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:
Cut a piece of 18gauge coppr wire 5 feet long
attach one end to a rafter.
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?
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.
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.
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.
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 here:
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Most antennas have two screws for attaching one side of a balun. Connect your coaxial cable to the other side.
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.
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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.
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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.
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
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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.
This article gives a great deal of information on HDTV as well as antennas theory and mounting.
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All antennas have definite calculation formulas for both the element construction and spacing. It varies widely for the type and frequency range in use. Television antennas generally use a range of element lengths and are calculated to a specific element spacing simply stringing random lengths of wire and soldering them to coax is both wasteful and grossly inefficient.
Here is a simple 4 element design that should be easy to build
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A normal outdoor antenna can easily be mounted inside the attic. older UHF antennas will work just as well as those that claim to be HDTV specific since the operating frequencies are not changed just the transmission mode.
As for aiming most antennas are directional with the strongest signal being received from directly in front of the antenna with an angle of 15-30 degrees capture. A few are designed to receive signals from the side but this mode is usually unreliable.
So you should be sure that you can aim the antenna directly toward the stations you want for best results and tacking wires on the rafters is unlikely to give you proper aim enough for good reception.
A copy of the amateur radio antenna gude will give more specific information if you wish to get a copy.
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.
The reason usually is physical strength and rigidity. The larger diameter also increases bandwidth, but often this isn't necessary to proper operation.
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.
Another important reason for using hollow tubes is structural weight.
18 AWG wire won't result in appreciable loss for nearly any antenna.
Square stock is slightly lossier than round, but the loss will be negligible when unsing any practical size.
Polishing won't make any detectable difference.
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.
If only that were true! But unfortunately it isn't.
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.
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.
If we're getting into traditional antenna design details... I've occasionally wondered why manufacturers still build VHF TV receiving antennas to match 300 ohm twin-lead wire. They could easily be made to match 72 ohm or 75 ohm coaxial line. One of these days I think I'll build a single-channel Quagi TV antenna just to try it out. Pick up those distant stations... Those balun transformers have to have SOME loss, don't y'think?
Name and callsign left off 'cause this is a public newsgroup. Multi-posting removed, 'cause I don't write to folks who can't reply.
I suspect that some of it is nothing more than tradition.
The traditional VHF beam for television is a log-periodic dipole array (often with a UHF corner reflector added). There are some notes in the ARRL Antenna Book which suggest that it can be tricky to build an LPDA with a low feedpoint impedance... the conductors of the beam-and-transmission-line must be spaced quite closely together. It's possible to do it, but I suspect that the construction becomes a bit more critical in dimensioning.
The older-style log periodic design (the one which forms a V in the vertical plane as well as the horizontal) may not be possible to build with a low-impedance feedpoint.
It's still necessary (or at least quite desirable) to have a balun of some sort, to decouple the feedline from the antenna itself. This will reduce the tendency of the coax shield to act as part of the antenna... which could cause some amount of ghosting. Some direct-feed LPDAs run the feedline through the inside of one of the feeder conductors from the rear to the front, where it comes out and is attached to both conductors... this serves as a balun. Or, a 1:1 toroidal balun can be used at the feedpoint.
I'm sure you can build a Quagi for direct connection. Ditto for a Yagi... a WA5VJB half-folded "cheap Yagi" design could be made for a
75-ohm direct connection, or you could use a gamma match on a more traditional Yagi.
Neither a Yagi nor a Quagi will have the broad frequency response of an LPDA, though.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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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.
Actually the cost of designing a 75 ohm unbalanced feed antenna would cause an increase in the overall cost of the antenna because of the added matching stub which also limits the frequency range of the antenna, that is why they still build them for 300 ohm balanced feeds.
I just bought a new 8 bay uhf antenna to repace my old rusty one and it has 300 ohm balanced feed with a 75 ohm balin installed for the down feed.
Yes the standard TV balin is lossy on the order of 2-5 db or more depending on design. They are seldom designed to high performance low loss levels used in military and high end commercial units.
Sadly there is no such thing as a TV ranged 300 ohm balanced amp or a TV with a 300 ohm balanced input any more. 300 ohm ladder line with a twist every two feet to stop ghosting was far superior and seldom used because coax was just easier. Ease won out over quality.
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) =3D sqrt( 2/(p*o*w) ) where p =3D magnetic permeability o =3D conductivity w =3D frequency in radians
for copper, p =3D 4 pi 10-7 o =3D 58 MS/m w =3D 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) =3D
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.
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.
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