Somewhere in the threads it was said that placing an FM transmitter in a window is a good way of sending the signal outdoors while keeping the unit safe from the weather. That raises a physics question.
Is glass transparent throughout the radio spectrum as it is to the light spectrum?
Ya but, what about reflection? At certain angles glass reflects light and creates a mirror effect.
Going farther, using actual mirrors, would it be possible to build an RF concentrator the same way it's possible to build a light concentrator? Putting a light bulb inside an open ended tetrahedron lined with mirrors creates three more light bulbs for a total of four.
An AM antenna inside a giant picture window might also reach the outdoors as if it were there.
Carl,
You ask some great questions. My first thought about "is glass transparent to radio waves" was "of course, everyone knows that." but as I thought more about this some things came to mind. Glass is "transparent" to visible light but not to UV or far infared. Hmmmm... If one looks at a plate of glass from the edge one sees a greenish color so even with visible light something is going on. We also know that some of the light (about 8%) is reflected so less than 100% is transmitted.
I happen to have a microwave source and detector which I have set up for other purposes and I thought why not measure the transmittivity of glass with this?
Here's what I did: The 15 mW Gunn diode source with a 15 dB gain horn antenna was placed 1 meter from the detector which is a HP thermistor microwave detector with a 10 dB gain horn antenna and is attached to a HP 432A power meter. The operating frequency is 10.55 GHz and both the source and detector were aimed for maximum indicated power.
Results:
Free space through air P = -7.6 dBm.
Window glass (3/32") in front of detector P varies with the position of the glass as it is moved toward and away from the detector from -12 to -8 dBm. The peaks and valleys of the power seem to be spatially related to the wavelength of the signal though I did not measure the actual distance from peak to nadir.
Rear surface silvered glass mirror P = -20 dBm.
Obviously, at microwave frequencies, the glass is not transparent as indicated by the reduced P observed. The signal loss could be due to reflection rather than absorption but my experiment cannot discriminate this. Further, the variation in the P with the glass position implies reflections and standing waves. My thought is that the glass reflected the signal back to the source which reflected it back toward the glass producing standing waves.
The mirror effectively isolated the source from the detector though some transmission was detected.
Though this observation only applies to the 10 GHz signal I used it does raise the question whether similar effects are seen at broadcast frequencies. With visible light the refraction and reflection we see with glass can be explained by using the fact that the wavelength of the signal is short compared to the dimensions of the glass, but at 10 GHz the wavelength is 3 cm which is long compared to the glass dimensions.
I am not equipped to do this experiment at BC frequencies but it would be a neat thesis project (and someone somewhere has probably done it). EDITED TO ADD: Actually I am. I have a thermistor mount which accepts a whip antenna and I have previously used it to measure the field from my FM transmitter. Maybe later if I get inspired I will try this. END EDIT
Guess I didn't prove anything except that at 10 GHz glass is not "transparent".
Neil
Neil radio 8z,
Your presentation is very informative and gives an extremely probable situation regarding the transparency of glass at different frequencies and, as you indicate, different distances between glass and transmitter. Your mention that glass is not so transparent at ultra violet or, down below light spectrum, infra red, strongly suggests that glass provides a distinct "window" in the light band.
For anyone interested, here is an electromagnetic spectrum chart on Wikipedia
Your microwave test took us below infra red and suggests a loss of 1 dB or more.
My intuition says that the reflections/standing waves are similar to those encountered in microphone audio, where hard surfaces cause reflections that cause a "comb-effect" between two mics (cancelation/exaggeration), that can be reduced by moving as close as possible to the plane of a surface, giving us "PZM - Pressure Zone Microphones," which lay flat on a table, floor or wall.
Therefore, I suggest, placing the transmitting antenna as close as possible to the glass might constitute best practice. If there are double storm windows the outer window would be the right better choice.
This makes me think of the new generation of auto antennas, some of which I think are embedded in the glass of the windshield.
Well, the emphasis is on the dum.
I just looked at a real window and guess what it has. It has a screen.
I'm drawing the shade now and pretending I'm not home.
I recall a seminar put on by one of the microwave vendors.
They demonstrated that microwaves could be focused with an optical lens placed between the transmitter and receiver horns. The lens was a common "reading glass" type.
All good insulators, such as glass, ceramics, and plastics, are very transparent to radio waves at 1 MHz. A pane of glass will not significantly attenuate a signal at 1 MHz.
A measure of relative transparency is the dissipation factor, which is found in tables of electrical properties of various materials. For glass, the dissipation factor varies from about .002 to .01, depending on the type of glass. PVC plastic has a dissipation factor around the upper end of the range for glass. HDPE (high-density polyethylene), which is a particularly good insulator at RF frequencies, has a dissipation factor of about 0005.
A vacuum has a dissipation factor of zero (by definition). For air, the dissipation factor is so low that it is difficult to measure accurately.
Higher frequencies have higher dissipation factors. At 100 MHz, it is still low enough not to produce significant attenuation with a pane of glass.
One thing, some glass may have tinting, some tinting can be metallic which would block radio.
Word is that incandescent bulbs are banned or at least discouraged because 80% of their energy is lost as heat.
Lost?
Most of us are deep in the deep-freeze and what becomes useful during cold weather? Why, heat, of course.
That's why every year during wintertime I store the LED bulbs and use all incandescent bulbs for their little extra heat boost.
When my Save-So-Much Store had a sell-out mound of incandescents I grabbed batches of them as in investment in winters of tomorrow.
I was reminded of this today because the wall switch discovered that all 3 ceiling bulbs had burned-out and, outdoor temperature +1 Farenheit, I quickly replaced them from my storehouse of incandescents.
Is it against the law? I assume everything is.
Wow! I had forgotten about this thread and it was fun to visit it again.
It is true that incandescent bulbs produce a high proportion of IR as heat which is not seen but is radiated. This will "heat" an area.
The question becomes one of economy. Electric heating per thermal unit is much more expensive than heating from other common sources such as wood, gas, coal, etc. so using bulbs to heat comes at a price.
But experience tells me that radiant heat is very pleasant to the senses and may allow comfort with lower air temperatures so there may be a savings in heating the air. Studies done by the Air Force show that humans prefer a warm back and cool front.
Neil