Author Topic: Homebrew TV antenna for ATSC or DTV  (Read 69 times)

Offline Hoofer

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Homebrew TV antenna for ATSC or DTV
« on: April 08, 2017, 09:17:37 AM »
Started to post this as part of another message, decided to do it here instead.

You've cut the cable, the only dish you want is wifey or a salad bowl - but... you'd *still* want to watch an occasional TV... you bought the darn thing, you're gonna use it for more than just DVDs and home movies.  Reception SUCKS!!!

First the BASICS, so we understand how Antennas works.

Regarding Antennas, for long range reception, you have a couple of options, all centered around S/N (Signal-to-Noise-Ratio).  Amplifiers are good at boosting signal AND noise, but antennas are especially good at picking out just the signal.  A directional antenna, Dish, Corner Folded, Yagi types, are especially good at "not hearing" crap from the wrong direction, thus, increasing your S/N ratio.  If ALL your transmissions come from one direction - you're half way there...

A Log Periodic antenna, one of those TV monsters with a long boom, and lots of tapered elements, picks up a broad range of frequencies/channels which don't carry DTV signals...  Don't bother with one, don't try to convert one, because it's picking up signals from EVERYTHING you can't use, effectively washing out the signal you really want.  For the doubters, if the Phase Angle between the elements isn't correct, because of the Velocity of the wave(s) across the connecting materials/harness/cabless, don't match or fall correctly on top of eachother, you'll get "Cancellation" - your signal from one element is cancelled by another element.   That's how those "noise cancelling" headsets work.  Two alternating signals, 180 degrees out-of-phase cancels eachother....  Yeah, there is some engineering involved, Array Phasing... actually fun stuff when it works, but also frequency specific.

Back to the fun stuff...

Think of a "tuning fork" as a single frequency "resonator".  The length of the fork determines the frequency (pitch).  If you had 20 tuning forks, all cut to resonate at the same frequency, and COMBINED, you'd have a much louder S/N ratio, they'd be louder than anything else in the vicinity.   Or think of a single solar panel, vs 20 solar panels - all collecting from the same source, the sun.  We want to do the SAME THING, there's a single transmission source (like the Sun), and we want to tie together a bunch of signal collectors (like solar panels).

Build an antenna with 20 or 30 similar length elements, all tied together to combine the signals of each to a single output, then amplify that already big signal, and you've got a great signal - instead of a weak signal, competing with noise/static.

This stuff isn't rocket science, anyone can do it, and it's been apart of broadcasting & listening forever.

If you have a favorite station, and know the broadcast frequency, use this formula,  234/frequency=Length of each element in feet.
In our area, most ATSC is around 450MHz.   234/450=0.52   So each element should be around 6-7".   Combining the signal from 32 x 6" elements is going to be quite effective, and it's so thin, it can fit in a pillow case, or behind a picture, or hung easily in an attic.  This guy is on the right track...  add enough elements, and your S/N ratio gets better and better.


This guy does a similar antenna & explains WHY he built it this way.


Another method of doing the same thing, is using very thin, parallel, insulated wires.  Remember those little loop antennas on the back of AM radios?  It looked like light tan cloth, wound up in a oval donut.  You'd turn the radio this way or that for the best reception.  Same idea as above, but the loop of wires is actually 200, 300, 400 individual insulated wires, each capturing the broadcast signal, and combining it to create a better S/N ratio for your table top or console AM radio.  Same principle, it isn't rocket science - increase just the good signal, and block out the noise/interference/static, etc.  The rest is done by the tuner in the TV, Radio - whatever, you're just making that job easier.

Regarding materials, Silver is best, Copper or Aluminum is good, Steel, Chrome, Stainless Steel, Gold, is mediocre.   Have you noticed those car antennas with a little wire wrapped around them...hmmm.   Avoid PVC, and instead soldier or braze together the wires.  Some PVC material conducts at VHF and even more at UHF frequencies (450MHz is in the UHF frequency range), use the wrong materials and lose some of that precious signal.   Some other materials make great resistors (generate HEAT when power is applied, or dissipate power/signal better than transmit/recieve) - Steel, Stainless Steel, Chrome, Zinc are not that good for antenna materials.

Before someone says, "Oh, it's always worked just FINE for ME!"   What you can't hear, is what you're missing.  My real education started when I realized how POOR my first antennas were as an Amateur Radio Operator.   This stuff isn't rocket science, doesn't take a bunch of test gear - unless you need proof-of-concept - and anyone can calculate the length of a 1/4 wave element with the formula 234 divided by frequency equals feet.  One other thing... while watching a Youtube thing - and hear "precise measurements" ... without any formula or rational, they're probably just copying someone else & haven't got a clue why it works.  It's OK, because here I am trying to condense down a whole bunch of stuff and not mention other stuff that'll scare off the casual experimenter, "Did you model that before you built it, son...?".   Trust me, engineers are really good at that, scaring off the casual builder with lots of theory - I'd rather see you build and get decent results.... but give away all my high performance, proprietary designs - HA!  not a chance.   We're just talking about TV, here.  Same stuff applies for AM, FM radio too... and is even easier!

My first choice for a high performance antenna, always, without question:  Silver.  There are a couple of forms, as solid stock, twisted wires, or Silver LITZ wire.    It is EXPENSIVE and boy-o-boy, you don't want to waste money making mistakes.  Copper LITZ is 1/10th the cost, and works nicely. The down side of LITZ wire, once it's "CUT" - it's over, folks... what you got, is what you got.  In this case, it's best to have or borrow the right test gear...  so, skip that idea, it works good at lower frequencies - especially AM radio and Short Wave, where "size really does matter".   I mentioned LITZ wire because the idea of parallel elements to boost S/N ratio is an old, very effective, efficient method, to transmit and receive.

Another KEY to a high performance antenna is to use a single piece of stock to avoid lossy joints, & dissimilar metals (which do not pass the signal at the same rate or efficiency).   Tin/Lead Soldier is not Copper.   Galvanized Steel nuts & bolts is not Aluminum or Copper.  Stainless Steel is not Copper, Aluminum, Silver or whatever ... get the idea?   Why?  Each material has it's own "Velocity Factor" - the rate or speed in which a signal travels along or through it.   Do sound waves travel at the same rate through water as air?  A Beverage Antenna is a good example.   A single continuous conductor makes for a better antenna than pieces joined by different kinds of metals.  The signal is flowing over the SURFACE of the metal, not through the center, also called, "Skin Effect".  Think of dissimilar materials as "Speed Bumps" that slow down and diminish the signal you really want.  So... that expensive TV antenna that was held together with what you thought were "cheap Aluminum pop rivets"... what you thought was "cost engineering" instead of using beautiful, rust resistant "stainless steel rivets" - and the antenna still worked, but not as good after you FIXED IT... meh, couldn't... naaaa...

The best choice is Aluminum, followed by Copper.  Granted Copper is a slightly better conductor than Aluminum, but both oxidize over time, and Aluminum always wins out, the oxidization layer is thin.  What difference does it make?  The signal travels over the outer surface of the conductor/wire.  Fresh, Bare copper is GREAT!  a few years later, it's all green with oxidation, and doesn't quite work the same...  Aluminum oxidizes REALLY, really fast, most of it in seconds and doesn't change much more - what you got today, is going to be nearly the same 10yrs later.  The signal you want, is travelling though that oxidized material.  So why not insulate it?  Insulation also changes the Velocity Factor, and finding UV resistant insulation that isn't conductive at 450MHz - meh, we're gonna compromise on this.

BTW... Aluminum is easy to cut, tap and thread, you could easily make a porcupine of an antenna with 1/8" rods...  WD40 is a great cutting/tapping/threading lubricant for Aluminum too!  Wash your hands after playing with it, wear gloves - you don't want to ingest Aluminum.   Cut all those threads...?   Put the Die in a vice, squirt some WD40 on it, Put the 1/4" Aluminum Rod in a drill, squirt some WD40 on it, and it can be done in about 5 seconds.  Same goes for the tapping, use a low speed drill, hold it steady, go slowly... use WD40 generously.

But, speaking of Skin Effect again, Adhesive Copper tape or Aluminum flashing tape, can turn fiberglass, a glass window, wall, ceiling, roof truss, rain gutter, flag pole, etc., into an effective, efficient antenna ... hidden away from view, in places that ban antennas altogether.

The take away can be summarized:
 
a.  Resonance hears best at ONE frequency and hears least at non-resonant frequencies, like a tuning fork. 
b.  Aluminum and Copper are the best, but don't mix them with other materials.
c.  The power of many antennas/elements put together is the way to build you Signal up over the Noise - S/N ratio.
d.  This isn't rocket science, 234 / frequency = length in feet. for each element.
e.  Experimenting is not just for RF Engineers (which come in 2 types, those who help, those who hinder experimenting)

Lastly, Antennas convert Electromagnetic waves of energy into electricity that your radio or TV converts into enjoyment.
Antennas can also carry power from Electrical sources, power lines, etc., that stuff can kill you in a split second, STAY AWAY from it.


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