How Can you Make a VHF TV Antenna for an Attic
Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic. I would have thought that I could easily find (simple) instructions on the internet but can't. Does anybody have a simple idea that just uses wire (wire should be easy to attach in an attic). I've seen some instructions (mostly UHF or DTV) and some of them do calculations for wavelength (let's say 5 feet). And then, with no explanation, the guy just says "I made it 10 feet for better reception". So I ask, can I not then just use the entire length of my attic for super-duper reception? Wire is cheap after all, and I only want to crawl up there once. I don;t have a PhD in antenna making, so a lot of the instructions/ terms don't mean much to me (dipole, balun, etc). I'm hoping for instructions such as: 1. Cut a piece of 18gauge coppr wire 5 feet long 2. attach one end to a rafter. 3. solder the other end to the centre wire of the coax 4 insert tab A into slot B etc etc. Also, I see instructions that say you should aim the antenna without defining "aim". Do you allign the wire in the direction of the transmission antenna, or should the wire by perpendicular? Thanks |
How Can you Make a VHF TV Antenna for an Attic
wrote in message ... Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to make a super-duper antenna for inside my attic. I would have thought that I could easily find (simple) instructions on the internet but can't. Does anybody have a simple idea that just uses wire (wire should be easy to attach in an attic). I'd like the answer to be YES, but it's NO.. If you wanted to make an antenna for just one channel, I'd say yes. I already did it for some guy who wanted a channel 2 antenna over a year ago. I've seen some instructions (mostly UHF or DTV) and some of them do calculations for wavelength (let's say 5 feet). It's a multi-step process. You have to look up the channel frequency for a TV channel. Then, you take the number 300 and divide it by the frequency. The result is the wavelength. The elements are then cut for approximately a half-wavelength. More details below, if you want 'em. Multi-channel antennas have multiple elements, all differing lengths. If you have one element, you can expect to receive one channel well and other channels maybe but not as well. A single channel antenna can be made of TV twinlead and attached to a piece of wood. It's called a "folded dipole." More below. And then, with no explanation, the guy just says "I made it 10 feet for better reception". So I ask, can I not then just use the entire length of my attic for super-duper reception? Nope. He's full of it to say that. The only thing that gets longer to make a better antenna is the boom, the center long rod of a long antenna, and it gets longer because additional elements are added to it to improve the performance. However, you have to know how many, how long and where to put them. That's why we study this stuff. Wire is cheap after all, and I only want to crawl up there once. Crawl up there once and bring a TV antenna with you ... a STORE-BOUGHT TV antenna. Hang it flat from the rafters. A balun is a little matching transformer with side-by-side wire connections on one side and a round coaxial cable connection on the other side. Picture he www.summitsource.com/images/products/COTRAN.jpg Most antennas have two screws for attaching one side of a balun. Connect your coaxial cable to the other side. Also, I see instructions that say you should aim the antenna without defining "aim". Do you align the wire in the direction of the transmission antenna, or should the wire by perpendicular? The outline of many TV antennas, viewed from above or below, resembles the outline of an arrowhead. That's it. The smaller elements are on the end that's nearer to the TV station. The signal arrives perpendicular to the alignment of the elements. http://www.radioshack.com/pwr/conten..._thumbnail.jpg is an antenna which illustrates the arrowhead concept. The stations are off to the right side in this picture. I have no idea whether the antenna in the picture is any good. If you make a single element antenna, you align it perpendicular with the arriving signal. These do work pretty well, by the way. http://www.wfu.edu/~matthews/misc/dipole.html has some step-by-step instructions for making a folded dipole with ordinary tools. One last thing: It's not beyond the realm of possibility to make one folded dipole attic antenna for Channel 6 and a second folded dipole attic antenna for Channel 9. The Channel 9 antenna just MIGHT also handle 7 and 13 if you're in a good reception area. You can cable both of them to the TV and switch between them. Sal |
How Can you Make a VHF TV Antenna for an Attic
On Tue, 14 Jul 2009 23:28:21 -0700, "Sal M. Onella"
wrote: [snippety snip some good info from Sal] Also, to the OP, if you're interested in playing with this some, even just to the point of seeing what some of the radiation/reception patterns look like (goes-out signal strength is the same pattern as the goes-in sensitivity, btw) hop over to http://home.ict.nl/~arivoors/ and d/l a copy of Arie's version of the 4nec2 antenna modeling software. There are example files that are similar to typical TV antennas, among others. You can get a list of channel assignments versus frequency on Wikipedia. -- Rich Webb Norfolk, VA |
How Can you Make a VHF TV Antenna for an Attic
On Jul 14, 9:55*pm, wrote:
Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to make a super-duper antenna for inside my attic. *I would have thought that I could easily find (simple) instructions on the internet but can't. *Does anybody have a simple idea that just uses wire (wire should be easy to attach in an attic). I've seen some instructions (mostly UHF or DTV) and some of them do calculations for wavelength (let's say 5 feet). *And then, with no explanation, the guy just says "I made it 10 feet for better reception". *So I ask, can I not then just use the entire length of my attic for super-duper reception? *Wire is cheap after all, and I only want to crawl up there once. I don;t have a PhD in antenna making, so a lot of the instructions/ terms don't mean much to me (dipole, balun, etc). *I'm hoping for instructions such as: 1. Cut a piece of 18gauge coppr wire 5 feet long 2. attach one end to a rafter. 3. solder the other end to the centre wire of the coax 4 insert tab A into slot B etc etc. Also, I see instructions that say you should aim the antenna without defining "aim". *Do you allign the wire in the direction of the transmission antenna, or should the wire by perpendicular? Thanks Just saw this thread and some very good suggestions. One thought. Unless your close to the TV Transmitters, stay away from using wire as the antenna. There is a good reason antennas are made of tubes, not wire. At high frequency, like television transmission frequencies, the current creates a repulsion field that pushes the current away from the center of the conductor. In other words, all the current travels on the outside surface of the wire. Look up the term, "skin depth". At frequencies as low as 20MHz, more than 99% of the current will be within 3 mils of the surface. The only easy way to lower the losses in the antenna is to use large diameter conductors, but since the inside of the conductor carries no current, you don't need metal there, so it is ok to use hollow tubes. Antenna manufacturers save themselves money by lowering material costs and shipping weight. They use hollow tubes. If you don't care about weight or material cost, go ahead and use solid rods, 1/4 inch, or even 3/8, but stay away from 18 Awg, way too small. One other thing, nature abhors sharp edges, that's why bubbles are round, so don't use square tubes or sharp bar stock either. Use rounded tubes or rods. Even smoothing and polishing the surface lowers the resistance. When you're done, passivate the surface of the conductors to prevent corrosion over time. [meaning: paint the antenna] Over time, corrosion will deteriorate your antenna's performance. Rounded surfaces also means make your connections smooth with nice transitions. As in, "if it looks good, it works good." You can use aluminum if the lengths are continuous and/or you make connections using constant mechanical pressure, like a "lots of teeth" star washer that has bitten down through the insulating oxide layer held with a bolt. All in all, it seems a lot easier to buy a fringe field antenna and put that in your attic. But if you do it yourself, hope you're successful, document what you built, and share it here. Robert |
How Can you Make a VHF TV Antenna for an Attic
I'm sorry, this response contains some misleading advice.
Robert Macy wrote: Just saw this thread and some very good suggestions. One thought. Unless your close to the TV Transmitters, stay away from using wire as the antenna. There is a good reason antennas are made of tubes, not wire. The reason usually is physical strength and rigidity. The larger diameter also increases bandwidth, but often this isn't necessary to proper operation. At high frequency, like television transmission frequencies, the current creates a repulsion field that pushes the current away from the center of the conductor. In other words, all the current travels on the outside surface of the wire. Look up the term, "skin depth". At frequencies as low as 20MHz, more than 99% of the current will be within 3 mils of the surface. The only easy way to lower the losses in the antenna is to use large diameter conductors, but since the inside of the conductor carries no current, you don't need metal there, so it is ok to use hollow tubes. This is true. However, in almost all cases the loss caused by using wire, even very small wire is still negligible. Exceptions are antennas which are very short in terms of wavelength, particularly at low frequencies. As frequency increases, the length of an antenna of equal performance decreases in direct proportion. However, the loss decreases only as the square root of frequency. So antennas of the same wavelength size become proportionally less lossy at higher frequencies. Antenna manufacturers save themselves money by lowering material costs and shipping weight. They use hollow tubes. Another important reason for using hollow tubes is structural weight. If you don't care about weight or material cost, go ahead and use solid rods, 1/4 inch, or even 3/8, but stay away from 18 Awg, way too small. 18 AWG wire won't result in appreciable loss for nearly any antenna. One other thing, nature abhors sharp edges, that's why bubbles are round, so don't use square tubes or sharp bar stock either. Square stock is slightly lossier than round, but the loss will be negligible when unsing any practical size. Use rounded tubes or rods. Even smoothing and polishing the surface lowers the resistance. Polishing won't make any detectable difference. When you're done, passivate the surface of the conductors to prevent corrosion over time. [meaning: paint the antenna] Over time, corrosion will deteriorate your antenna's performance. It depends on the type of corrosion. But it would have to be severe before becoming so bad as to cause an appreciable reduction in performance. Aluminum, tin, and some other metals passivate themselves by forming a hard insulating oxide layer on the outside. Unless you're in a maritime climate, copper won't deteriorate in a way that matters, either. Insulated wire is an easy way to prevent corrosion in an unfavorable climate. Rounded surfaces also means make your connections smooth with nice transitions. As in, "if it looks good, it works good." If only that were true! But unfortunately it isn't. You can use aluminum if the lengths are continuous and/or you make connections using constant mechanical pressure, like a "lots of teeth" star washer that has bitten down through the insulating oxide layer held with a bolt. This can cause more problems than it solves, if the bolt and washer are the wrong metal such as steel. A good book on Yagi antenna construction will tell you about techniques for working with aluminum. All in all, it seems a lot easier to buy a fringe field antenna and put that in your attic. But if you do it yourself, hope you're successful, document what you built, and share it here. With that I agree. Roy Lewallen, W7EL |
How Can you Make a VHF TV Antenna for an Attic
Roy Lewallen wrote:
I'm sorry, this response contains some misleading advice. Robert Macy wrote: Just saw this thread and some very good suggestions. One thought. Unless your close to the TV Transmitters, stay away from using wire as the antenna. There is a good reason antennas are made of tubes, not wire. The reason usually is physical strength and rigidity. The larger diameter also increases bandwidth, but often this isn't necessary to proper operation. At high frequency, like television transmission frequencies, the current creates a repulsion field that pushes the current away from the center of the conductor. In other words, all the current travels on the outside surface of the wire. Look up the term, "skin depth". At frequencies as low as 20MHz, more than 99% of the current will be within 3 mils of the surface. The only easy way to lower the losses in the antenna is to use large diameter conductors, but since the inside of the conductor carries no current, you don't need metal there, so it is ok to use hollow tubes. This is true. However, in almost all cases the loss caused by using wire, even very small wire is still negligible. Exceptions are antennas which are very short in terms of wavelength, particularly at low frequencies. As frequency increases, the length of an antenna of equal performance decreases in direct proportion. However, the loss decreases only as the square root of frequency. So antennas of the same wavelength size become proportionally less lossy at higher frequencies. Antenna manufacturers save themselves money by lowering material costs and shipping weight. They use hollow tubes. Another important reason for using hollow tubes is structural weight. If you don't care about weight or material cost, go ahead and use solid rods, 1/4 inch, or even 3/8, but stay away from 18 Awg, way too small. 18 AWG wire won't result in appreciable loss for nearly any antenna. One other thing, nature abhors sharp edges, that's why bubbles are round, so don't use square tubes or sharp bar stock either. Square stock is slightly lossier than round, but the loss will be negligible when unsing any practical size. Use rounded tubes or rods. Even smoothing and polishing the surface lowers the resistance. Polishing won't make any detectable difference. When you're done, passivate the surface of the conductors to prevent corrosion over time. [meaning: paint the antenna] Over time, corrosion will deteriorate your antenna's performance. It depends on the type of corrosion. But it would have to be severe before becoming so bad as to cause an appreciable reduction in performance. Aluminum, tin, and some other metals passivate themselves by forming a hard insulating oxide layer on the outside. Unless you're in a maritime climate, copper won't deteriorate in a way that matters, either. Insulated wire is an easy way to prevent corrosion in an unfavorable climate. Rounded surfaces also means make your connections smooth with nice transitions. As in, "if it looks good, it works good." If only that were true! But unfortunately it isn't. You can use aluminum if the lengths are continuous and/or you make connections using constant mechanical pressure, like a "lots of teeth" star washer that has bitten down through the insulating oxide layer held with a bolt. This can cause more problems than it solves, if the bolt and washer are the wrong metal such as steel. A good book on Yagi antenna construction will tell you about techniques for working with aluminum. All in all, it seems a lot easier to buy a fringe field antenna and put that in your attic. But if you do it yourself, hope you're successful, document what you built, and share it here. With that I agree. Roy Lewallen, W7EL Just don't use stranded wire ... |
How Can you Make a VHF TV Antenna for an Attic
Correction:
Roy Lewallen wrote: . . . This is true. However, in almost all cases the loss caused by using wire, even very small wire is still negligible. Exceptions are antennas which are very short in terms of wavelength, particularly at low frequencies. As frequency increases, the length of an antenna of equal performance decreases in direct proportion. However, the loss decreases only as the square root of frequency. So antennas of the same wavelength size become proportionally less lossy at higher frequencies. . . . Loss increases, not decreases, with frequency, in proportion to the square root of frequency. But the conclusion stated in the last sentence is correct. If you quadruple the frequency, wires become four times shorter for the same type of antenna. Assuming you keep the same wire size, this length change results in one quarter the loss resistance. The decrease in skin depth due to quadrupling frequency causes an increase of loss only by a factor of sqrt(4) = 2. The net result is that quadrupling the frequency cuts the total loss in half. Roy Lewallen, W7EL |
How Can you Make a VHF TV Antenna for an Attic
On Jul 14, 9:55*pm, wrote:
Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to make a super-duper antenna for inside my attic. *I would have thought that I could easily find (simple) instructions on the internet but can't. *Does anybody have a simple idea that just uses wire (wire should be easy to attach in an attic). I've seen some instructions (mostly UHF or DTV) and some of them do calculations for wavelength (let's say 5 feet). *And then, with no explanation, the guy just says "I made it 10 feet for better reception". *So I ask, can I not then just use the entire length of my attic for super-duper reception? *Wire is cheap after all, and I only want to crawl up there once. I don;t have a PhD in antenna making, so a lot of the instructions/ terms don't mean much to me (dipole, balun, etc). *I'm hoping for instructions such as: 1. Cut a piece of 18gauge coppr wire 5 feet long 2. attach one end to a rafter. 3. solder the other end to the centre wire of the coax 4 insert tab A into slot B etc etc. Also, I see instructions that say you should aim the antenna without defining "aim". *Do you allign the wire in the direction of the transmission antenna, or should the wire by perpendicular? Thanks Good grief. Just buy an antenna and open it up in the attic. I've done it several times and it will cost a lot less in time and aggravation. Or is your time not worth anything? G² |
How Can you Make a VHF TV Antenna for an Attic
wrote in message ... On Jul 14, 9:55 pm, wrote: Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to make a super-duper antenna for inside my attic. snip Good grief. Just buy an antenna and open it up in the attic. I've done it several times and it will cost a lot less in time and aggravation. Or is your time not worth anything? Oh, what a shame! Is there no joy in experimenting any more? I have always felt the essence of our hobby has been somebody saying, "Let's try this and see what happens." Sometimes it's an enjoyable QSO and sometimes it's a cloud of acrid smoke. I've had both. I, too, suggested a store-bought antenna but I went on to suggest other things to try. The OP seems to have an adventurous spark. I vote for ENCOURAGE. 73, Sal (KD6VKW) |
How Can you Make a VHF TV Antenna for an Attic
On Jul 16, 10:58*am, Roy Lewallen wrote:
I'm sorry, this response contains some misleading advice. Robert Macy wrote: Just saw this thread and some very good suggestions. One thought. *Unless your close to the TV Transmitters, stay away from using wire as the antenna. *There is a good reason antennas are made of tubes, not wire. The reason usually is physical strength and rigidity. The larger diameter also increases bandwidth, but often this isn't necessary to proper operation. *At high frequency, like television transmission frequencies, the current creates a repulsion field that pushes the current away from the center of the conductor. *In other words, all the current travels on the outside surface of the wire. *Look up the term, "skin depth". *At frequencies as low as 20MHz, more than 99% of the current will be within 3 mils of the surface. *The only easy way to lower the losses in the antenna is to use large diameter conductors, but since the inside of the conductor carries no current, you don't need metal there, so it is ok to use hollow tubes. This is true. However, in almost all cases the loss caused by using wire, even very small wire is still negligible. Exceptions are antennas which are very short in terms of wavelength, particularly at low frequencies. As frequency increases, the length of an antenna of equal performance decreases in direct proportion. However, the loss decreases only as the square root of frequency. So antennas of the same wavelength size become proportionally less lossy at higher frequencies. Antenna manufacturers save themselves money by lowering material costs and shipping weight. *They use hollow tubes. Another important reason for using hollow tubes is structural weight. *If you don't care about weight or material cost, go ahead and use solid rods, 1/4 inch, or even 3/8, but stay away from 18 Awg, way too small. 18 AWG wire won't result in appreciable loss for nearly any antenna. One other thing, nature abhors sharp edges, that's why bubbles are round, so don't use square tubes or sharp bar stock either. Square stock is slightly lossier than round, but the loss will be negligible when unsing any practical size. *Use rounded tubes or rods. *Even smoothing and polishing the surface lowers the resistance. Polishing won't make any detectable difference. When you're done, passivate the surface of the conductors to prevent corrosion over time. [meaning: paint the antenna] *Over time, corrosion will deteriorate your antenna's performance. It depends on the type of corrosion. But it would have to be severe before becoming so bad as to cause an appreciable reduction in performance. Aluminum, tin, and some other metals passivate themselves by forming a hard insulating oxide layer on the outside. Unless you're in a maritime climate, copper won't deteriorate in a way that matters, either. Insulated wire is an easy way to prevent corrosion in an unfavorable climate. Rounded surfaces also means make your connections smooth with nice transitions. *As in, "if it looks good, it works good." If only that were true! But unfortunately it isn't. You can use aluminum if the lengths are continuous and/or you make connections using constant mechanical pressure, like a "lots of teeth" star washer that has bitten down through the insulating oxide layer held with a bolt. This can cause more problems than it solves, if the bolt and washer are the wrong metal such as steel. A good book on Yagi antenna construction will tell you about techniques for working with aluminum. All in all, it seems a lot easier to buy a fringe field antenna and put that in your attic. *But if you do it yourself, hope you're successful, document what you built, and share it here. With that I agree. Roy Lewallen, W7EL My advice was NOT misleading. Yours was a very poor choice of word. Misleading means the suggested effort would result in moving away from an optimum solution. ALL of my advice leads to better solutions and is therefore "not misleading" and I stand behind my suggestions. You are very correct on catching the lack of thoroughness addressing 'mixed metal' contacts. Yes, a lot of electrolytic action happens at the junction of dissimilar materials. One must be extremely careful when making such contacts. Had you criticized my comments by suggesting that many of the efforts involved will not yield noticeable improvement [especially to a novice], I would have accepted that. After working for years in low noise, high performance systems these techniques have become de rigueur for initial construction. The OP probably would not notice improvements except in the most fringe of conditions. Yes, the diameter of the rod will broaden bandwidth, but elements having ratios on the order of 80:1, the effect on bandwidth won't be very noticeable. Length variations and spacing will have more impact. Regarding skin depth of a conductor: Always keep in mind that the skin depth equation is based upon the assumption of PLANAR wave. The equation is extremely simple and easily memorized as the square root of 2 divided by three terms: skin depth(in meters) = sqrt( 2/(p*o*w) ) where p = magnetic permeability o = conductivity w = frequency in radians for copper, p = 4 pi 10-7 o = 58 MS/m w = 2*pi*f, with f in Hz results are in meters, so I suggest using an Excel spreadsheet formula. skin depth of copper at 80MHz is 0.3 mils! 99% of the current is in less than 1 mil of the conductor. Using finite element analyses [femm 4.2] techniques it is easy to calculate the impedance of a conductor as long as the dimensions stay below 1/10 of wavelength. For 80MHz that would be 1.2 feet. At 80MHz, 18 Awg copper wire is approx 220 milliohms per foot and 3/8 inch aluminum tubing is approx 36 milliohms per foot. Neither of these impedances would have much impact to the signal coming from the 377 ohm source impedance of free space. Normally we would have predicted the decrease in impedance by applying the ratio of the increased perimeter reduced by the less conductive aluminum. The ratio of perimeter is 0.375/.04 or approx 9.4, but aluminum is not as conductive as copper so the conductivity ratio is 25/58 for a total change in resistance by approx 0.375/.04*(25/58) = 4.04 improvement. Finite element analyses calculates the improvement to be more than 50% higher than that. [It's caused by the small radius of the wire.] Plotting the current density down into the conductors shows what happened. The 'effective' skin depth in the 18 Awg wire is about 50% less than in the aluminum, all due to the reduced radius of the outside of the conductor. As I said, nature hates sharp edges. Again, as the element 'taps' into free space the 377 ohms of free space predicts a difference of less than 0.005 dB on the signal. So there will indeed be an extremely small effect from using 18 Awg wire or 3/8 tubing on the received signal strength. If the antenna were to be much smaller than wavelengths and capacitance were added to resonate the elements, THEN the impedance dfference would become noticeable and affect how much signal is available to the receiver. Regarding bar stock? picture the current concentrating at each of the four corner edges, with the current not being uniformly distributed around the perimeter. That would almost be equivalent to having 4 parallel small diameter wires mounted on 3/8 inch centers! Really wasted the metal. Regarding the importance of smoothness: My outside antenna became badly pitted from atmosphere, even with aluminum developing an oxide layer, it still corroded. The roughness lowered the gain of the antenna enough to notice it on the reception from fringe stations. Years ago [and at higher frequencies] we plated metal in our resonators with silver *and* polished the silver to get the impedance down. We measured huge differences in impedance [measured as improved Q] as we polished the surface down to mirror finishes. Whoever suggested the ARRL Antenna handbook is right. Great book. Regards, Robert |
How Can you Make a VHF TV Antenna for an Attic
On Fri, 17 Jul 2009 16:16:35 -0700 (PDT), Robert Macy
wrote: We measured huge differences in impedance [measured as improved Q] What were the Qs? "Huge differences" is not a quantified, and thus verifiable claim. 73's Richard Clark, KB7QHC |
How Can you Make a VHF TV Antenna for an Attic
Hi - I need to receive VHF TV (channels 6,7,9,13) and would like to
make a super-duper antenna for inside my attic. I would have thought that I could easily find (simple) instructions on the internet but can't. Does anybody have a simple idea that just uses wire (wire should be easy to attach in an attic). There was an article in Electronics World in December 1967 by Harold Pruett titled "Designs for Log-Periodic FM & TV antennas". He used two lengths of hookup wire, attached to a wooden frame in a zigzag pattern, and gives all the dimensions needed. I built one then and it has worked fine ever since, though now there's nothing to receive in this area so I've switched to a UHF-only antenna in the attic. I can mail you xeroxes of the article. I'm adouglas (at) gis.net. Alan |
How Can you Make a VHF TV Antenna for an Attic
On Jul 17, 4:45*pm, Richard Clark wrote:
On Fri, 17 Jul 2009 16:16:35 -0700 (PDT), Robert Macy wrote: We measured huge differences in impedance [measured as improved Q] What were the Qs? *"Huge differences" is not a quantified, and thus verifiable claim. 73's Richard Clark, KB7QHC Memory serves Q went from 400/600 range up to the 8000 range. |
How Can you Make a VHF TV Antenna for an Attic
On Sat, 18 Jul 2009 08:31:36 -0700 (PDT), Robert Macy
wrote: We measured huge differences in impedance [measured as improved Q] What were the Qs? *"Huge differences" is not a quantified, and thus verifiable claim. Memory serves Q went from 400/600 range up to the 8000 range. For what band? 73's Richard Clark, KB7QHC |
How Can you Make a VHF TV Antenna for an Attic
The questioner asked: "Do you align the wire in the direction of the
transamission antenna or should the wire be perpendicular?" I saw no answer to his question. So reluctantly I respond. Most antennas are broadside to the direction of transmission but not all. One simple and effective TV receiving antenna has its wires on a diagonal to the direction of the transamitter. The antenna has a diamond shape and is called a thombic. Its long axis is aligned in the direction of the station. Each of the four sides of the rhombic is 2 or more wavelengths. As size increases so does the antenna gain. Bandwisth is very wide covering several TV channels. Channel 2 was the lowest frequency TV channel widely used so it had the longest wavelength (50 MHz & 6mtrs. or 68 ft.). Nothing is critical about a rhombic. One was proposed as a "Super TV Antenna" in the March 1967 erition of "Electronics Illustrated" magazine. It is 64x24 ft. Each of its 4 sides is about 34 feet. An 820 ohm, 2 watt carbon resistor is used to terminate the junction of the 2 sides of the rhombic at the end which is pointed at the station to be received. The rhombic is narrower than long. The opposite end of the rhombic is connected to 300 ohm twinlead which extends to the receiver, but higher impedance line would be marginally better as the rhombic is effectively a wide place in an 820 ohm transmission line. I`ve built and used many rhombics. They all were satisfactory in spite of somewhat ragged patterns. They are capableof high gain and extreme bandwidth. They require much space and 4 supports but construction is simple for the minimalist model. Best regards, Richard Harrison, KB5WZI |
How Can you Make a VHF TV Antenna for an Attic
Richard Harrison wrote:
"Channel 2 was the lowest frequency TV channel widely used so it had the longest wavelength (50 MHz & 6 mtrs. or 68 ft." The multiplier is 3.28 to convert meters to feet, so I should have written 19.68 ft.. I also intended to warn that an attic is usually unfriendly to antennas due to the proximity of metal pipes and wires which interfere. Best regards, Richard Harrison, KB5WZI |
How Can you Make a VHF TV Antenna for an Attic
|
Quote:
Look for this product; Phillips MANT-950. It is a amplified DTV antenna which is very small in size and big in performance. I just installed one! There are also product reviews from consumers who have installed this in the attic and they have included pictures! This antenna takes little time to put up and includes a 18db. amplifier which works well. My only additional suggestion,is that you use the RG6 "quad shield" cable sold separately at walmart,because the short 20' length that comes with it is RG59 and not shielded well at all. The photos in the product reviews show a typical attic install too. Good luck! N9ZAS. |
How Can you Make a VHF TV Antenna for an Attic
wrote in message ... In article , (Richard Harrison) wrote: I`ve built and used many rhombics. They all were satisfactory in spite of somewhat ragged patterns. They are capableof high gain and extreme bandwidth. They require much space and 4 supports but construction is simple for the minimalist model. How critical was the aiming of the antenna? I'm asking because if I build one, many wavelengths long, it probably won't be easy to move the poles. The bigger it is, the more critical. Longest rhombics have the highest gain and, by necessity, the narrowest beamwidths. Beamwidth is usually specified as the angle between the two 3 dB points on the left and right sides of the beam. Most designs either specify the beamwidth or show you a polar plot from which you can arrive at the beamwidth by inspection. It is not inconceivable to make a rhombic structure that pivoted about a single point at the feed and was supported at the terminated end. An insulating structure would be needed but if you were to confine it to UHF, a five-wavelength rhombic would be about 15 feet long. Awkward but not imposssible; we already do it (or close to it) with some of the larger quad antennas |
How Can you Make a VHF TV Antenna for an Attic
Rikoski wrote:
"How critical was the aiming of the antenna?" Depends on the number of wavelengths in a side at the frequency you are trying to receive. Arnold B. Bailey in "TV and Other Receiving Antennas" gives data for a typical rhombic on page 528. He shows a beamwidth of about 30 fegrees with deep nulls on either side of the center lobe. Gain is 14.5 db over a dipole or less. The rhombic is comparable to the parabolic antenna Bailey shows on page 525. Best regards, Richard Harrison, KB5WZI |
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