse-power the antenna is required to accommodate would also help. JeffL did touch on the under-defined problem.
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