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Several people on this forum have noticed that a vertical antenna above ground near a house or mobile home with aluminum siding has very low range. This is not very surprising, because reflections from the aluminum siding cause a virtual image antenna to be created. The current in the virtual antenna is in the opposite direction of the current in the actual antenna, causing the fields in the actual antenna to be cancelled out. This causes the radiation from the antenna to be very low.
The radiation improves as the vertical antenna is moved further from the aluminum siding. Fairly good results are obtained when the antenna is about 1/6 wavelength from the siding. Not much more improvement is obtained when the separation is more than about 1/2 wavelength.
I have noticed poor range when a verical antenna was placed close to a house with concrete block construction. The concrete blocks are a lot less conductive than aluminum siding, but a similar effect takes place anyway. A part 15 AM vertical antenna at ground level should be placed as far away as possible from any structure, to avoid interference from said structure. Unfortunately, the lots of most homes usually do not allow much separation between the antenna and structures. It is necessary to elevate the antenna above the structure to get reasonable range.
One of the possible configurations of an elevated antenna that has not received much attention lately is the center-fed vertical dipole. It actually works about as well as a vertical monopole, of the same length, above ground, that is remote from any structure.
Near the top of the AM BCB, a 3 m monopole above grounf (remote from other structures) has a radiation resistance of about 0.1 ohms and a capacitive reactance of about 3000 ohma. The efficiency of an antenna is the radiation resistance divided by the loss resistance of the antenna system. The loss resistance comprises ground resistance and loading coil resistance.
A 3 m center-fed dipole in free space has a radiation resistance of about 0.05 ohms and a capacitive reactance of about 6000 ohms. However, an AM antenna is never high enough to be considered to be in “free space,” and the earth ground always has an effect. At the roof level of a typical house, the radiation resistance increases to about 0.1 ohms (same as for a 3 m monopole above ground), but the capacitive reactance remains at about 6000 ohms. This means that the elevated vertical dipole requires a loading coil with twice the inductance that is needed for a monopole above ground. So, the dipole has more loss resistance due to the loading coil than the monopole above ground; but the dipole also has no ground resistance (since there is no connection to ground). The increased loading coil loss just about balances out the absence of ground loss; and so the elevated dipole works about as well as a monopole above ground without any nearby structures.
Total posts : 45366Ermi,
Interesting article but I do have a question.
You wrote: The current in the virtual antenna is in the opposite direction of the current in the actual antenna, causing the fields in the actual antenna to be cancelled out.
Why would the current be 180 deg. out of phase in the virtual antenna if it is close (in small fractional wavelength terms) to the primary antenna?
Neil
Total posts : 45366A linear antenna near a reflecting surface forms a complete circuit with said reflecting surface because of capacitive coupling with the reflecting surface.
If the antenna is in parallel with the reflecting surface (such as when a vertical monopole is in parallel with aluminum siding on a wall), a portion of the current in the antenna goes into the reflecting surface (because of capacitive coupling) and returns into the antenna. The return path of the antenna current through the reflecting surface is in the opposite direction of the current in the antenna. The current path is like in any electric circuit. This is why the direction of the current in the virtual antenna is in the opposite direction of the current in the actual antenna.
Any reference book about antennas has illustrations of the direction of the currents in the actual and virtual antennas for various angles of orientation of the the antenna with the reflecting surface.
Total posts : 45366To All,
What follows is not a rebuttal of what Ermi posted but rather a bit of high powered theory for those who have interest. Ermi’s point that nearby reflectors will decrease your signal is valid but it, as with most things in the practice of technology, depends. He is correct but not necessarily for the reason given. It is not this simple.
Ermi,
Thanks for your reply. It forced me to do some homework and it appears to be worthwhile.
Your statement regarding the opposite current would certainly be correct if there were a lumped capacitor at the top of the antennae and another at the bottom with an EMF source in series, but we are dealing with a distributed parameter system here.
The closest example in the reference I have spoke of directors and reflectors. Though the wavelength separation is probably wavelength wise approximately the same as we are discussing a critical point in the operation of the virtual antenna as a director or reflector is it’s resonant frequency. The radiating antenna may be resonant in practice but an aluminum siding wall is certainly unpredictable. This would be speculative and I am going to defer it unless you want to pursue it.
I did find an equation which shows that the current in the virtual antenna can be out of phase as you described but it also shows that it could be in phase or any phase relationship depending on the coupling and virtual antenna parameters.
From “Electronic and Radio Engineering” by Terman the current in the virtual antenna, I2 induced by the radiating antenna current I1, is:
I2= -I1(Z12/Z22)
which does indicate a 180 degree phase shift, but since Z12 and Z22 are complex numbers the phase of I2 is affected by the coupling Z and the Z of the virtual antenna. This can thereby produce either reflection or direction effects and phase relationships other than 180 degree inversion.
Another way I look at this is that the energy applied to the driven antenna has to go somewhere. Now it gets interesting because, neglecting losses, this means it is distributed in a pattern, favoring some directions and neglecting others.
A very important point which you alluded is that the radiation resistance of the primary antenna changes with the presence of a virtual antenna and this certainly can reduce the power radiated due to losses and have exactly the same effect of killing the signal as you described, but it is not simply due to the fields canceling due to 180 degree phase shift.
Thanks for the chance to go back to school.
Neil
Total posts : 45366I used to be heavy into c.b. radio right up until about 5 years ago. ( 18 yrs)
One thing I did for friends and even new cb’ers was help install mobile antenna’s. Alot of times I had to convince some hard headed operators that their install was not ideal and that radio they spent X amount of Dollars on was still good but their choice of positions on their car or truck was not optimal for 11 meters.One thing i saw a lot of was pickup trucks with fiberglass whips mounted almost flush with the cab of the truck. They would complain they could not be heard farther than a mile or less but had the quietest receiver they have every heard. While i was on a base station with a noise level of S+20 over 9 they were receiving S 5 and could only copy stations close to them or that were running linear’s.
After some lengthy discussions and much aggravation, I was finally able to get the operator to move their whip away from the cab of the truck. Suddenly they had a better appreciation for their new cb radio but after two days of listening to hash and trash they wished they left the antenna mounted against the cab. lol
In effect, this was the same thing you have explained here.
Although I would venture to say their pattern was so narrow due to the close proximity of the cab and antenna, the signal coming in was nulled by the cab of the truck, same goes for transmitting too.If i’m off base with my analogy then I apologize, it just makes sense to me and seemed relevant to this discussion.
Total posts : 45366I’m especially interested in this portion:
This means that the elevated vertical dipole requires a loading coil with twice the inductance that is needed for a monopole above ground. So, the dipole has more loss resistance due to the loading coil than the monopole above ground; but the dipole also has no ground resistance (since there is no connection to ground).
Total posts : 45366My mention of capacitive coupling between the linear antenna and a plane conductive surface was intended to give an intuitive feel for the direction in which the current in the image antenna flows. Current flows along the length of a linear antenna, but it does nor flow only along the antenna. Like with any electric current, there must be a return path through a circuit.
I was puzzled by radio8z’s mention of “directors and reflectors.” This seems to me to be a reference to directional arrays, such as Yagis. What I was referring to has very little relationship with the theory of Yagis and the like. What I was referring to is the theory of image antennas.
Figure 7 in the “Antennas” chapter of Terman’s Radio Engineers’ Handbook gives the images of several linear antennas with various orientations to conductive surfaces. Similar illustrations are in many other antenna reference books. According to Terman’s Figure 7, if a linear antenna is in parallel with a conductive surface, an image antenna exists in which the current flows in the opposite direction of the current in the actual antenna.
In the theory of image antennas, the fundamental assumptions are that the conductive plane surface has perfect conductivity and is infinite in extent.
Finite conductivity does produce the phase shift radio8z mentions, but at medium frequencies, earth conductivity is high enough for the phase shift to be minimal. The conductivity of aluminum is very high, and practically no phase shift is produced.
While the extent of aluminum siding is hardly infinite, if the separation between the antenna and the siding is small compared to the extent of the siding, a rough approximation to infinite extent is produced. This tends to explain the often-observed fact that a vertical antenna close to aluminum siding gives poor range.
A vertical antenna next to a low-conductivity surface, such as a cement block wall, probably has phase shift, and also energy absorption by the wall. This is a more complicated situation than with a high-conductivity material, like aluminum siding.
Total posts : 45366When a vertical dipole is used, and the transmitter and and audio source are battery powered, and there is no cable to the studio, there is no ground connection required.
Cables to a power source and the studio would complicate the installation considerably. Then there could be RF applied to the cables.
As for the absence of a lightning ground, well, the transmitter will be toast in the event of a strike–similar to when there is a lightning ground.
Total posts : 45366Ermi,
We have different books by Terman but I suspect we are looking at the same figures. They are for antennae above a perfect ground and show the image antennas for several orientations. You noted “In the theory of image antennas, the fundamental assumptions are that the conductive plane surface has perfect conductivity and is infinite in extent” which agrees with the discussion Terman uses in my reference. Also, the conclusions used for the antenna field above ground would apply to one located near a vertical conductive surface, with the planes rotated. So far so good.
The problem I see is that the model fails because a typical home wall is a small fraction of the MW wavelength and cannot be considered infinite in expanse. With this in mind, I sought a more suitable model which brought into play the idea of directors and reflectors where the virtual antenna current would be in the reflector rather than in a modeled image antenna behind the surface. This also fails since the wall is much wider than a reflector or director on a beam antenna and these elements’ contributions would be re-radiation effects and neglect reflections.
This reminds me of diffraction effects in optics where mirrors made of sub-wavelength wide strips (diffraction grating) do not produce images.
Neil
Total posts : 45366“As for the absence of a lightning ground, well, the transmitter will be toast in the event of a strike–similar to when there is a lightning ground.”
In the case of the Rangemaster and possibly the Chezz radio product, there is a gas discharge device connected between the antenna and ground. This is to bleed off any unwanted charge build-up mitigating a direct strike. An earth ground is required.
Total posts : 45366radio8z’s last post clarified his viewpoint considerably for me. I now understand that he considers the extent of aluminum siding on a typical home to be insufficient to produce the countercurrents necessary to substantially reduce the radiated field from a nearby vertical antenna over ground.
These countercurrents occur even for surfaces very small compared to wavelength if the separation between the linear antenna and the conductive surface is small compared to the extent of the surface. (Remeber that the Part 15 AM antenna itself is quite small compared to wavelength.) Lentz’s law requires that the magnitude and phase of the currents induced in the surface must reduce the magnitude of the incident field. The closer the antenna is to the conductive surface, the more the radiation from the antenna is reduced.
Total posts : 45366There is nothing wrong with adding a gas discharge tube to a transmitter circuit. It would do no harm, and might even do some good. I think it is a bit optimistic, however, to hope that it would protect against a direct, or even an indirect, strike.
Total posts : 45366Ermi,
In response to your post: “radio8z’s last post clarified his viewpoint considerably for me. I now understand that he considers the extent of aluminum siding on a typical home to be insufficient to produce the countercurrents necessary to substantially reduce the radiated field from a nearby vertical antenna over ground.” It appears that you have clarified my viewpoint in your own mind but did so in error because I did not state what you apparently assume. What I did say is that you presented a model for the situation (the image model) which I believe to be flawed. I also attempted to present a model which, I admitted, also fails. At no time did I say that the effect of a metal wall was not detremental. My point is that we don’t have an accurate model, not that the effect is not negative. In fact if you read back through my previous comments you will see that I agreed with you regarding the effect of proximity of the antenna to a wall in reducing the radiation. I stated that this should happen, just not for the reason you proposed.
I find it interesting that you are now thinking along the line as was I with your mention of the countercurrents in the siding. That was my original approach.
Models can be useful for general guidance which I think yours provided, but considering these models and pushing them beyond the founding assumptions can be misleading.
Neil
Total posts : 45366I’m glad that we agree about the reduction of the radiated field of a linear antenna next to a conducting wall by a reverse current induced in the wall.
I consider the image antenna viewpoint to be a very useful concept for understanding various antenna configurations. Without it, many very ordinary antennas would be very difficult to comprehend. Strictly speaking, the image antenna viewpoint applies only to perfectly conducting plane surfaces of infinite extent. It has nevertheless shown itself to be useful for cases where the conductivity is rather low, and the extent of the surface is small (even subwavelength).
Let’s look at a particular example: Some people think that the “inverted L” antenna is horizontally polarized, since most of its length is oriented in the horizontal direction. Actually, the inverted L antenna is vertically polarized, and the image antenna viewpoint explains why. The current in the horizontal portion of the image antenna flows in the opposite direction of the current in the actual antenna, causing the fields generated by the horizontal portion to be mostly cancelled out. The currents in the vertical portion of the inverted L antenna flow in the same direction in both the actual and image antennas, and nearly all of the radiation is vertically polarized. In this case, the image antenna wiewpoint works at medium frequencies, although, the earth ground conductivity is quite low. At higher frequencies, where the earth ground conductivity is a lot lower still, the inverted L antenna will not work.
In the situation described in this thread, a linear antenna in parallel with a perfectly conducting plane surface of infinite extent produces an image antenna with the current flowing in the opposite direction of the flow in the actual antenna. As shown in the previous paragraph, the assumption of a perfectly conducting plane can be relaxed by quite a lot. Also, as we will see, the assumption of a plane of infinite extent can be relaxed by quite a lot–even into the subwavelength region.
radio8z raised the objection that, by optical analogy, an image would not be created if the reflecting surface is subwavelength in size. The optical analogy is not applicable to the case where the antenna is subwavelength in size, and the spacing between the antenna and the reflecting surface is subwavelength in extent.
One must be careful in using optical analogies in radio, because optics usually deals with the far field, and not the near field. Diffraction (specifically Fraunhofer diffraction) is applicable only to the far field. In most cases, since optical wavelength is in the vicinity of only about a half micrometer, optics mostly deals with the far field. In the near field, optics texts refer to the existence of “evanescent waves,” which are called the “induction field” in radio.
The scanning probe optical microscope scans an object in the optical near field, and greatly exceeds the Fraunhofer diffraction-limited resolution limit of an ordinary optical microscope. This is called the “superresolution principle.” The superresolution principle also applies when a subwavelength antenna is located at an even smaller subwavelength distance from a conductive plane. The Fraunhofer diffraction limit implied by the optical analogy of a subwavelength mirror does not apply in this case. The image antenna viewpoint can apply for subwavelength reflecting surfaces under the restrictions of even smaller subwavelength spacing, and a subwavelength antenna..
Of course, the image antenna assumption should be tested by other methods; but I do not know of an antenna analysis method that is more intuitively evocative or pedagogically useful.
Total posts : 45366I was contacted privately by a guest on this website who told me that my estimate of a capacitive reactance of 6000 ohms for the elevated dipole is too low. I looked into the matter, and saw that the 6000 ohm estimate is about right for a large dipole diamater in the vicinity of 3 inches. For 1/4 inch dipole diameter, the capacitive reactance is about 16000 ohms, which would require a really big loading coil for tuning. So, it is apparent that the elevated dipole should have a big diameter to be able to use a reasonably-sized loading coil.
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