10/20/04

The 7.5” loop is representative of a large class of indoor antennas built around a loop or something similar. These may include units with amplifiers or switched “tuning circuits”, and frequently VHF rabbit ears are included.

Simple loops

If this is all you need, great! Some simple improvements are possible :

• You can add three feet of twin-lead (300 ohm ribbon cable) to it. This will get it away from the body of the TV set and allow you to rotate it. But twin-lead must always be kept one inch from anything metal and must not touch any substantial thing that is plastic.
• You can rescale the loop for your weakest channel. Use this formula : dia=4537/(6*N+389) where N is the channel number (14-69). (Plugging 36 into this formula will give you 7.5. Multiply that by 3.141 to get the total wire length.) 12 or 14 gauge copper wire is fine. Remove any thick insulation.
• Higher gain is possible with a double-loop (J2 on the gain graphs). However it has a narrower bandwidth. This “figure-8” antenna should look like two tangent circles, one above the other, fed at the bottom. Keep the wires a quarter inch apart at the crossover point. This formula gives the diameter of each circle : dia=4244/(6*N+389) Multiply this by 6.283 to get the total wire length.

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 a loop antenna is what is shown in the net gain graph, not what the maker claims.

Most receivers have a UHF noise figure somewhere in the range 3-10 dB, with 5 dB being typical. If the antenna amplifier has a noise figure below 3 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 noise figure below 3 dB.
2. you need a longer cable to reach a good reception spot.
3. you want to feed two sets using a splitter.
4. you want to recover the 1 dB loss in the 300W-to-75W balun and the coaxial cable.
5. your TV has 300W UHF 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 in some cases it is mainly for the VHF channels and does a poor job on UHF. (There is no way to tell before you buy.) A UHF-only antenna with a tuner is a good buy. You may find you can tune it to your weakest station and forget about it.

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