Sunday, February 03, 2008

Why Buy When You Can Build? - Part II

Here are the materials I used to build my DB4 antenna:

  • (1) piece of 1/4" pine sheet cut to 16"x30"
  • (1) short 2x4 (only need about 12")
  • 10' of 12g copper wire
  • Approx. 3' of romex electrical wire
  • At least 32" of 18"-wide aluminum foil
  • (12) 10D x 1" wood screws
  • (5) 8D x 1-3/4" wood screws
  • (1) 5' length of 2x2
  • (1) 300Ohm-to-75Ohm balun (Radio Shack for about $5)
  • (12) 1/4" washers

And here's how I built it:

Step 1

I marked the centerline of the 16"x30" reflector and then covered the other side with the aluminum foil. As it turns out I got really luck here...I just barely had enough to cover the whole length of the board. I then used duct tape to hold down the edges.


Back of the reflector

Step 2

Measuring along the centerline on the back of the reflector, I very carefully drilled holes for the element mounts every six inches:

|-6-6-6-6-|

I then drilled a hole between the middle two to make a mount for the balun, so the measurements from left to right are:

|-6-6-3-3-6-|


The screw holes along the centerline of the reflector

Step 3

I cut the 2x4 into 2" pieces for the mounts (total dimensions for each piece: 3-1/4" x 1-1/2" x 2"). I then drilled a pilot hole in the center of one 1-1/2" x 2" side for each block. Using the 8D x 1-3/4" screws I connected the five mounts to the reflector. This was rather tricky. I kept the reflector face down on the sawhorses and started the screws through until there was enough poking out the front to catch the holes in the mounts. I then held the mounts in place from the bottom while screwing them down from the top with the drill. What makes it most difficult is that you want the mount to turn as little as possible while it tightens down because twisting can cause the aluminum foil to tear. Fortunately it worked out just fine.


Setting the screws to attach the element mounts on the other side


The element mounts secured to the reflector

Step 4

Once the mounts were secured I needed pilot holes for the screws and washers that would hold the elements and wires in place. The holes need to be far enough apart so that the washers do not touch as this would screw up the reception. I marked in at 5/8" from each side and then drilled so that the inside edge of the pilot hole would be right on the marks. Once drilled, I started a washer and 10D x 1" screw in each of the mounts, with two washers on each of the center screws.


Screws and washers set for the element and balun mounts

Step 5

Before I go any further, I need to mount the reflector to some sort of mast. What you mount it to is entirely up to you and will depend on how you will be using it. The reflector is just that...a reflector. I reflects any signal that might pass through the elements back to them from the other side. It doesn't even have to be solid, as evidenced by the oven-rack style reflector used by the ChannelMaster 4228 to cut down on wind resistance. In fact, many people have built these DIY DB4 antennas using oven racks they bought at the dollar store so that they can mount them outside.

Anyway, my point is that running something as small as a couple 10D screws isn't going to affect anything, so that's how I mounted it to the 2x2 mast. I used 5' of 2x2 knowing that I will be cutting it down shorter once I actually take it up into the attic and find a mount point.


Mounted 2x2 mast. Attached by screws at 9" from each end

Step 6

Now it's time for the wiring. I cut the 12g copper wire into 14" sections for the elements. Each of these sections were then bent into a V at about 26degrees (3" apart at the tip). You can use a variety of different things for this part. Many people use coat hangers which are a little heavier gauge. If you do use them, be sure to use sandpaper to clean off any coating, especially at the contact points near the screws.


The 12g copper wire I used for the elements.

For the wiring between the elements, I stripped the outer coating off the romex electrical wire. I used the unshielded ground and white wires. Whether you leave one of the wires shielded or not is not important, but what is important obviously is that you strip the shielding off where the wire makes contact with the elements. Essentially, what we're doing is assembling two DB2 antennas that connect in the center at the balun, so the wiring for one set of mounts is separate from the other set. This is why the balun is in the center as opposed to on the end where it would seem to be more convenient. The wires need to be long enough to run between two of the mounts and to the middle mount where the balun is, with a crossover in between.


A picture of the crossover from the finished antenna

I can't remember exactly what the purpose is for the crossover, but I do know it is important. It is very important though that neither the washers nor the wires touch, so you need to have some sort of shielding between the wires when they cross over.

To assemble, I ran the wires between the appropriate screws. I then added the copper V elements so that they sit on top of the wire. When they screw down they will maintain contact with the wire.


First set of wires and elements connected

Once the wiring was run and the elements were in place, I tightened down the screws. Now obviously this causes the elements to angle upward since the wires only run under one side, so I simply applied a little pressure to bend them down parallel to the reflector again. I didn't tighten the center mount screws yet because we're going to run the other set of wires and the balun into those first.


Second set of wires and elements connected

Step 7

After running the other set of wires, it's time to attach the balun. For the sake of simplicity, I ran each set of wires so that they met on opposite sides of the center screw.


Balun mount (right) showing the separate sets of wires

When I attached the U connectors on the balun, I laid them in at an angle so that they made connection with both sets of wires.


Balun connections set at an angle to connect with both sets of wires


Balun connected and secured to the balun mount in the middle

When everything was in place, I tightened the screws and made sure everything was secure. That's basically it. We now have our DB4 antenna!


The finished product


A closer look

Testing and Summary

Since I finished this at about 11PM and everyone else was asleep, I didn't really feel like climbing in and out of the attic to test it. Instead, I simply took a 25' length of coax I had in the basement and hooked the antenna up to the TV in our upstairs bedroom. The results varied from channel to channel, but overall there was definitely an improvement. The Toledo stations came through very well, and when I rotated it towards Detroit I got a pretty decent signal from some Detroit stations that were almost 70 miles away!

Now a couple of things to clarify. First, this type of antenna is a UHF antenna. I remember knowing that there was a difference between VHF and UHF when I was a kid, but in the age of cable television and satellite such differences don't get cited often. Basically, your channels up to 13 are VHF, and from there up you're in the UHF range. Most stations that were traditionally in the VHF are moving to UHF as part of the switch over to digital in 2009. So, while I did see decent gains in reception for channels 11 and 13 it wasn't because of design.

Second, the location I tested in...the master bedroom...is not the ideal place for signal reception. The garage roof is on the other side of the east wall and basically runs diagonally across about 2/3 of the wall. If you were to trace the signal path it essentially has to pass through the shingled garage roof at a very severe angle, so the signal reception is adversely affected. The upside to this though is that the eventual mounting pointing the attic has no such obstruction. The signal need only pass through the vinyl siding and plywood wall, which it can do with very little loss.

Third, as I said in my previous post this is a directional antenna. While most of the stations I want are in the same direction, I wouldn't mind having stations out of Detroit. The signal strength from the Detroit stations was not ideal, and depending on where I mount the antenna in the attic I may still have to receive through the shingled roof, which will affect signal strength. If I were hell bent on getting the Detroit stations in HD, I would almost certainly have to mount this externally. Regardless, I have two main options to get signals from different directions. I can either install a rotor (yes...the old rotor like your grandparents had) or I can mount a separate DB4 antenna pointed in a different direction from the first and combine the two signals. The latter is a little more complicated but still by no means impossible. Truth be told, I could tie in a VHF antenna the same way if I still had channels in the VHF range that I wanted.

All in all though the antenna was easy to build and showed a visible improvement in reception even under less than ideal conditions.

Next steps: final mounting and calibration, running RG6 coax and seeing if I need a signal amplifier.

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3 Comments:

Blogger admin said...

Where are you located?
I built a antenna last week in Kingsville..
I found placement outside getting all the major feeds but one move and I lose NBC,Fox...

If I point towards Toledo I get a different set but cant find NBC...

Right now I get NBC, ABC, FOX, CBS...

8:55 AM  
Blogger frumanchu said...

I'm out west of Toledo in Fulton County. I'm quite considerably closer to Toledo than you are :)

11:02 AM  
Blogger admin said...

Wow. Interesting. I should show you some pics of mine.

I assumed by your post you were in a similar area on the Canadian side of the border :-)

Very similar. I watched a video on youtube, coathanger HDTV I think.. Anyways, I cant believe the HD quality.

Interesting to hear what these things are picking up.

11:57 AM  

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