Receiving Portland, OR Digital TV Stations
Click on any photo for a larger view
|The map at right shows where I am (marked Wpt4), relative to the transmitting towers near downtown Portland (marked Wpt2). Only 22 miles away, but far enough -- far from the crowding of the Portland Urban Growth Boundary, and the ever-increasing density the regional government is imposing on the poor unfortunates that live within it. But a relatively short distance as TV transmission goes. Should be easy to receive the Portland digital TV stations, right? Well ...|
|Looking at a topo plot reveals the problem. My house is on the left, the Portland transmitter location in the West Hills is on the right (specifically, for the joint Ch. 40/Ch. 43 tower). On the right, the additional vertical line accounts for the height of the transmission tower. The red line shows the line-of-sight path from the transmitting antenna to my house (note the vertical and horizontal scales are different on this plot). As can be seen, there are a number of obstructions between the transmitter antenna and my house. Since all the Portland DTV are in the UHF band, non-Line-Of-Sight makes things difficult: those short wavelengths don't diffract around obstructions very readily. As a result, the received signal is composed of a bunch of reflections of similar amplitude, and the resulting multipath represents a "challenge" to the receiver. Even with a Dish Network Model 6000 receiver, which uses the well-regarded NxtWave Nxt2000 chipset, I wasn't able to get anything with a rooftop-mounted antenna.|
View looking east from mast -- height needed!
|I figured a letter-writing campaign to the local stations,
asking them to switch their analog and digital channel assignments (which
for most of them would put the digital stations in the VHF band), would be
Instead, I sought to eliminate as many of the obstructions as possible. With a 50+ ft. mast, I could clear alot of the local "ground clutter" (black line in topo plot).
Unfortunately, short of putting up a 250 ft. tower, there was no way to avoid the obstructing hill about 4 miles away. However, by locating the mast toward the south end of my property, I was able to do a little better by "shooting" through a low spot in the obstructing hill (this effect is not shown in the topo plot). Still not Line Of Sight, but the best I could do.
|I started out with a Channel Master 4248 on a 50 ft.
mast. With this, I was able to receive 4 digital stations (ABC, CBS,
UPN, & Fox), but two (NBC and PBS) were not receivable.
I added a second yagi antenna, stacked vertically, and added another 10 ft. section of mast. The reception on the original 4 stations improved somewhat, but the other 2 could still not be received. But that's the current state of the antenna system, shown in the photo at left (wintertime).
Main components seen in this photo are:
'sounds like an ad for Channel Master, but I have no connection with the company!
|Close-up of the top of the mast. The guy ring at
bottom of photo is the top of the 50 ft. telescoping mast, which is made
up of five 10 ft. nested tubes Guys wires are attached every ten feet,
which is what gives the mast its rigidity.
Above the guy ring is an additional 10 ft. section of mast I added, to bring the height of the stationary part of the mast to 60 ft. About half-way up this mast is the rotator. Further up is the 9523 alignment bearing, with the top guy wires connected to it. The ten foot mast that the antennas are mounted on passes through the bearing, and fits into the top of the rotator, overlapping the stationary mast by several feet. This makes for a very stable arrangement that takes alot of the stress off the rotator. It's a real shame Channel Master no longer sells the 9523.
I could probably have achieved better antenna performance by stacking the antennas horizontally. This would considerably narrow the horizontal beamwidth; the vertical beamwidth is pretty narrow on one of these antennas as it is. But such an arrange is mechanically unwieldy, and would require a "real" triangular section tower such as a Rohn, rather than just a simple mast.
|This shows how the guy wires that secure the mast are
anchored. I used 1/8" galvanized cable for the guy wire.
It's fairly cheap at Home Depot, more than strong enough, and very
flexible and easy to work with.
The wires are attached to screw-in ground anchors, available at any farm supply store. I used ratchet-type tensioners, made for high-tensile wire fencing and also available at farm stores, to tighten the wires. They're cheaper than turnbuckle-type adjusters, and have alot more adjusting range.
The mast needs three anchoring locations for each of three radial directions from the mast, at distances of 10, 20, and 25 ft. from the mast, for a total of 9 anchors.
|This is a close-up of the wire tensioners. Copper
compression sleeves and stops (available where you get the cable) are used
to join cables, and to prevent the cable from inadvertantly slipping out
of the tensioner. The tensioners are easily tightened with a wrench.
Raising procedure was a follows: First, I set the unextended 50 ft. mast by itself in place, and plumbed it vertically by tightening the bottommost set of three guy wires. This set of wires is then left alone. Next I raised the next-to-the-bottom section of mast into position and adjusted the guy wires connected to it (i.e., the wires that connect to 20 ft. height on the mast). I them loosened up these wires a bit, and lowered this second section. I then extended the third section, adjusted its guy wires to proper length (at the 30 ft. level), loosened them, and brought them back down. I repeated this again for the 4th section, and finally the 5th and top section. Doing things incrementally like this ensures that the lower levels of guy wires are at about the proper length before the next section is raised.
Finally, I added the topmost 10 ft. stationary section, together with rotator, bearing, and rotating mast with the 2 antennas, to the top of the unextended mast. Then the whole thing was raised to the final position.
NOTE: It is essential, during the raising and lowering, that there be one person at each of the three guy wire radials, keeping the wires properly taut so that the mast sections stay reasonably vertical (in my case, this consisted of my wife, my brother-in-law, and my daughter, while my wheelchair-bound mother-in-law "supervised" and entertained the 3- yr.-old -- think of it as "Quality Family Time"). The mast is very strong once it is raised and vertical with all the guy wires tightened, but the mast will bend if it is allowed to get too far from vertical during the raising process.
|Essential tools. On the left is a Nicopress tool I used to crimp the sleeves onto the cable. These can be rented from farm supply stores that sell high tension fencing gear. The tape measure is shown for scale. To the right of that is a small bolt cutter, necessary for cutting the 1/8" steel cable. The photo to the right shows the ground anchor. The size I used is 30" long, with a 4" diameter anchor plate. These screw into the ground fairly easily, but provide a very secure anchor. Incidentally, the mast bottom sits on a small poured concrete pad, which prevents the mast bottom from sinking into the ground as the wires are tightened.|
|When combining or "stacking" antennas, it's
important to do it properly. The easiest way to do this is to just
put a 75 ohm to 300 ohm matching transformer on each antenna, and connect
them to a 2-way signal splitter (using it "backward" as a
combiner). The two cables to the splitter must be the same length,
and the transformers should be the same make and model.
It's also important to get the phasing right. Hook up the 2 antenna signals out of phase, and they'll cancel -- no signal will be received. To make sure I got the phasing correct before putting things 60 ft. in the air, I used the test fixture shown in the diagram to the right. The "test signal input" on the left is connected to a TV antenna, or any other source of a UHF TV signal. The 2 matching transformers are connected to the fixture as shown. If you get an output signal, fine, you're good to go. If you get nothing, reverse the "A" and "B" labels on the leads on one of the transformers. Once everything is working OK, if the "A" lead on one transformer is hooked up to the left terminal on one antenna, then the "A" lead on the other transformer must be hooked up to the left terminal on the other terminal. Similarly for the "B" leads and the right terminals.
|At left is a spectrum analyzer plot for channels 22 to 32,
as seen at the receiver input. The analog stations have two
"spikes" for the visual and aural carriers, while the digital
spectra are flat across the 6 MHz channel. Interestingly, the
strongest signal, KWBP, is also the farthest away -- about 60 miles!
Of course, it happens to be the highest power (5 Megawatts ERP), and the
transmitter is at a very high altitude in the Cascade foothills (about
A Hewlett-Packard 8591E spectrum analyzer was used (every DTV viewer should have one).
|This plot shows channels 40 to 49.
Not surprising, the two unreceivable channels in these plots, Chs. 27 & 46, are the lowest in amplitude, and the most "ragged"-looking (indicating alot of multipath). These two channels are on the same transmitting tower, not very far from the other transmitters, with a compass bearing that's only 1 degree further north than the others.
I've tried extra amplification, which raises these two channels well above the noise floor of the spectrum analyzer, but the 6000 still can't lock onto them. Perhaps a receiver with a more-capable equalizer will be successful.