Building an HDTV Antenna db2, db4

We finally cut our cable last summer.  The low-cost, home-built, attic or closet antennas shown below and their appropriate positioning allow us to receive 11 free HDTV VHF/UHF broadcast channels in our area -- Nearly as many (real) channels as we were getting for $30/month from our local basic cable package.

DB4 

DB2

Getting the best signal for all the channels requires some planning prior to building your antenna(s)* so I am going to recommend doing this in the opposite order in which I did.

*antennas is correct plural for radio broadcasting, antennae is correct plural for biology.

STEP 1:  Tower Locations
First, find all the possible stations that you can receive in your area and their distance and direction relative to your home.  Anything outside of  30 miles or so is probably going to need an outdoor antenna. The transmission towers which provide the 11 stations that I receive range in distance from 2-24 miles.

The best resource for locating your stations and towers is http://www.rabbitears.info/search.php


Be sure to enter your actual address with your zip code because this will give you distance and the direction (azimuth) of the station's transmission towers to within a degree of azimuth.  Reading the direction (or azimuth) in degrees of each tower is simple.  Facing due North is 0 degrees, due East is 90 degrees, due South is 180 degrees, and due West is 270 degrees.  360 degrees and 0 degrees are of course the same azimuth.

Don't be surprised when the location of the DTV transmission tower for a particular station is completely different from the location of their local TV studio.


One thing that may confuse you at first is difference between the true broadcast (RF) channel and the PSIP channel.  Among other things, the PSIP channel is the channel that your station carries with its call letters and is the channel that the station tells your TV to display.  Each RF VHF/UHF broadcast channel has a bandwidth of 6MHz which is further subdivided into parts.  Many stations will broadcast two to three PSIP channels or streams from within one allocated RF channel.  For example, my local PBS station channel 5 broadcasts three PSIP channels: 5-1 PBS, 5-2 Create, and 5-3 World.  This is how my TV displays them, but the actual physical channel which the antenna receives is RF channel 36 (602-608MHz) which is subdivided into 36.1, 36.2, and 36.3.

This difference becomes very important if you are designing an antenna to resonate for a particular channel frequency.  From the PBS example above, RF channel 36 has a UHF wavelength of about 50cm (20in).  If you mistakenly thought the PSIP channel 5 was the same as the RF channel, then you would be designing an antenna to receive an RF channel  5 with a VHF wavelength of 3.8m (150in).  With a 1/4 wavelength antenna design, this is a difference in antenna elements of 4in vs nearly 40in.

wavelength = c/frequency (c=speed of light)

or

wavelength = 300/freq in MHz

Fortunately, the antenna design below does a pretty good job of picking up a good spectrum of RF channels including both of these extremes.

Here is a table of the physical radio frequency VHF/UHF channel allocations.  Notice that there is a huge gap in the VHF frequency range between channels 6 and 7 which is carved out for the U.S. FM frequency range of 87.5MHz-108MHz.  This is why you can sometimes hear a television station broadcast at the very low end of the FM Radio dial.

After printing a list of stations with details, I mapped out the azimuth (direction) of each tower on the ceiling right below where one of my antennas is placed.  You are going to need a directional compass (app) or you could use Google maps to get a satellite picture of the top of your house and figure out the axis direction of your house by comparing to the North arrow.  A protractor and straight edge were also for this. The axis of my house is 10 degrees off of due North.

Azimuth (Direction) Map

Of course, estimating will probably get you close enough, but remember, the antennas detailed here are basically directional, despite being sold online as multi-directional antennas.

My transmission tower directions were rather easy to deal with.  I have two different general directions:  Mostly South and Mostly West. I placed two antennas in the same attic location with one pointed at about 180 degrees and the other pointed at about 260 degrees.

STEP 2:  Antenna and TV Locations and Connections
Next you need to figure out where you are going to place your antenna(s) and how you are going to run the coax cable to your TV.  Use the shortest length of digital-grade cable possible.  I used a 25' length to run from the antenna location in the attic to our LCD TV.

The antennas each use an inexpensive balun (300ohm balanced-75ohm unbalanced) transformer which can be purchased at Walmart or RadioShack for about $5.

Balun Attached to Balanced Antenna Array Feed Points

If you are older than thirty then you probably remember screwing a balun into your old CRT television to make your 13 channels of coaxially-delivered 75ohm cable compatible with the 300ohm antenna terminals on the back of the TV.  Now you do the whole thing in reverse.  You may even still have one laying around in a drawer if you are a packrat like me.

The baluns are sometimes sold in a packaged pair with one standard balun and one F-type (pictured below).   Both types of baluns can be used to transmit the balanced VHF/UHF radio signal received by your antenna to the unbalanced, shielded coax cable to your TV.

F-type Balun with Female to Female Jack.

Both Types of Baluns Packaged Together for $5.

Unlike your old cable signal from the cable company, the RF signal which your antenna transmits is very susceptible to interference and degradation before entering the shielded coax cable.  It cannot be reliably split to serve more than one TV without some additional expensive boosting unless the signal is particularly strong.  The coax cable must run the complete distance from the TV to the balun to avoid signal problems and should not be split.



When you are done constructing your antenna (STEP 3), attach the balun directly to the antenna feeds and the coax directly to the balun. Do not attempt to extend the length from the antenna to television with anything other than coax.

While you probably won't be able to split the signal coming from the antenna to your television, you can use your old splitter in reverse as combiner to "join" two different antennas pointed in different directions for maximum reception.  The splitter is a passive device with no active electrical components so it can work in either direction.

Technically speaking when you join two antennas, you are supposed to use a properly filtered antenna device called a combiner or Jointenna to avoid multipath interference.  Antenna engineers will tell you not to use a splitter in reverse as a combiner,  but I found this cheap setup to work quite nicely for both double antenna setups in my house.

At this point it is worth mentioning that our federal government including Congress deserves some credit for making this so inexpensive.  First the 2009 law which forced migration to DTV broadcast and second the law requiring a digital tuner on all HDTVs made after 2007 to be sold in the US.(Citation Needed)


STEP 3: Constructing the Antenna
There are several options for the materials to be used for your antenna.  Both the DB2 and the DB4 designs require three basic elements:
1. Metal wire or rod material (coat hanger, copper wire, steel rod) -- At least 12 feet for the DB4
2. Screws or bolts for connections (wood screws or machine) -- Need 10 for DB4
3. Body material for spine and connections (wood and/or pvc pipe)

I have built several (six in all) of both types of antennas using the materials listed above with varying degrees of success.  My best and most recent antennas have been constructed from 1/8 in steel rod, PVC pipe and fittings, and 10-24 x 2in bolts with nuts and fender washers.

1/8in Plain Steel Rod

One general rule for receiving antennas is that thicker, more straight conductive material works better than thinner--It provides more metal material surface area and therefore more free charge for the radio waves to oscillate back and forth along the antenna and therefore a higher gain for your DTV signal.

10-24 x 2 Machine Bolts w/Nut and 1/4in Fender Washers

I found these to be best all purpose connectors for this purpose.  They are thick enough to provide a very secure connection without the need to solder and thin enough to accept the clips for the balun at the feed points.

Here is the basic plan for the DB4 (Dipole Bowtie 4-bay).  It is made of two electrically isolated vertical arrays (forming a dipole) onto which there are attached bowties in four evenly spaced bays .  The arrays must isolated from each other and cannot touch.  The two crossover points (where blue meets red in diagram) must not be allowed to touch electrically.

This antenna is based on 1/4 wavelength dimensions.  VHF broadcast wavelengths typically range from 1-5m (39-200in) and UHF wavelength range from 10-100cm (4-39in).  This antenna will pick most of those wavelengths if constructed carefully.

Additionally, each connection node from the right should be insulated from the one on the left.  This is why PVC is a good choice for the body of the antenna.

In this case I have used a short 1-inch half section of PVC to insulate the left and right arrays from each other.  The tension created from the 1/8in steel rods is enough to keep it in place.

Each of the connection nodes protrude from the back without connecting to each other.

Each of the bowties is made from one 18 inch length of 1/8 in steel rod which was carefully angled in the middle by hand by pulling each end around the thin edge of some rigid, stationary object. The bowties are fed between the two fender washers at each node.

Each side of the array is connected via one ~22in length of 1/8in steel rod.  These rods are woven underneath the bottom fender washer at each node and also alternate sides from top to bottom which creates  a good tension on each connection node.

Once the antenna is built, gently connect the balun to the feed points and connect your coax to the the balun.  The balun matching transformers are usually pretty sturdy, but you can twist on them too hard and ruin the internal connection.

To connect an F-type balun, simply run two short, evenly spaced wires of equal length from the feed points to F-type terminals.  Then you can connect the coax to the F-type balun with a female-female coupler jack.

As mentioned above, two antennas can be joined together.  The DB2 antenna is good enough for signals that are close and powerful, but the DB4 will be necessary for signals that are farther away.

Be sure to position the antenna vertically with the bowties pointing horizontally such that they are perpendicular to the azimuth direction of the station antenna. Most station antennas have a slight electric beam downtilt of about 1 degree with another 1 degree angle of depression from tower down to ground level.  Of course, the angle of depression becomes more pronounced the closer the tower is.  This means that you will probably get a better signal by angling the antenna slightly upwards if possible.

One thing that I have left out is a reflector.  The DB4 and the DB2 are often fitted with a reflector made of metal mesh, wire caging, or even aluminum foil to increase the gain.  The reflector should be spaced 1/4 wavelength behind the antenna arrays (about 3-5in in most cases).  I have found the addition of various types of reflectors to have little impact on my antenna performance.

Below are a couple of pictures of a professionally designed and manufactured DB2 with a reflector which I stumbled upon recently while visiting a friend.  It is made of aluminum and is (seldom) used in the break room of an office setting in a large metropolitan area.

Extra Notes:
Antenna physics and design is a very complicated topic, but it is probably true that many people who are willing to build their own antenna to save a little cash are also interested how the thing actually works. There are three basic parts:
1.  How the transmitter antenna radiates its signal
2.  How the signal propagates through space
3.  How the receiver antenna "catches" the signal

A very simple (and perhaps simplistic) way to imagine this whole process is to revert to the analogy of water waves.  Imagine a lake in which there is a fish at the bottom.  In this case though we are not trying to catch the fish, but simply trying pass a message to it from the shore.  The fish cannot hear us, but someone else whose fishing line broke has left their bobber at the surface and a fishing line with a weighted hook which drops down to the bottom right in front of the fish.

We can pass a coded message to the fish via the bobber, line, and hook by simply creating water waves with our hand.  If we create a water wave by smacking our hand on the top of the water, then the bobber will transmit the motion of the wave to the hook making the hook move up and down in front of the fish.

Our hand transmits a signal(transmitter)to the water (space) which carries that signal to the bobber line (antenna).  The driving signal at a transmitter antenna (your hand above) causes electrons to flow back and forth along the axis of the transmitter antenna which causes electric and magnetic fields to emanate from the antenna as electromagnetic radiation in the radio spectrum.  The radio waves propagate through space as oscillating electric and magnetic fields (water waves above) eventually enveloping your antenna (bobber above) and causing the electrons in the metal antenna to flow back and forth along the antenna elements thus creating the same signal.

You can carry this analogy a bit further by using it to explain different antenna sizes, wavelengths, and interference.  If you change the size of the bobber, then you can change the size of the waves that will drive it.  A really large bobber will only adequately respond to large waves--Little ripples won't move it enough.
Multi-path interference occurs when the water wave you created reflects off of the shore or some other object interfering with your original water wave.


The real explanation of the operation of a simple dipole antenna whether transmitting or receiving starts with electrons flowing through a wire.  I will be elaborating on this further when get more time.

A visual of of the electric and magnetic fields emanating from a dipole transmitter antenna is helpful, but the propagation of the resulting electromagnetic wave through space requires some background in electrodynamics.