Friday, December 30, 2011

Making a homebrew HDTV antenna

Since I work for a TV station now, and we're web-casting a lot of University sports, it behooves me to get more familiar with sports broadcasting, and one of the ways I'm doing that is to watch sports on TV.  Now, I'm still too cheap to pay for cable TV, I'm on my holiday break, and most importantly I haven't built an antenna since my J-pole, so I figured I ought to build myself a TV antenna. 

I'm basing this antenna on plans I found for a UHF bowtie antenna here.  Now, being the sort of guy I am, I can't just blindly follow the recipe. :)  I need to screw around with it a bit, and see if I can improve on the design based on my own particular situation. 

I went to TV Fool's signal locator and put in my address.  It gave me a handy list of all the TV broadcast stations in the area, along with their channel assignments (if you're following along at home, note that you need the actual channel assignment, not the virtual channel.  Sometimes these two things are different).  Next, I consulted Wikipedia for a list of TV channels and associated frequencies.  This told me that all the TV stations in town were in the range of 494 MHz to 698 MHz. 

I think it's safe to assume that the original design is tuned for a broader frequency range than I need.  If I tune the design for my narrower range, maybe I can get another dB or two of gain, which will improve my signal quality.  Also, it's more fun that way. :)

Now for the parts.  I went to the hardware store and bought some deck screws, some washers, and a 2 x 4.  They didn't have any 1 x 3's or 2 x 3's, and I didn't much feel like going somewhere else to see what they had.  I also bought a 75Ω to 300Ω balun, as called for in the design (this design has a 300Ω characteristic impedance).  For the antenna bays, I've got some old wire hangers laying around that need to be put to a good use.  I've also got wire laying around that I can use to interconnect the bays.

In the design, each antenna bay is a pair of 14" wires, folded into a "V" whose ends are 3" apart (so an angle of about 25°).  This is a bowtie antenna, so each side of the bay is a dipole 1/2 wavelength long.  14" gives a wavelength of about 420 MHz, well below the minimum frequency I'm looking for. 

Let me pause a moment to detail how I'm calculating the length of the dipole.  Wavelength (λ) is equal to the speed of light (c) divided by the frequency (f).  So:

λ = c/f

An important thing to note here is that c isn't always c, if you take my meaning.  Generally speaking, if someone mentions c in the context of the speed of light, they mean the speed of light (or other electromagnetic radiation) in a vacuum.  In (e.g.) a metal, however, electromagnetic radiation moves more slowly.  So we introduce a velocity factor, a number that describes just how much slower electromagnetic radiation is going to move in the material than in free space.  If we call the velocity factor v, then we get:

λ = cv/f

This is good.  By introducing the velocity factor into the equation, we can more precisely model how electromagnetic radiation (e.g. radio waves) is going to behave in the wire of the dipole.  But I don't have a velocity factor for steel wire (which is what they make wire hangers out of), so I'm just going to use a value of 1 for v

So, back to the antenna, if I want a minimum frequency of 494 MHz, I want to start with a dipole about 12" long.  I can always trim it down later if I need to.

Now, let's get to building!  To start with, I'm going to cut my wire hangers into 8 12" pieces.  And this is tougher than it sounds.  A pair of dikes (diagonal cutters) won't cut it.  Tin snips won't do it.  A stout pair of side-cutters (or "Kleins", electrician's pliers) might do it, but I don't have a pair of those.  What I'm doing instead is taking a big pair of dikes and gripping the wire where I want it to break, then just wiggling the hanger back and forth until it breaks.  Metal fatigue FTW!
8 12" dipoles

As you can see, some of these aren't exactly straight.  A number of wire hangers came from the dry cleaner, and they only have metal on top - the bottom is just thick paper.  So I straightened those as best I could.  I also sanded the center of each dipole because  I'm going to short all the dipoles along each side together at the center.

Dipoles bent so that the distance between each leg is roughly 3"

Here are the bent dipoles.  I've tried to keep the spacing between the two legs to about 3 inches.

Now comes the fun part: drilling the holes and mounting the dipoles.  I got a cordless drill as an early Christmas present; time to put it through its paces!

As per the plans, I drew a line across the board at 2" from one end, then 3 more lines at 5 1/4" intervals (so marks at 2", 7.25", 12.5", and 17.75").  These lines are where the two dipoles in each antenna bay will be placed.

Next, I made two perpendicular marks on each line, 1" apart.  I like things to be even, so I centered the marks, placing them 1.25" from each edge (a 2 x 4 is actually 3.5" wide).  I then drilled pilot holes at each mark for the deck screws.
Here you can see the board with the deck screws already part of the way in.  This makes it easier to place the elements and the connection wire.  I also have two small steel washers on each screw.  The lower holds the dipole element to the board.  The upper will pinch the connection wire between it and the lower washer, providing electrical connectivity.

Next, I added the connection wires between each element. I used insulated wire, and I only stripped those parts of the wire that were going to be in electrical contact with an antenna element.  I've also stripped a place in the center of the antenna to connect the balun.

Here you can see the deck screws where the balun will connect.  Like the other screws, these have washers on them to improve the electrical connection between the wire and the balun.
And this is the antenna with the balun attached.  This is a usable antenna right now, but it's omnidirectional.  Adding a reflector, as recommended in the original plans, will increase the gain of the antenna, but you might want to try this antenna as-is and see how well it works for you. 

I mounted the antenna, sans reflector, in my basement and attached it to my TV.  I'm able to pull in all the stations that TV Fool says are available in my area, including the two analog low-power stations.  But none of those transmitters are more than 30 miles from me, and I don't have any channels from further away.  Once I get the antenna mounted outside, and the reflector attached, I'll report back.

No comments: