Iowa Public Television

Tuesday, August 26, 2008

Are You Receiving Me?

I have been on the road a lot doing DTV information sessions across the state of Iowa. As part of the presentation I bring along my Philips Silver Sensor antenna and few different converter boxes and generally hook them up to an analog television so I can show the attendees how the boxes work. The reason I bring along the Silver Sensor is because it is small and easy to set up but I frequently have people ask me about it because a number of them have tried traditional rabbit ears and are not entirely satisfied with the results. Many folks marvel that the Silver Sensor is not amplified and yet it works in areas where their amplified rabbit ears don't. I am often asked where to acquire this antenna and if I recommend it and it is those questions that I would like to answer in this message.

First, a simple statement of fact. Indoor reception in inherently difficult and unreliable. The reason for this is that in order to just get to the antenna, the transmitted signal has to pass through walls and windows and all of the trees and anything else that is a ground level just to get to the antenna and all of these items degrade and disrupt the signal. In addition everything in the room with the antenna is a potential source of reflection which further degrades the receivability of the signal. On top of that add to the mix that anything in the room that moves, like the viewer, the kids, the dog or the cat will also dynamically disrupt the signal and further degrade received service and unlike analog television where the picture on the screen gives you a clear indication of how poor the incoming signal actually is, digital gives no indication until the decoder realizes it doesn't have enough information to make a picture and it either freezes, macro blocks (blockiness) or goes away. So I will never recommend indoor reception or an indoor antenna.

So why then does the Silver Sensor work when I am doing demonstrations? It is pretty simple really, the Silver Sensor is a directional antenna where as rabbit ears are omnidirectional. Plainly stated the Silver Sensor receives very well from one direction at a time so in a room where there may be signal coming from all directions, it ignores them and delivers a much less disrupted signal to the receiver than rabbit ears in the same location. Directional antennas also act a little like a satellite receive antenna (dish) in that they tend to gather more of the signal coming in from a specific direction and add it all together. This is referred to as the gain of the antenna and is specified in decibels (dB) which is a ratio. In the case of most antennas, the gain specified in dB is a comparison to the directional antenna and a dipole and guess what, rabbit ears are a dipole. In the case of the Silver Sensor, it has a gain of about 6 to 7 dB compared to a dipole but don't jump the gun, that doesn't mean it is 6 to 7 times better. The way the math and physics work out, the Silver Sensor probably collect about 2.5 times as much signal as a dipole (rabbit ears) and probably as important it ignores signals coming from other directions. That is why it works better than rabbit ears. I have had people say but my rabbit ears have a 10 dB amplifier built in so that should overcome the gain improvement of your Silver Sensor. This is not entirely true because remember that dB is a ratio so an amplifier with 10 dB of gain is taking the signal coming in to the amplifier and increasing the amount of signal by 10 dB. Okay but the amplifier is after the antenna so that the signal that is being amplified in the rabbit ears has already been degraded by all of the items I mentioned above and therefore all you are doing is increasing the size of the degraded signal but the receiver and decoder still can't figure out what the message is no more than yelling at someone who doesn't understand English makes them understand.

So I can't recommend an indoor antenna with any degree of certainty because the number of things that will disrupt the service using an indoor antenna are virtually unlimited and constantly changing. I will always tell people to look at outdoor antennas as their best option. I have had a number of people tell me that they are not permitted to put up an antenna because of local restrictions. I would suggest that you follow this web link (http://www.fcc.gov/mb/facts/otard.html) and I believe you'll find that many of those restrictions are superseded by federal rules that in most cases will allow you to install an outdoor antenna, even in rental properties. So you might want to look at antenna reception again. Remember, rabbit ears work great on rabbits but not so well on television signals.

Bill

11 comments:

Anonymous said...

Thank you for sharing this information on indoor reception. In the future, I hope that you do the same for outdoor reception. As a family, we are not interested in subscription services, but we want to install the best reception system that fits our geographic location, the lower-half of a slope with trees.
-- Cedar Rapids

Anonymous said...

Thank you for this very helpful post on indoor antennas.
At the meeting in Davenport, you stated that you were about to test a Winegard Sharpshooter SS-3000. How does this antenna compare to the Silver Sensor antenna? Would you care to give your opinion as which gives thr best performance?

Anonymous said...

Bill, I have a attic mounted small antenna. Just got done yesterday running new RG6 to all the TV's that will be receiving DTV. I have noticed that there is a difference in the reception from a true DTV (Sanyo) to my new Zenith converter box. The Zenith box has better reception. Is it true that the stations are not broadcasting at full power on the digital side right now?

Anonymous said...

How can we tell (now) if we need an outdoor antenna? Are there certain channels we should be receiving? For example, we don't get any sound from KCWI. Is this an indication that we need an outdoor antenna?
--Ankeny, IA

Anonymous said...

When does the repeater in ottumwa convert to digital. It seems to not broadcast anything now in analog. The digital signal from Des Moines is hit/miss even with a directional antenna.

Bill Hayes said...

I always recommend an outdoor antenna if possible. Indoor reception is always a challenge and due to the all or nothing nature of digital decoding, things that impair an analog signal and make it ghosty or noisy may actually cause complete failure of the digtial service. Also unlike analog television where the picture and sound are essentially two separate services multiplexed together, in digital television everything is data so service impairments and failures are complete rather than audio or video only failures.

Bill

Bill Hayes said...

The Ottumwa translator changed channels some time back and is now on channel 18. As for when it will go digital, we plan on converting it to digital very quickly after the analog shutoff. We have everything we need to do the conversion. However, there is now a strong possibility that the analog shutoff will be delayed until June 12, 2009 according to some legislation currently under discussion in Congress. Regardless, our translators will be converted to digital operation very quickly after the analog shutoff. I'm estimating less then 60 days and Ottumwa will probably be the first one we do.

Bill

Anonymous said...

I have an old Philips TV and I am using a Magnavox converter to get the digital signal. But the TV makes some noise. TV does not make any noise while I watch movies on my DVD. Do I have to change the converter box?

Bill Hayes said...

I'd really need to know what kind of noise the television is making with the converter box. Also, are you using video and audio outputs from the DVD player?

Bill

Anonymous said...

About the Ottumwa translator, 18, is it a very low-power broadcasting source so that 22 miles east in Fairfield I won't get it even with a directional antenna? Ditto the same with the Keosaqua translator on 54, though only 20 miles south of Ff?

Bill Hayes said...

The Ottumwa translator really isn't all that low powered. We operate on channel 18 with a radiated power of 100,000 watts. If you are using an outdoor antenna, you should receive a pretty good signal from the Ottumwa translator. The Keosauqua translator site was just rebuilt and went on the air on channel 24 at a similar power level and should also put a very good signal over Fairfield. I think you should give it a try and see what you get for reception.

Bill