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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 |
#2
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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 ... |
#3
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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 |
#4
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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 |
#5
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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 |
#6
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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. |
#7
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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 |
#8
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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. |
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