|
|
Post by keith on Feb 24, 2012 12:58:50 GMT -5
Thanks for the links, I think I understand a bit better now. Yes, the thread has probably gotten a bit over the top.
|
|
|
|
Post by Dave on Feb 25, 2012 9:08:52 GMT -5
My fault, I just love this stuff, although I don't understand it or follow it as I once did. Part of the draw of radio for me was always trying to figure out what the hell was going on when I hung a wire from a tree and let it do what it was supposed to .... throw RF up at the ionosphere.
|
|
|
|
Post by Dave on Feb 25, 2012 9:20:54 GMT -5
Hah! Here is an exchange I copied from a Yahoo radio group a few years back. I barely remember Chris, but he was quite knowledgeable.
Mag vs elec wave components
signal components
>
> Are all radio frequency signals always composed of equal magnetic and
> electrical fields? Are there ever any types of signals that have
> unequal fields?
>
> I'm experimenting with a small loop antenna which should be more
> sensitive to the magnetic field and less sensitive to noise propagated
> in the electrical field. An answer to the above question might in
> some way help me to distinguish between man-made and natural noise.
>
Some sources have a stonger electrical component than magnetic. One in
particular is cloud-to-ground lightning discharges. They travel between the
ionosphere and the earth's surface as a ducted wave and have a variety of TE
môdes. Cloud-to-cloud lightning has a stronger magnetic field and is more
likely to excite TM môdes.
Electrical noise that originates close to the earth's surface also has a
stronger electrical field component, and these are commonly refered to as
Zenneck Waves. They travel along the earth's surface much further than they
would in free space by way of the boundary condition of the earth's surface
and the atmosphere.
>
> Also, exactly why is a small loop less sensitive to the electrical
> field of a signal? (A large loop antenna has the opposite
> characteristics.)
>
The magnetic field of the signal induces equal and oppostite currents on
the outer surface of the shield halves, which then couple to the inner
surface at the shield gap. The inner surface currents then couple to the
inner conductor by way of TEM.
The shield gives protection from low frequency electric fields. The
electric field induces a voltage on the two shield halves, and as the
frequency of the signal decreases the two shield voltages become equal in
magnitude and phase. In turn, the currents on the shield halves become
equal and opposite and the currents coupled to the inner conductor cancel.
Chris
|
|
|
|
Post by Dave on Feb 25, 2012 13:37:07 GMT -5
|
|
|
|
Post by keith on Feb 27, 2012 17:32:35 GMT -5
The assumption that the most interesting thing I could find to do on a Saturday night was review old physics texts is sad. The fact that the assumption was valid is even more sad. That I spent another hour on Sunday night doing more reading on EM radiation may warrant an intervention.
After that I still don't believe there can be an E wave or an M wave in free space. Any solution to Maxwell's equations require EM radiation.
Lightning is an electric field caused by a charge separation exceeding the resistivity of the matter between the charges. The fields around the current flowing through a channel of ionized air are like those of a big current flowing through a big wire. I don't see how it matters if the charge separation is cloud-ground, intra cloud or inter cloud other than differences in scale or distance.
Energy is transported by the current and radiated by changes in the current. Since lightning bolts are messy, it's a chaotic mixture of wavelengths and amplitudes but it has to be EM radiation.
|
|
|
|
Post by Dave on Feb 27, 2012 19:23:14 GMT -5
I'm not sure about lightning, but if I understand your quandary, I think the references we've been reading do not imply that in a given wave the E can be more or less than the M components. I think the references speak to the sensitivity to those components in a given antenna.
I cannot visualize the E versus the M components. I know I've seen pictorial representations, but they do not help me.
|
|
|
|
Post by Dave on Feb 27, 2012 22:08:13 GMT -5
 This is my second most favorite Harvey Well Transmitter, the T-90, manufactured in the late 1950's. Meant to be used at home or your car. Cars were much bigger then, but a transmitter, receiver and antenna tuner, even if a compact design, took up a lot of space on the transmission hump. It was difficult to get transmitters smaller, except in aviation instances where "odd" power supplies were used. Even in your radio shack in the attic, basement or bedroom, at the time you needed a transmitter capable of 50 to 100 watts output, which was necessary because receivers were not as sensitive as today's and you wanted to be heard by the ham on the other end. For that kind of power, you needed vacuum tubes that had to be run at 600 to 800 volts DC. So you needed a pretty big and heavy power transformer in the power supply to step up 110 from your wall outlet at at a hefty level of current. You also needed a hefty transformer for modulation if you used voice communications. And then you needed something like a transformer called a choke to to keep the radio signal from getting back into the power supply. These were all Big hummers and had to be mounted on a substantial platform or chassis, which was more weight. These components running that kind of current got quite hot and so you needed to space them farther apart than today's electronics. More chassis, more weight, more space. The T-90, above, had a look-alike twin, a receiver and they were stacked one above the other. I had a set of twins, plus the separate power supply which was about the same size, and I ran them often at a reduced power, about 35 watts. |
|
|
|
Post by keith on Feb 27, 2012 22:09:09 GMT -5
Fun with Atmospheric Electricity, stuff I still retain from my grad school days.
We live in a fairly large electrical field, nothing to do with modern generation it's the potential difference between the earth and the sky. Near the earth's surface it is on the order of 1000 v/m. It's sort of static. By that I mean that it is normally in the same direction but varies according to time of day, season, local weather and 11 year sunspot cycle.
At Minnesota Duluth we built devices to measure this field called field mills. The working element was a metal disk with notches machined into the outer edge. This would chop the field and produce a signal which could be processed. Normally changes would be slow and predictable. All that changed if a thunderstorm came over the horizon. The field would begin to swing rapidly plus and minus off the chart (in this case an Esterline Angus paper chart recorder).
For the next lesson we'll cover Air-Earth current.
|
|
|
|
Post by Dave on Feb 27, 2012 22:23:57 GMT -5
 One of my favorite shortwave receivers. General coverage, I used it for ham work as well as general listing to shortwave broadcast stations on both sides of the Atlantic and a few powerful stations from the Orient. It had terrific audio and the tuning mechanism was as smooth as silk. The National NC 173 was popular in the early 1950's. There were a few problems with these old radios, even those in good shape that I tuned up and peaked. For one thing they drifted off frequency. Even the high end models drifted for the first half hour after they were turned on. They were not as sensitive as modern radios. That was not a problem with broadcast listening, but when trying to work another Morse Code station on a night when the conditions were rough it could be tough going. They were also not as selective, meaning that you couldn't narrow down the listening frequency to eliminate nearby signals, a crucial aspect of ham work. And when computers and computerized devices began to take over our homes, along with dimmers, lcd screens, etc. the old radios were not as able as their descendants to reject all the digital noise that's in the air today. They did have absolutely lovely audio, however. So although it was fun to tune around with these old radios and work other hams when conditions were good, I often used filtering units and other outboard devices to improve reception. I sometimes ran them through a computer and applied more sophisticated audio filtering and notching programs to their output. And when conditions got bad enough, I turned them off and fired up my modern radios. |
|
|
|
Post by Dave on Feb 27, 2012 22:26:29 GMT -5
Fun with Atmospheric Electricity, stuff I still retain from my grad school days. We live in a fairly large electrical field, nothing to do with modern generation it's the potential difference between the earth and the sky. Near the earth's surface it is on the order of 1000 v/m. It's sort of static. By that I mean that it is normally in the same direction but varies according to time of day, season, local weather and 11 year sunspot cycle. At Minnesota Duluth we built devices to measure this field called field mills. The working element was a metal disk with notches machined into the outer edge. This would chop the field and produce a signal which could be processed. Normally changes would be slow and predictable. All that changed if a thunderstorm came over the horizon. The field would begin to swing rapidly plus and minus off the chart (in this case an Esterline Angus paper chart recorder). For the next lesson we'll cover Air-Earth current. Doesn't it produce some sort of modulation that some folks listen to in the very low frequency band? Like below 20 KHz? I can get down to 9 KHz with my current equipment, but I've never heard anything down there. |
|
|
|
Post by keith on Feb 27, 2012 23:17:35 GMT -5
I don't remember. I was looking at a paper output filtered through the mechanics of a chart recorder. A 1 Hz signal would have been pretty high. We generally looked at 15 minute intervals averaged by eye. The fundamental frequency we generally looked at was 24 hour.
We did make field mills for NASA which were placed in an array around the Kennedy Space Center. (A sudden thunderstorm & a fueled rocket on the pad are not a good mix.) Of course they could afford electronic networking rather than data on a military surplus chart recorder, analyzed and compiled in a month or so.
|
|
|
|
Post by Dave on May 24, 2012 8:29:31 GMT -5
I meant to post further on magnetic loops. In my new location, the Home Owner's Association does not allow outside antennas, so I considered a number of alternatives for both receiving a wide variety of frequencies and transmitting at low power on 2 or 3 amateur radio shortwave bands. And end-fed wire simply strung out the window and under the eaves was one alternative, made from fine speaker wire or similar material. Another was a specially designed flag pole that also acted as a vertical antenna. (There is no proscription against antennas, per se, rather "none showing.") Flagpole Antenna, one example: www.texasantennas.com/index.php?option=com_content&view=article&id=114&Itemid=124DoItYourself Flag Pole Antenna However, because the low-noise small loop I spoke of earlier was sensing quite a bit of near-field noise at my new location, I knew that either of the arrangements I just mentioned, especially a vertical antenna, would be swamped with noise as well, and do a much worse job of "filtering" it out than the small loop.  (Above is a graphic example of signals out of phase. These are supposedly audio signals from microphones, but the idea of phase also applies to radio frequency signals.) One solution is to use the best performer regarding noise (the magnetic loop) as a separate receiving antenna and transmit separately on an antenna best suited for transmitting. A "phaser," which I believe I may have mentioned earlier, can be inserted in the line from the receiving antenna to phase out noise. A phaser connects to two antennas (one may be just a simple length of wire inside the house along the floor next to a wall) and combines the incoming signals against each other such that the noise is cancelled out and (if you're lucky) the desired signal remains. When they work, they're amazing, but they're rather touchy. The noise and the desired signals must be naturally out of phase (normally not a problem) and their strengths must be fairly equal for the phaser to work. If you're thinking ahead and wondering why two interfering SIGNALS can't be separated by phasing, they can and I've done it on the medium wave band (U.S. AM band) where ground waves are can be combined and phased. However, on short wave, we're most often dealing with skywaves bouncing down to us from the ionosphere and sky waves normally change their phase continuously. This makes the job of separating them by phase almost impossible. But where one of the signals is noise and is relatively fixed regarding phase, it can be tuned out while listening to any part of the spectrum, from low bands to the shortwave international bands. More later on the antenna arrangement I eventually chose. |
|
|
|
Post by Dave on May 24, 2012 9:25:41 GMT -5
The small loop (about one meter diameter) receiving antenna I spoke of above does a great job of reducing noise on the signal (from the point of view of the listener) but can still be overwhelmed by a great deal of noise. Another aspect of my small loop is that it cannot be used for transmitting, even at low power. Small loop arrangements can be designed so that they are "broadband," that is to say that tuning them to match the listening frequency is not necessary. You can make a small loop transmitting antenna, too, but they almost always need to be tuned and this cannot be done efficiently at the operating position inside the radio shack. Remote tuner arrangements are the norm for small loop transmitting antennas. After perusing the literature and advertisements, I finally settled on a small loop transmitting/receiving antenna from a company named MFJ in Mississippi. The company is a major marketer in ham radio equipment, but their name is somewhat tarnished by often shoddy construction practices. However, their pricing is terrific and they will replace and/or work with the customer to resolution. Most hams open up the lids on MFJ products and after inspection, re-solder a few joints and merrily go on their way.  I found the MFJ loop to be truly amazing. It consists of a one meter loop and a remote tuning controller. Inside the black enclosure fitted across the loop is a very large butterfly capacitor turned by a small motorand controlled from the radio operating position. Built in to the controller is a combination cross-needle power and SWR meter. You push the buttons to carefully adjust the remote capacitor while viewing the meter, adjusting for minimum SWR to indicate the best tuning. (Standing Wave Ratio is too large of a topic for this thread, but suffice to say it is a measure of the efficiency of a typical transmitting network consisting of transmitter, transmission line and antenna. Maximum power is passed to the antenna when the impedances of each of the three elements are matched to each other. Adjusting the capacitor at the antenna matches the antenna to the line and transmitter.) Two big surprises awaited me as I temporarily set the loop antenna up on the top of a computer armoir that today houses my mini radio station (quite a come-down from my cellar room full of radios!) The first surprise was that the new tuned MFJ loop was much quieter than the small receiving loop. The MFJ loop does not have the sophisticated circuit design of the small receiving loop, but evidently the High "Q" of the MFJ loop that allows such a narrow piece of the spectrum through also contributes greatly to noise reduction. The second surprise was that the damned thing really produces long distance contacts. Even with the MFJ loop temporarily sitting only five feet off the floor on the armoir, and using only 50 watts of transmitting power, I've worked other hams on CW (Morse Code, my preferred mode of radio) from Arkansas to Utah from South Carolina to the west, and east across the Atlantic to Spain, UK and Croatia so far. And I've received fairly good signal reports! I'm now unsure I even want to go to the bother of trying to get a transmission line through to the attic where I had planned to mount the antenna. |
|
|
|
Post by keith on May 27, 2012 5:32:05 GMT -5
Always a wonderful surprise when something works better than expected. If you can avoid working in the attic during the summer it sounds like a good thing. I've run TV & computer cables (pre WiFi ) through my attic during the summer and it was not pleasant.
If you can open up MFJ boxes and fix cold solder joints I assume they must be through hole circuit boards, none of that surface mount wave soldered stuff.
|
|
|
|
Post by keith on May 27, 2012 9:16:20 GMT -5
Of course using the antenna hidden in the flagpole has a 007 feel about it, Dave with his headphones keying coded messages to his contact in the mountains of Utah.
|
|
