Amplifying a weak OTA signal - AVS Forum
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post #1 of 12 Old 01-15-2002, 09:35 AM - Thread Starter
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I am in Chicago and I get most of the DTV channels available in my area. However the WBBM-DT is only registering about 28-35 on the antenna strength meter. Every now and then it will pop up just over 40 and I will actually be able to see a picture. Is there a relatively inexpensive amplifier I can buy that would boost this signal? For some reason the 2 types of amps I have tried Both from Radio Shack, haven't worked. They actually kill the reception of all my other dtv signals. Please help!!!
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post #2 of 12 Old 01-15-2002, 09:58 AM
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A bigger antenna is in order.
Or
More Height?

Studio Broadcast Engineer
KET
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post #3 of 12 Old 01-15-2002, 01:24 PM
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If the range of human hearing extended to 1 GigaHertz instead of 20 kHz, then your RatShack amplifier would make a dandy little fuzz/distortion gizmo for your local grunge bands' hyper-electric guitar. Otherwise they aren't of much use. Has anyone with a spectrum analyzer on hand measured their utility as a frequency multiplier, intermod mixer or the like for some ham project? I suspect that they are good for something, just not sure what yet.

My favorite still being Winegard, just 'cause that's what I have experience with, but Channel Master is probably good also. I did discover something VERY interesting last night about my 20 year old Winegard. The UHF antenna is a really cool design where the pre-amp is built into a cartridge that plugs into a special slot built into the antenna, or else you can plug in a simple "pass-through" card. Either way this card is what matches the antenna from 300 ohms to 75, whether amplified or not. The thing I like about this is that you don't have to have a 300 to 75 ohm balun attached to the antenna - together with the almost certain mismatching and stray coupling that you are likely to see in the vicinity or even such a short undisciplined piece of twinlead.

On the other hand - this totally great and remarkable piece of antenna engineering came with a really cheapo piece of crap aluminum box that feels like someone's junior high school science project --- that they had the nerve to call a power inserter. The box says something to suggest that it inserts 18 volts AC onto the wire - so I don't know off hand if I could power it off the power to an LNB, not even gonna go there.

So what I did last night is to take a standard RatShack UHF-Satellite diplexer and use that as a replacement power inserter! I did this by taking the 18 volts from the cheapo aluminum box and feeding that to the Satellite connector on the diplexer, that is the connector that blocks UHF but passes DC and Satellite signals. Then I hooked the UHF antenna port to the TV - since that port only passes UHF, but NOT DC! (or AC). Hooking the 18 Volts AC to the other port doesn't hurt the diplexer because it is a completely passive biderectional device. Be careful if you try anything like this with an active device or you may seriously regret it! Finally I connected the Satellite+UHF+power port to the feedline to the UHF antenna -- and shazzam -- I know had an ultra high quality power inserter for my UHF antenna, with in all likelihood a very low SWR and insertion loss - especially as compared to the old cheapo/flimsy aluminum box from before.

Of course the old cheapo aluminum box is still being used as a power source, but now that it is piping the power into the diplexer port it is only being used as a power source as it no longer has the UHF passing through it.

Results:

5-1 on UHF 29 was 40-50 units, now 60-72.
7-1 on UHF 24 was 83-90 units, now 93-100
20-1 on UHF 19 was 60-70 units, now rock steady 79.

5-1 went from unwatchable to sometimes watchable. 20-1 went from sometimes watchable to usually watchable. 7-1 and others were already pretty good, but its nice to have more margins.

Moral: it don't matter how good the antenna is, and the coax and all the rest - if you piss it away at the TV with anything that mismatches the line or whatever the like -- you're still screwed. So if I were you - I would definitely look at the quality of ALL of the components and make appropriate adaptations ...

I'm a lot happier today than I was over the weekend when I raised the antenna to 41 feet and re-guyed it only to find that channel 5-1 had gotten worse! Maybe its time to replace that antenna with a fresher one?

No warranty is either expressed or implied. The universe is a figment of its own imagination. Your milage may vary. Not responsible for lost homework, missed bus transfers, or forgetting to buy a lottery ticket the day they run your birthday. Have fun.

Have spectrum analyzer, will travel.
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post #4 of 12 Old 01-15-2002, 02:31 PM
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I agree the whole system must match. If you have a very long antenna lead, a pre-amp can help with the loss from cable, but will also insert noise into the signal. The more noise, the harder it is for your STB to decode the signal. The meter on your STB does not measure signal strength, it is measuring the amount of error in the signal. Reducing noise will improve the quality of the signal. 100% = no errors. Going to a larger and better quality cable can give you a boost in signal without degrading it with noise. Also every connection can reduce the signal, so use high quality fittings.

A 20 year old antenna could have degraded performance due to corrosion. I don't think the power supply has much effect on the signal, as long as you are getting the correct voltage. If you are adding extra connections to the cable with the power inserter, you might be better off with a new antenna, amp, and cable(if it's 20 yr old too).

-Glenn
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post #5 of 12 Old 01-15-2002, 03:24 PM
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Yeah, well I got the HD reciever in December and there are these things called rain and work that limit my time on the roof, darn!! The feedline from the antenna is RG-11, but yes I am ashamed to admit that its about 20 years old also. I did have the sense however to whack a couple of feet off of each end in order to get rid of the pieces which are most likely to have absorbed mosture over the years. The dielectric material did look kinda yellowish for about three inches I guess - so I whacked off a couple of feet to be sure - and put on new F connectors also.

About a month ago I gave the preamp module a good rinse with WD-40 to get off some of the gunk - there was surprizingly very little, and then cleaned off the residual WD-40 with a good long rinse in warm sudsy water since the WD-40 might have some undesirable dielectric properties that could detune things. I should mention that the old carthrige amplifer is actually a completly open circuit board with visable soldered components - not a sealed unit like you would expect these days. It is after all made to plug into a cooresponding moduler slot in the antenna. And dont laugh too hard -- a bunch of computer motherboard manufacturers have had to come up with water based board cleaning/rinse procedures in production because of the various bans on chloro-flourocarbons!

I suppose would still say that the cobblers children have no shoes. I do afterall have an Engineering-Physics degree from UC Berkeley, had a year of senior coursework/labs in microwave/RF electrical engineering in that program - and worked for various broadcasters as an RF engineer before going to work for an Internet company. But there I was last month, scrubbing boards and all the rest. Cool, huh?

My biggest problem I think at this point might be that I can get amost cable grade reception on a couple of Modesto or thereabouts stations on channel 19 and 29 when pointing the antenna the east, vs channel 5 on 29-1 and 20 on 19-1 to the west. So I've looked at the .pdf files from Winegard giving the specs, polar plots, gain and front/back ratios for their current line - and I've noticed that their highest gain UHF antenna does NOT have the highest front/back ratio! (I think its 14db) So maybe I really want the medum gain antenna which is more like 19db.

So there I go again, ready to plunk down the credit card to try a fresh antenna - but its a bit uncertain as to whether I want something super directional with a lot of gain for best noise figure, or if I want a higher front/back ration for NTSC co-channel rejection. I've already spent a LOT of money as it is.

BTW: it is best to minimize the number of splices like you said - but using a diplexer as a UHF power inserter only passes the UHF-RF through the combiner port and the UHF port, which is exactly the same is passing the whole signal through the crappy old aluminum box. The third port of the diplexer is only being used to insert DC from the crappy aluminum box, and the second connector on the crappy box is not used, i.e, the one that would normally go to the set .... and all of this did result in an improvement.

No warranty is either expressed or implied. The universe is a figment of its own imagination. Your milage may vary. Not responsible for lost homework, missed bus transfers, or forgetting to buy a lottery ticket the day they run your birthday. Have fun.

Have spectrum analyzer, will travel.
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post #6 of 12 Old 01-15-2002, 05:38 PM
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The problem with buying a new antenna is that it may take 3 or 4 before you find the right one. If you still have the specs on the old one, try to match it or find one that improves on its weakness. What model do you have now?

Inside a Channel Master pre-amp is just a open circuit board. I just ruined mine by allowing it to collect water, it quickly corroded the copper windings around one of the small coils and died.

http://atechfabrication.freeservers....de_amp_001.jpg

-Glenn
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post #7 of 12 Old 01-15-2002, 05:52 PM
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I'm using a channel master 0064 DSB with good results.

http://www.channelmaster.com/pages/ss1.htm
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post #8 of 12 Old 01-16-2002, 05:11 AM
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Glen,
Thanks for the testing info. I have a question. Your site lists 3 CM antennas, yet you only provided testing data for the 4248. Do you have data for the 4228? I'm very interested on how these two compare. Thanks.
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post #9 of 12 Old 01-16-2002, 10:08 AM
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glgorman wrote:
[quote] About a month ago I gave the preamp module a good rinse with WD-40 to get off some of the gunk - there was surprizingly very little, and then cleaned off the residual WD-40 with a good long rinse in warm sudsy water since the WD-40 might have some undesirable dielectric properties that could detune things.

WD-40 is not the best stuff for cleaning electrical contacts. Deoxit is considered the best by people who restore expensive old gear. Read about it at : http://www.caig.com/descript/deoxit.htm


Ernie/NE6D
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post #10 of 12 Old 01-16-2002, 11:01 AM
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gocubz51:

Try hooking a portable t.v. set to your antenna downlead and judge quality of reception on chs. 2 & 5.

If they're okay, then WBBM-TV's digital signal on ch. 3 should be okay, too.

Repeat after wiring the amplifier between the antenna and portable set to see how the amplifier affects analog reception. (The digital signals are similarly affected.)

All the other Chicago digital stations are transmitting on UHF, so be sure your antenna and preamplifier are rated to cover the VHF frequencies.
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post #11 of 12 Old 01-16-2002, 11:46 AM
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Im not sure how far away the tower your trying to pick up is, but after spending 100's of dollars and many man hours on the roof with friends and family, I found out that weather and the time of day influences the signal more then anything I/we could do. Of course Im trying to pick up a signal about 60 miles away. I eventually gave up...

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post #12 of 12 Old 01-16-2002, 10:32 PM
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Tim,

That is a very good question. I have gotten so caught up in the cutting edge of antenna design that I lost focus on the more common and readily available antennas.

Please see this post for test info regarding the the CM 4228

http://www.avsforum.com/avs-vb/showt...0&pagenumber=1

The 4228 is a great antenna for locations that have a fairly good line of sight with transmitters. It was the top-of-the-line in the 50's and is still a top performer. There is no such thing as a "HDTV" antenna.

-Glenn
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