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Question about Ferrite Antennas
I enjoy AM dxing, and I've noticed that the radios with larger ferrite rods
seem to be better for dxing than those with small ones. I did some research on the subject, and I kept running into terms such q-factor and flux. My 45-year-old mind only has room for so much, and I'm afraid that anything I try to learn about electronic theory will just push out some of the more important stuff already stored in there such as anniversary dates and current wife's name. So maybe somebody can answer a couple of questions for me in simple English, and maybe help me save what few brain cells I still have left. 1) Is there an optimal length for an internal ferrite antenna? 2) If so, then why don't all radios have one that size? 3) Is ferrite expensive or something? 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? Thank you in advance for all who reply. |
On Thu, 10 Jun 2004 19:53:30 -0500, lsmyer wrote
(in article ): 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? How much difference does the "DX/Local" switch make? In other words, what effect does the switch have on a strong local station and what effect does it have on a weak, distant station. The "Local" position will make it just about insensitive so that the "Local" stations won't overload it. In the "DX" position, "Local" stations will overload the radio. It's a good feature but ya have to remember to switch. Gray Shockley ---------------- DX-392 DX-398 RX-320 DX-399 CCradio+ w/RS Loop Justice AM Antenna Torus Tuner (3-13 MHz) Select-A-Tenna --------------------- Vicksburg, MS US |
"lsmyer" kirjoitti viestissä
... I enjoy AM dxing, and I've noticed that the radios with larger ferrite rods seem to be better for dxing than those with small ones. I did some research on the subject, and I kept running into terms such q-factor and flux. My 45-year-old mind only has room for so much, and I'm afraid that anything I try to learn about electronic theory will just push out some of the more important stuff already stored in there such as anniversary dates and current wife's name. So maybe somebody can answer a couple of questions for me in simple English, and maybe help me save what few brain cells I still have left. 1) Is there an optimal length for an internal ferrite antenna? 2) If so, then why don't all radios have one that size? 3) Is ferrite expensive or something? 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? Thank you in advance for all who reply. All the receivers have a limited space for the ferrite antenna. I am no expert in antennas, but what little I know is that 1) The total physical length of an antenna (ref: wavelength) is important: usually the longer the better 2) The tuning of the antenna is important, too. Is the antenna "fitted" to the "front end" of the receiver and how well... e.t.c. ? If a receiver is "tuned" to work fine on shortwave it may not work fine on MW 3) Ferrite itself is not expensive, though the retail price for a piece of antenna can be high 4) My little Kenwood TH-F7 is more sensitive with its tiny ferrite rod and my old Grundig YB207 is not with its longer piece of ferrite. Kenwood, though, is too sensitive for all kind of noise and "parasites", too. 73's, Matti Ponkamo, Naantali, Finland |
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Ismyer, I'll be glad to help you with ferrite antennas. 1) No, there is no
optimal ferrite rod length. For your Sangean 505 try a ferrite plus coil antenna such as the Terk or Select-a-Tenna. These antennas couple to your receiver inductively (magneticially-no wire connections). 2) Ferrite rods are expensive so the bigger rods are on the better receivers. But, you can easily add one. But, you really don't need to if you use one of the above MW antennas. See Univesal Radios web site & look up the Palomar LC-1 & RF Systems AA-2 add on ferrite rods. 3) Yes, Ferrite rods are expensive & hard to find. For MW you can easily make one. See the web site of "National Radio Club", a MW DX Club with rod designs. 4) If you use a Select-A-Tenna the 505 will out perfrom the smaller one. It is all in knowing how to couple an exterior loop to the receiver. You need an exterior loop that couples (transfers signals to) your 505. 5) Flux is the type of ferrite used. It is sold by an assigned number. #61 is good for MW. 6) "Q" stands for the "Quality" of the circuit, not always agood thing. For example a "Hi-Q" provides a narrow band width. You want a wider band width. Need more help let me know. You really need a Select-A-Tenna with the mini jack on it's base, or the locally available Terk. 73 John T. Wagner, Ohio |
Thanks to everyone for their help about ferrite antennas. Now I know enough
to start experimenting, and that's what a hobby is all about! |
Now I know enough
to start experimenting, and that's what a hobby is all about! Damn straight. I've been into antenna tinkering for years, and I have as much fun failing as I do succeeding (well, within reason, haha!) with any given project. I learn from both, and get a lot of help from this group too. Have a blast- Linus |
On Thu, 10 Jun 2004 20:53:30 -0400, "lsmyer"
wrote: I enjoy AM dxing, and I've noticed that the radios with larger ferrite rods seem to be better for dxing than those with small ones. I did some research on the subject, and I kept running into terms such q-factor and flux. My 45-year-old mind only has room for so much, and I'm afraid that anything I try to learn about electronic theory will just push out some of the more important stuff already stored in there such as anniversary dates and current wife's name. So maybe somebody can answer a couple of questions for me in simple English, and maybe help me save what few brain cells I still have left. It's been years so the erudite here will please set me straight. The coil-antenna combines signal gathering with tuning and directivity. The ferrite core just increases the antenna-coil inductance. This results in a smaller coil for tuning frequency coverage but less antenna (the physical coil proper) for actual signal reception. It adds to core losses but there is less wire loss. It makes a much smaller transformer. So, there are several tradeoffs. The most efficient AM band ferrite coils for external antenna use were only about an inch and a half or so as I recall. They are variable inductance too for tweaking. Some old radios may have them. 1) Is there an optimal length for an internal ferrite antenna? 2) If so, then why don't all radios have one that size? 3) Is ferrite expensive or something? 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? Thank you in advance for all who reply. |
In article ,
H. Dziardziel wrote: On Thu, 10 Jun 2004 20:53:30 -0400, "lsmyer" wrote: I enjoy AM dxing, and I've noticed that the radios with larger ferrite rods seem to be better for dxing than those with small ones. I did some research on the subject, and I kept running into terms such q-factor and flux. My 45-year-old mind only has room for so much, and I'm afraid that anything I try to learn about electronic theory will just push out some of the more important stuff already stored in there such as anniversary dates and current wife's name. So maybe somebody can answer a couple of questions for me in simple English, and maybe help me save what few brain cells I still have left. 1) Is there an optimal length for an internal ferrite antenna? 2) If so, then why don't all radios have one that size? 3) Is ferrite expensive or something? 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? Thank you in advance for all who reply. It's been years so the erudite here will please set me straight. The coil-antenna combines signal gathering with tuning and directivity. The ferrite core just increases the antenna-coil inductance. This results in a smaller coil for tuning frequency coverage but less antenna (the physical coil proper) for actual signal reception. It adds to core losses but there is less wire loss. It makes a much smaller transformer. So, there are several tradeoffs. The most efficient AM band ferrite coils for external antenna use were only about an inch and a half or so as I recall. They are variable inductance too for tweaking. Some old radios may have them. This sounds right to me. I don't see how a larger ferrite rod would necessarily work better other than change the number of turns for a given frequency range. A larger rod would have more inductance per turn. I would add the type of ferrite is important, which will determine the inductance per turn and core loss. 1. No optimal length. 2. The antenna rod works with other components chosen in the design of the radio and so must be different. 3. Ferrite is not expensive but it is brittle and the longer the rod the easier it is to break. 4. The design of the front end of the radio of which the rod antenna is just one part of determines the difference in performance. -- Telamon Ventura, California |
In article
, Telamon wrote: In article , H. Dziardziel wrote: On Thu, 10 Jun 2004 20:53:30 -0400, "lsmyer" wrote: I enjoy AM dxing, and I've noticed that the radios with larger ferrite rods seem to be better for dxing than those with small ones. I did some research on the subject, and I kept running into terms such q-factor and flux. My 45-year-old mind only has room for so much, and I'm afraid that anything I try to learn about electronic theory will just push out some of the more important stuff already stored in there such as anniversary dates and current wife's name. So maybe somebody can answer a couple of questions for me in simple English, and maybe help me save what few brain cells I still have left. 1) Is there an optimal length for an internal ferrite antenna? 2) If so, then why don't all radios have one that size? 3) Is ferrite expensive or something? 4) How can my little Degen 1102 (tiny ferrite antenna) do so well on MW while my ATS505 does so poorly? Thank you in advance for all who reply. It's been years so the erudite here will please set me straight. The coil-antenna combines signal gathering with tuning and directivity. The ferrite core just increases the antenna-coil inductance. This results in a smaller coil for tuning frequency coverage but less antenna (the physical coil proper) for actual signal reception. It adds to core losses but there is less wire loss. It makes a much smaller transformer. So, there are several tradeoffs. The most efficient AM band ferrite coils for external antenna use were only about an inch and a half or so as I recall. They are variable inductance too for tweaking. Some old radios may have them. This sounds right to me. I don't see how a larger ferrite rod would necessarily work better other than change the number of turns for a given frequency range. A larger rod would have more inductance per turn. I would add the type of ferrite is important, which will determine the inductance per turn and core loss. 1. No optimal length. 2. The antenna rod works with other components chosen in the design of the radio and so must be different. 3. Ferrite is not expensive but it is brittle and the longer the rod the easier it is to break. 4. The design of the front end of the radio of which the rod antenna is just one part of determines the difference in performance. Hey check this out they have an electronic kit for just about anything. http://www.kitsusa.net If you want to experiment with different rod antennas you could build this kit and try different rod antennas. Connect an inexpensive DVM to measure the AGC voltage from this little breadboard radio and you will be able to tell if the experimental rod is better or worse. http://www.kitsusa.net/phpstore/inde...8&prevaction=c ategory&previd=2&prevstart=12 Then you will know more than the blow hards that recently cross posted in here that endlessly talk out their you know what. -- Telamon Ventura, California |
"H. Dziardziel" wrote in message
... It's been years so the erudite here will please set me straight. The coil-antenna combines signal gathering with tuning and directivity. The ferrite core just increases the antenna-coil inductance. This results in a smaller coil for tuning frequency coverage but less antenna (the physical coil proper) for actual signal reception. It adds to core losses but there is less wire loss. It makes a much smaller transformer. So, there are several tradeoffs. The most efficient AM band ferrite coils for external antenna use were only about an inch and a half or so as I recall. They are variable inductance too for tweaking. Some old radios may have them. You're right about the ferrite core increasing the inductance of the antenna coil, however, it does something else as well - it provides a 'low resistance' (in a magnetic sense) path for the signal's magnetic field to travel through. Suppose you're facing the transmitter, and suppose the magnetic field lines from the station's signal are horizontally oriented (which I think is likely to be the case for a signal traveling along the ground). This means that a particular field line would approach you from the right and leave you going to the left (or vice versa since these are alternating fields - it just depends on the instant when you look). Given all of this, the MW antenna should be oriented such that the maximum number of field lines pass through the inside of the coil (which will maximize the current that gets induced in the coil windings). If the antenna uses a ferrite bar, this orientation means that the length of the bar is lined up with the magnetic field lines. Now for the interesting part ( :-) ). If there were no ferrite bar, the magnetic field lines would travel in fairly straight paths (in your local vicinity), and the coil would intercept a cross-section of the field determined by the cross sectional area of the coil. However, when a ferrite bar is used, many of the magnetic field lines in the vicinity of the antenna take a detour so as to pass through the ferrite bar, because travelling through the ferrite bar is easier than travelling through the air. This has the effect of increasing the number of magnetic field lines that pass through the inside of the coil, thereby increasing the antenna's signal pickup. If the bar is short, only a small number of field lines will decide it's worth their while to take the detour. If the bar is longer, a larger number of field lines will end up travelling a shorter distance *through air* as a result of taking the detour and travelling through the ferrite bar for its length. So a longer bar does work better because it creates a bigger distortion in the local magnetic field, and, in effect, catches a larger number of magnetic field lines and sends them through the center of the coil. So there you go - I may have left a few physicists cringing over some of the terminology, but you get the general idea... -- Stephen -- Please remove no and spam from my email address if replying by email. |
On Thu, 17 Jun 2004 11:38:23 -0400, "Stephen"
wrote: rotated out portion You're right about the ferrite core increasing the inductance of the antenna coil, however, it does something else as well - it provides a 'low resistance' (in a magnetic sense) path for the signal's magnetic field to travel through. Suppose you're facing the transmitter, and suppose the magnetic field lines from the station's signal are horizontally oriented (which I think is likely to be the case for a signal traveling along the ground). This means that a particular field line would approach you from the right and leave you going to the left (or vice versa since these are alternating fields - it just depends on the instant when you look). Given all of this, the MW antenna should be oriented such that the maximum number of field lines pass through the inside of the coil (which will maximize the current that gets induced in the coil windings). If the antenna uses a ferrite bar, this orientation means that the length of the bar is lined up with the magnetic field lines. Now for the interesting part ( :-) ). If there were no ferrite bar, the magnetic field lines would travel in fairly straight paths (in your local vicinity), and the coil would intercept a cross-section of the field determined by the cross sectional area of the coil. However, when a ferrite bar is used, many of the magnetic field lines in the vicinity of the antenna take a detour so as to pass through the ferrite bar, because travelling through the ferrite bar is easier than travelling through the air. This has the effect of increasing the number of magnetic field lines that pass through the inside of the coil, thereby increasing the antenna's signal pickup. If the bar is short, only a small number of field lines will decide it's worth their while to take the detour. If the bar is longer, a larger number of field lines will end up travelling a shorter distance *through air* as a result of taking the detour and travelling through the ferrite bar for its length. So a longer bar does work better because it creates a bigger distortion in the local magnetic field, and, in effect, catches a larger number of magnetic field lines and sends them through the center of the coil. So there you go - I may have left a few physicists cringing over some of the terminology, but you get the general idea... -- Stephen Your nearly poetic license disclamer is well appreciated although I am mystified at how the flux "knows" it should detour? Most intriguing and Nobel prize stuff perhap?. This site has a dandy handbook style lightly theoretical description and fine images.: ..http://www.st-andrews.ac.uk/~www_pa/...rt7/page5.html Keeping in mind the facts that: transmission and reception is in theory reciprocal, and essentially so in reality for low power applications: the meaning of radiation resistance, effective area, and gain, the ferrite results in higher output for the identical input i.e. the impinging EM energy on the coil. It's more efficient: a better antenna impedance match thus minimizing effects of the coil etc losses. A larger core and coil, just like a larger loop only etc thus gives more output. A longer bar _only_ has no effect as it is insignificant compared to the total flux path which is all air. It will change the inductance however. Note too that the coil (the ferrite only enhances this) changes the loop reception from electric to magnetic field and thus the orientation for maximum by 90 degrees. That gain could be up to a million explains why my nifty 20 year old great sounding credit card thick Sanyo AM/FM (stereo too) has sensitivity nearly equal to my 2010. Regards |
In article ,
H. Dziardziel wrote: On Thu, 17 Jun 2004 11:38:23 -0400, "Stephen" wrote: rotated out portion You're right about the ferrite core increasing the inductance of the antenna coil, however, it does something else as well - it provides a 'low resistance' (in a magnetic sense) path for the signal's magnetic field to travel through. Suppose you're facing the transmitter, and suppose the magnetic field lines from the station's signal are horizontally oriented (which I think is likely to be the case for a signal traveling along the ground). This means that a particular field line would approach you from the right and leave you going to the left (or vice versa since these are alternating fields - it just depends on the instant when you look). Given all of this, the MW antenna should be oriented such that the maximum number of field lines pass through the inside of the coil (which will maximize the current that gets induced in the coil windings). If the antenna uses a ferrite bar, this orientation means that the length of the bar is lined up with the magnetic field lines. Now for the interesting part ( :-) ). If there were no ferrite bar, the magnetic field lines would travel in fairly straight paths (in your local vicinity), and the coil would intercept a cross-section of the field determined by the cross sectional area of the coil. However, when a ferrite bar is used, many of the magnetic field lines in the vicinity of the antenna take a detour so as to pass through the ferrite bar, because travelling through the ferrite bar is easier than travelling through the air. This has the effect of increasing the number of magnetic field lines that pass through the inside of the coil, thereby increasing the antenna's signal pickup. If the bar is short, only a small number of field lines will decide it's worth their while to take the detour. If the bar is longer, a larger number of field lines will end up travelling a shorter distance *through air* as a result of taking the detour and travelling through the ferrite bar for its length. So a longer bar does work better because it creates a bigger distortion in the local magnetic field, and, in effect, catches a larger number of magnetic field lines and sends them through the center of the coil. So there you go - I may have left a few physicists cringing over some of the terminology, but you get the general idea... -- Stephen Your nearly poetic license disclamer is well appreciated although I am mystified at how the flux "knows" it should detour? Most intriguing and Nobel prize stuff perhap?. The permeability of ferrite is much greater than air. Ferrite become a path of least resistance to magnetic fields. The Nobel prize was given out long ago on this subject. This is just a basic concept. Nothing new here. -- Telamon Ventura, California |
On Sat, 19 Jun 2004 07:47:46 GMT, Telamon
wrote: The permeability of ferrite is much greater than air. Ferrite become a path of least resistance to magnetic fields. Yes.. The Nobel prize was given out long ago on this subject. This is just a basic concept. Nothing new here. Only gravity bends EM waves. GUT .. The higher permeabillity does not "suck" in EM waves. Nor do EM waves have a uncanny ability to seek out the path of least resistance. The higher permeabillity means a greater flux density is possible within that medium. So, for example, once absorbed they stay in that medium, the path of least resistance and more of the energy can be transformed. First however, the EM waves must arrive at the core-coil.. Just like a black body absorber. And the reciprocal case when the ferrite loop is used for transmission: The denser field is created by the coil -core medium and radiates out into space. What the permeability does is improve EM and electrical power _transformation_ efficiency within it. |
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