Request for antenna/preamp recommendation

[spacedman]

Hi,

I was hoping for an antenna recommendation to service my elderly parents home.

I picked up a couple Zenith DT900 digital converter boxes and set top rabbit ears only pick up 2 stations and none of the big 4 networks. If possible I would prefer to do an attic mount in the crawlspace of their single story ranch as they are opposed to an outside antenna. There are 3 different televisions that need a hookup to the antenna.

I'm reasonably geeky in that I've built my own Mythtv system and made a DB2 clone for my own use but I've never done an install with multiple ~100ft cable runs to separate televisions so I'm unclear on amp/preamp choices and it seems like I would also need something like a distribution amp to service 3 tvs.

I've attached tvfool data for the location. My intention is to take my homemade DB2 up into their crawlspace as a test run to see if I can get anything at all. Initially it seems like a CM4228 might be a good candidate as most of the stations are in the same general direction and there are 2 must have(Fox8/CBS10) VHF Hi stations.

I'd appreciate any experienced advice.

 

[dish_in_the_sky]

OTA reception...

I'd recomment getting the antenna outside, above the roof.

You'll lose significant signal (3-15 dB) in an attic. The wide variation in loss is due to many variables involved, from multipath to frequency-dependent RF loos to detuning wires, etc fount in an attic, to water absorption.

Every dB is precious. An antenna with good gain outside, using RG6, will help a lot. A preamp, such as the channel master 7777, at the antenna, will preserve the S/N ratio through splitting. I suggest a 15 dB gain VHF(hi)/UHF antenna placed high and outside with a channelmaster 7777 preamp for your case. If you want everything you can get, add a 10 dB distribution/line amp at the splitter (better: 5-10 dB variable gain here.) If 15 dB means too big an antenna at Ch 8 and 10, 8 dB gain would make it, but not by much.
Ideally use a VHF-Hi and separate CM-4228 for VHF and UHF, with the 4228 at the top.

A CM4228 sounds like a reasonable antenna to try, EXCELLENT at UHF, but might be insufficient on Ch 8-10 for you. A 16 dB S/N ratio is required for reliable ATSC reception, 15 dB S/N is right at the edge. 14 dB, forget it.

Given the receiver sensitivity of -83 dBm, 4-way split of 5-6 dB, coax loss of ~8 dB, you need a minimum signal strength of -83 +6 +8 = -69 dBm at the antenna.

As practical antennas run ~15 dBi gain (12-19dBi depending on antenna), you can subtract that: -69 dBm -15 dB +1 dB (balun loss)= -83 dBm minimum signal on tvfool (I assume tvfool means isotropic dBm=antenna with no gain.)

Now add 3dB for *some* margin (rain, airplanes, leaves, wind moving antenna) and you have -80 dBm minimum signal.

As your "must-have" signals are -81 and -87 dBm, they are below -80 dBm, and too low for reliable reception with just an antenna.

A higher gain (19 dB) antenna is expensive, LARGE and hard to aim (undesired? rotater needed..) but would get you 4 dB more, for -84 dBm.

Skip the larger antenna idea....

A preamp? Channelmaster 7777 2.8dBNF at ~23 dB gain:
Now noise floor comes in. The 2.8 dB number is how much noise the preamp adds compared to a 75 ohm resistor at 290 deg. Kelvin (about 17 deg C). The resistor noise is -95.2 dBm, our reference level. The preamp output noise is equal to -95.2 dBm + 23 dB (gain) +2.8 dB. Converting that to input noise (the resistor and preamp noise are uncorrelated, and it's a complex calculation) yielda a preamp input noise floor of -98.5 dBm.

Your antenna signal must be 17 dB above this noise floor (1 dB loss through balun, 16 dB S/N for digital signals.)

With the antenna gain of 15 dB and a signal of -87 dBm, you have -87 + 15 = -72 dBm at the antenna terminals. -1 dB through the balun, now -73 dBm.

98.5 [-dBm] - 73 dB [-dBm] gives an S/N of 25.5 dB, adequate S/N for solid reception at -87 dBm.

Now, overload: Your strongest signal is at -49.9 dBm (UHF ch 23), add 26 dB (UHF) gain, 15 dB (antenna) -1 dB (balun loss), and you have -9.9 dBm output. The rated max output is 51 dBmV (1 dBmV=-48 dBm) or +2.25 dBm. You're 12 dB under this, so TV channels won't overload you.

I only considered one channel for overload because the others are at least 2 dB lower, and this will result in <1 dB error, insignificant in your case. (I assume the nax output rating is for 1 channel)

But you might get overload at FM or other frequencies. This is best tested with an analog TV for channels present on (actually) empty channels, but could also be noticed if digital quality drops on any channels when the only system change is adding a preamp.

BTW: Your minimum TVFOOL signal on VHF is -97.5 dBm (hairy edge, decoding problems), and minimum reliable is -96.5 dBm), assuming 15 dB antenna gain, 1 dB balun loss, and the C-M 7777 preamp at the antenna.

Note that with a 0.5 dB NF preamp, 0.5 dB balun loss and 17 dB antenna gain, the noise floor for digital signals on tvfool is -106 dBm. This is hard and expensive. From here, only cryogenic cooling and BIG antennas will help.

Hope this helps!

 

[salemtubes]

For the most part, Dish_in_the_sky's advice is excellent. I live in Salem and receive the Cleveland stations using a CM 4228, a Winegard YA-1713 high band VHF antenna and a CM 7777 preamplifier. I'm about 55 miles from the Cleveland transmitters.

Based on the TV Fool information, I'd guess that your actual location is on relatively low ground (1 edge and 2 edge at 40 miles from the Cleveland transmitters and 2 edge at about 30 miles from WNEO), which means you'll need a large outdoor antenna setup mounted as high as possible. You might also want to consider an Antennas Direct 91XG for UHF reception. It has a much lower wind load than the 4228. If I were to do my setup over, I'd use the 91XG instead of the 4228. You will also need to use a high band VHF antenna like the YA-1713. The 7777 has inputs for both a VHF and UHF antenna. I do not recommend a combination UHF/VHF for your location. I realize that your parents do not like the idea of an outdoor antenna, but they may not have a choice if they truly want to receive WOIO and WJW via an antenna. WKYC, channel 3 will broadcast its digital signal on channel 2 until 2/17/09 and will then move to channel 17 on that date. You probably will not receive channel 3 with the YA-1713/4228 or YA-1713/91XG setups until 2009. If your parents don't want to wait until 2009 for channel 3, you can use an antenna like the Winegard HD-4053P instead of the YA-1713.

WOIO, channel 19, is broadcasting its digital signal on channel 10 at very low power, 3.5 kW ERP. They will stay on channel 10 after the analog shutoff with no increase in power. On 2/17/09 WJW, channel 8, will move its digital broadcast from channel 31 to channel 8. WTRF, Wheeling and WTOV, Steubenville will be moving their digital broadcasts to channels 7 and 9 respectively in 2009. WTRF's transmitter is about 60 miles from Canton and will not be at full power until after the analog shutoff. There are some FM stations in the Canton area; so you'll want to make sure that the FM trap switch of the 7777 is in.

You'll need a rotator if you're interested in receiving more than the Cleveland and Akron stations. You will probably be able to receive at least two of the Youngstown stations (WKBN and WFMJ). Youngstown's WYTV will probably be a problem due to its low power and low transmitter antenna height.

With multiple long coax runs, you will probably need a distribution amplifier with multiple outputs placed between the 7777's power supply and the TVs. You'll then run a separate coax from the distribution amplifier to each TV. Make sure you use high quality quad shield coax.

 

[spacedman]

Wow! Outstanding and thoroughly informative posts guys. I really appreciate the effort and time. I'll post back w/ results and such once I've completed the install.

 

[dish_in_the_sky]

Glad to be of help... Let me know what I missed so I can do better next time.

I'm an RF engineer, no experience in receivers, but read up on OTA TV DXing. Any advice from those more knowledgeable is appreciated and I will use it to give better advice, as well as use it for my own project.

Incomplete or inaccurate advice can be expensive, maddening or discouraging.

Here's what I'm (slowly) working on...

I've been researching OTA VHF/UHF DXing in order to try to grab some Canadian stations from my location near Seattle, a challenging task at best. My wife might be able to get some Chinese-language programming that way, in addition to FTA satellite.

The biggest challenges are a shoestring budget and very limited time with three small children!

I'll be building a ~0.5 dB NF UHF preamp (can do Ch 7-up on VHF as well) with some Agilent PHEMT FETs. Since I don't have adequate tools available to properly simulate or test these devices out to 10 GHz, I'll simply duplicate a 100-500 MHz 20 dB reference design Agilent (Avago) has--they DID test (or at least simulate) it to 10 GHz! Even though they claim 500 MHz is the upper limit, from their data, it performs very well to 1 GHz, though I might have to filter out at >750 MHz to remove cell phone interference (I'll certainly find out!)

I plan to use a pair of equal delay transmission line transformers (6.25:1 Z ratio, 5:2 voltage, 25:4 impedance) for fairly close impedance matching the 300 ohm antenna Z of a double-bay Gray-Hoverman antenna (UHF only) to the 50 ohm preamp input impedance, running this preamp up at the antenna. Of course a 50 ohm 1:1 balun at the preamp will be required. The preamp's output will be run through another equal delay transmission line transformer to bring it up to ~75 ohms for launching into RG-6. The practical limit for receiving digital signals with this will be ~ -106 dBm (isotropic) for Channels 28-51 if I recall the antenna gailn plots correctly. The noise floor (except manmade/optical noise) will be 16 dB lower. At the gain peak of the antenna, I can probably reach -109 dBm isotropic. A second (switchable) preamp will be necessary to get 20 more dB of gain for the weakest signals.

Unfortunately, my location has some hills and trees :-( I can't do much about that, even with my antenna on the roof. A 1000 foot tower would help...but not on my land or budget!

 

[spacedman]

This install was delayed longer than expected but I finally completed the project w/ much better than expected results. As an outside roof mount was ruled out for aesthetic reasons and also the additional labor required to follow code by getting grounding from an antenna mast/grounding rod over to the house's electric service grounding I decided to cross my fingers and do an attic mount.

This single story ranch house has a trussed roof w/ plywood, felt and standard asphalt shingles. I mounted an Antenna's Direct 91XG UHF and separate Winegard YA-1713 Hi-Band VHF to a 5' antenna mast section. The mast is mounted w/ two 4" wall-mount brackets attached to two 2x4 boards attached to the truss cross members.

I also went w/ the Channel Master 7777 preamp which is then fed into a Channel Master 3044 4-way distribution amp, both mounted in the basement and patched into the house's pre-wired coax. I used Belden 7915A RG6 and Thomas & Betts SnapNSeal compression connectors for the 100+ foot run from the attic to the basement. Unfortunately the house's pre-wired coax was done w/ the world's cheapest RG59 but there wasn't much I could do about that.

The end result was about as perfect as I could hope for. The install gets a lock on all the Cleveland transmitters except WKYC NBC channel 3 which isn't too surprising given that a Hi-Band VHF antenna is being used. Once they switch to UHF 17 they should be received easily. The most marginal station was WOIO CBS 10, presently broadcasting w/ 3kW. The install receives this station w/ about 75-80% signal strength. Once they pump the power to 8.75kW in February reception should be even better.

It remains to be seen how well this setup will perform w/ a layer of snow on the roof but thus far things look great.

Thanks so much for the detailed posts above that put me on the right track.

 

[Don_M]

It remains to be seen how well this setup will perform w/ a layer of snow on the roof but thus far things look great.

Don't worry. We've had an attic antenna for eight years and had no reception problems with snow on the roof -- not even after the 42-inch blizzard we had five years ago. My impression is that, if anything, reception gets slightly better during and after snowfalls. We only had DTV for the second half of last winter, but reception patterns didn't seem to be any different versus analog during that limited period of time.