5/16/06

The above diagrams are for a “V” shaped dipole (rabbit ears) 40 inches long (80 inches total), adjusted to 45°, and connected directly to 300W ribbon cable without any balun or adapter. If this is all you need, great! Rabbit ears often come with amplifiers or switched “tuning circuits”, and a UHF loop is common. Ribbon cable loss is easier to avoid for VHF than for UHF. For VHF, ribbon cable should not be up against anything metal.

Units with amplifiers

In most cases all the amplifier does is allow the maker to claim a high gain number. As was explained in the section “Antenna Basics”, the amplifier will compensate for loss in the cable but will not improve the antenna. The loss in the four feet of cable that comes with the antenna is probably not significant. The true gain for rabbit ears is what is shown in the net gain graph, not what the maker claims.

Most receivers have a VHF noise figure somewhere in the range 4-10 dB, with 6 dB being typical. If the antenna amplifier has a noise figure below 4 dB then you can assume it is probably quieter than the receiver. In this case, the difference between these two numbers represents an improvement provided by the amplifier, and the amplifier is a good idea. But few indoor amplifiers are that good. If the noise figure is not listed on the packaging then you can assume the amplifier is not quiet enough to be an improvement over the receiver.

It may be reasonable to buy an amplified indoor antenna if :

1. the amplifier has a VHF noise figure below 4 dB.
2. you need a longer cable to reach a good reception spot.
3. you want to feed two sets using a splitter.
4. your TV has 300W VHF terminals. The amplifier can overcome high ribbon cable loss where the rules can’t be followed.

Units with tuners

Many indoor antennas have a rotary switch on them that you must set each time you change channels. This switch improves the match between the antenna and the feedline. When effective, it makes the net gain as good as the raw gain. But it is not generally obvious how well it will work. (There is no way to tell before you buy.) You may find you can tune it to your weakest station and forget about it.

Getting the most out of rabbit ears.

If your rabbit ears do not have a tuning switch, you can achieve some tuning by adjusting the length of the elements. The following lengths are for rabbit ears set to 45° and connected directly to a 300W ribbon cable, no amplifier. These are total lengths measured with the elements horizontal. Keep both elements equal in length. If the elements have metal (not plastic) “eye protectors” on the ends, subtract an inch or two from the following lengths.

2 3 4 5 6 7 8 9 10 11 12 13

113” 103” 94” 83” 78” 44” 44” 43.5” 43” 42” 41.5” 41”

Additional gain is achieved by making the elements horizontal, making the antenna a straight dipole. Assuming your living room has space for this and that you can keep the antenna far away from anything metal, the following lengths (subtract an inch or two for metal eye protectors) should be used.

2 3 4 5 6 7 8 9 10 11 12 13

109” 99” 91” 80” 74.5” 84” 81” 78” 75” 72” 69.5” 67”

The straight dipole has nulls to the sides that can eliminate interference.

If for channels 2, 3, and 4 you cannot achieve the lengths recommended above then connecting large metal balls to the ends of the elements will cause a big improvement. These are called “capacitive hats” and will match the antenna as if it is much larger. Balls an inch or more in diameter can be made of loosely crumpled up aluminum foil. Make both balls the same size, then experiment to find the best antenna length.

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