1/24/07

The Silver Sensor

Zenith and others sell this antenna. Zenith calls it the ZHDTV1. This UHF Log-Periodic Dipole Array is a very popular indoor antenna. It looks good. If you don’t think any antenna looks good then this is your antenna: It has high gain with minimal visual impact. Reception out the back is very weak, which may help in multi-path situations. But because the designers chose to angle the elements back 15° (to give it an airplane look) there are no nulls at +90° and –90°. Amazon.com, Buy.com, Hififorless.com, knbelectronics.com, and many others sell the Silver Sensor. Terk has a similar antenna that also has rabbit ears for VHF.

Urban Ghosts (inner-city multi-path)

This is a particularly vexing problem. The signal is strong, but the DTV just won’t lock-on. Multi-path is caused by reflections off of buildings and some hills. These signals arrive late. On analog channels they appear as ghosts. Actually all channels have them. But unless the direct path is blocked, they are usually too dim to see.

To determine if you are haunted by these specters, observe analog channels, close in frequency, from the same directions. If you see ghosts that are more than very dim, then they are blocking your DTV reception. The ghosts are usually to the right of the actors. (If any are to the left then a reflection is stronger than the direct signal and you may benefit from pointing the antenna at the reflection.) If there is more than a faint trace of snow then you also need a bigger antenna.

Assuming little snow, your first step is to acquire a Silver Sensor or a DB-2 antenna. While watching the analog station, try to find a position that minimizes the ghosts and the snow. This will teach you what you need to know to make the DTV station work. Search the whole room, especially near windows facing the tower. Rotate the antenna while keeping in mind the overhead view diagram above. The angle where a ghost fades out will tell you the direction it is coming from.

If you still have no luck getting a DTV lock then strong ghosts are arriving from the front. The following procedure may provide a working solution. This procedure will only work if there are one or at most two strong ghosts (and maybe a lot of weak ones). If you have three or more strong ghosts then your only hope is a long cable to a place more advantageous.

The Two Antenna Trick (indoor version)

Acquire a second Silver Sensor. Two identical antennas are required. A desktop or other flat surface will be required for them. The desk to should not be up against a wall that the antennas have to point through. For the antennas to be equal they must be away from other objects. Join them with a 2-to-1 splitter and two identical short cables of about 2 or 3 feet :

The radiation pattern for the combined antennas has the same shape as for a single antenna, except now there are a lot of nulls in the pattern. The strategy is to make nulls coincide with the directions of the ghosts.

Set the antennas beside each other and pointing at the strongest signal. Keep them parallel. The separation distance (S) between the antennas determines how many nulls are in the pattern :

Moving one antenna slightly ahead of the other (A) will rotate the nulls to the right or left in the overall pattern :

As you adjust A, you should see each ghost dim and then re-intensify. For your brightest ghost, go back to the dim spot. If it does not completely disappear, it is because the antennas are no longer in equal strength fields. Rotating one of the antennas to reduce its gain should make the cancellation complete.

There is a combination of S and A that will make two ghosts go away. You must search for it. Do this first with an analog channel. If its frequency is close to that of the DTV channel then a very similar S and A will work for the digital channel. If there is only one strong ghost then there is an S and A that will work for two channels.

Don’t be too surprised if you can’t get this to work. Canceling two ghosts is difficult. The two diagrams above assume both antennas are in identical fields. Indoors, this is seldom the case. If this system does work then all you have to do is convince your wife to let you keep it.

There is an outdoor version of the Two-Antenna Trick.

This page is part of “An HDTV Primer”, which starts at www.hdtvprimer.com