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Help with 2nd antenna and aiming please!

968 Views 7 Replies 4 Participants Last post by  gjvrieze
Heh... So I have mostly got my DB8 aimed in the right place. Seems that tvfool's bearings are a LOT more accurate than anyone else's transmitter bearings. Now I run into the issue of "tweaking" the aim. By tweaking, I mean, having to move the antenna by as little as 1/16 of an inch, climbing back down the latter, checking signals, and then going back up the ladder to adjust again. I've been doing that for three days now.

I finally have it aimed where all but the local FOX comes in at about 50-70%, and I'm thinking that that is going to be the best I can do for now, until they go full power in February. What's really eating at me is that the Fox station is in and out pretty bad, while everyone else is fairly strong. And, they're all on the same tower.

So my first question is this: Can anyone recommend a cheap, but usable, hand held meter for aiming antennas? Or does anyone know how to do it using a multimeter? Back when I had satellite, the satellite guy had a nice one that told him when the dish was aimed for the best reception, and I have seen mention of hand held meters here and other places, but no idea where I could find one and buy it.

Now, after getting that one aimed, I have a second antenna. I just learned the hard way that stacking two UHF antennas back to back is not the best way to go about doing that.

SO, I will now go and buy a second roof mount and mount the next antenna. The antenna in this case is a Radio Shack U-75R. I want to point this one at about 306 Deg. to pick up stations in Greensboro, NC. The DB8 is pointed at 93 deg. to get the local Raleigh-Durham stations.

My question here is: How close can I put the second UHF antenna so it won't interfere with the DB8? The idea is to put both antennas onto a duplexer and run one RG-6 run into the house to feed the DTV tuner. Obviously, having them on the same mount, and seperated by no more than 18 inches is a bad idea. So how close CAN they be and not cause reception issues with each other, especially since they'll be pointed in opposite directions?

For what it's worth, here's my tvfool report so you can see what I mean WRT the RDU and Greensboro stations (Keep in mind that I am about equidistant from each set of transmitters):

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You also need to do your tvfool for post-transition. Your ABC station WTVD-DT will move to VHF-hi CH11.

It's not a problem to mount your 2 UHF antennas back-to-back. The problem is how you combine them. If you use a splitter/combiner the two antennas will each pick up some of the same signals which will cancel when combined. If you don't want to rotate your antenna, you will have to use an A/B switch to select which antenna you want. If you only had one channel in the other direction there are filters to combine them all into one downlead.

A satellite meter or multimeter is not going to work. I use an old Sadelco analog signal level meter which is battery operated but quite heavy. It is not designed for digital signals but it's OK for comparative readings. The newer handheld meters for digital signals are lighter but more expensive.

What I have been doing is do the rough aiming with the Sadelco meter and then do the final "tweaking" with the signal quality bar of an APEX DT502 CECB. The signal quality bar is a more sensitive indicator for aiming because it indicates the level of bits errors (BER) that are caused by multipath problems. You could use the APEX box for both signal strength and signal quality because it has 2 bars. I have recently done some experiments using the DT502 for this type of problem:
http://www.avsforum.com/avs-vb/showt...6#post15431676 post #124

There is one shortcoming of the APEX box: you can't do a manual add of a channel after the scan. You will have to do a separate scan for the other direction.
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Ugh... Forgetting a 2nd antenna and such for a moment... I can get every one of the RDU channels BUT Fox 50. I don't quite get it, because every one of them is on the same tower. All are at or above 2000 feet too.

I had Fox tuned in and got a decent, though still spotty signal earlier today and it faded as it got dark... which meant I had to hook up the analog rabbit ears and watch the orange bowl in analog...

Any thoughts? For one, I noticed that the reception greatly improved on the DB8 as long as I was standing within 2 feet of the antenna (standing behind the screen). It's around 20 feet up, but only about 2 feet off the roof, SO, I'm thinking I am going to have to buy some mast and put it 10 - 15 feet above the roof.

Sound like a good plan? I really hate to think I'm going to have to actually hire someone just to point an antenna for me. That's really embarrassing because I'm a ham, so I know at least something about directional antennas and gain and LOS signals...
Have you tried moving the antenna up/down a few "inches", or are you just re-aiming? I have seen a move up or down as little as 6 inches make a "huge" difference; even with more than adequate UHF signal levels.
I forgot to tell you why the multimeter or satellite meter will not work---the front half of the measuring system is missing: the part that tunes in the channel that you want and rejects all the others.

There is a small handheld meter called the DigiAir. The narrow input level range of 25-100 dBuV might be a problem for some users, if the specs are correct. I haven't used it, but EscapeVelocity has. I think he found that it didn't meet his needs for testing indoor antennas for his thread and ended up using the Zenith DTT900. The Zenith has a signal bar that seems to combine signal strength and signal quality in one bar. He divided each of the 3 sections of the bar into 3 parts, giving 9 steps.

No need to feel embarrassed. I've been a ham since the early 1950s, doing antenna experiments since I was 8, so I know how unpredictable they can be. And it's worse now with digital because of the "cliff-effect."

If you decide to use the Apex DT502 to tweak your antenna aim you will probably need to temporarily insert an inexpensive attenuator to keep the signal strength on the box from maxing out (but the signal quality bar should be OK without the attenuator):

The 2 Apex boxes that I measured maxed out at +12.5 and +2.2 dBmV. If you mean the FOX on CH49, you would probably need about 20 or 30 dB of attenuation, allowing for the gain of your antenna:

-36.9 dBm + 48.8 = +11.9 dBmV

But, your problem is not signal strength, it's signal quality. Note how critical IDRick says that signal quality is for his computer capture card:
Originally Posted by IDRick /forum/post/15431676

While I certainly agree that there are far better tools for measurement, the CECB's can be very useful for finetuning antenna location. In my case, I have broadcast towers in two directions (200 and 270 degrees). My DIY 4-bay does a nice job of receiving signal from both directions with proper placement in the attic. The APEX 502 was invaluable for optimizing signal strength of my two lowest channels (1 at 270 and 1 at 200 degrees) and deciding final antenna location/orientation.

Signal quality readings from the APEX also had practical application. My antenna is connected to two tvs and a computer capture card. Anything less than 100 on signal quality scale results in audio and video dropouts with the capture card. I get marvelous recordings with proper placement of the antenna (optimized signal strength and signal quality = 100). I'm sure top quality equipment could fine tune my setup even further. I'm satisfied with a $20 tool versus a several hundred dollar tool, especially for a one time use.

Your FOX signal acts like it's right on the edge of dropout, which happens when the error correction system (FEC) is almost unable to keep up with the increasing error rate (BER). The factors that reduce signal quality and cause a higher BER are:

1. Improper signal level: A weak signal will cause a poor signal-to-noise ratio; a signal that is too strong can overload a tuner or preamp. A nearby FM transmitter can also cause overload, which would require an FM trap.

2. Reflections from multipath problems.

3. Impulse noise in the reception area.

I agree with Digital Rules. The location of the antenna is just as important as the aim. Try a different spot and monitor the signal quality. You have plenty of signal strength.

When I was testing the DT502 with my CM4221 antenna on my marginal channel (13.1 on RF41) I got:

Signal Quality 60%

Signal Strength 55%

I had aimed the antenna with my SLM, but when I rotated the 4221 slightly to the right I got:

Signal Quality 100%

Signal Strength 56%
Note the BIG change in signal quality with only a slight change in signal strength.

It seems that the signal quality indication is a more sensitive aiming tool than signal strength, because it shows the increase in BER from multipath reflections. In my situation the BER is affected by the weak signal, the fixed multipath reflections, and the changing multipath reflections from cars passing by (Which shows the need for the new ATSC M/H standard).

Downunder in .au land, they have a good understanding of the importance signal quality, BER, and MER:
http://www.dtvforum.info/index.php?showtopic=18627 see post #11
http://www.dtvforum.info/index.php?s...=360&start=360 #366 & 369
http://www.hillsantenna.com.au/arbitrage/pages/61 BER
http://www.hillsantenna.com.au/arbitrage/pages/60 MER
http://users.tpg.com.au/bitek500/Dow...M%20instal.pdf p4 DVB-T digital cliff
http://www.bbc.co.uk/rd/pubs/papers/.../crn-dvbtm.pdf ATSC vs DVB-T, excellent charts on p3
http://www.wowvision.tv/signal_strength_meters_BER.htm Signal strength meters and BER
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OK... found out some more info. The Fox channel (50) is on a backup antenna lower on their tower due to still ongoing work by WRAL which is far higher up the channel. SO, for now, I may have as good as I'll get until WRAL finishes their antenna swap. ALL my local HD channels are on the same tower it seems (what great planning that was).

HOWEVER, I now have good signal on some channels, not so much on others... sigh...

I am going to have to get a couple pieces of mast and raise the antenna about 10 feet off the roof top, I think. It's only about 2 feet off the roof right now. I noticed that the fox channel was perfect, near 100% signal and quality, as long as I was standing directly behind the mesh on the DB8. As soon as I backed away a few feet, the signal went back into the crapper... any ideas?

Anyway, Until I can find a new way to mount the antenna (I'm thinking 10 - 15 feet of mast and a tripod roof mount for now), I'm stuck. BUT at least they are all coming in better across the board (other than Fox 50).
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By standing behind the antenna, your body reflected the signal towards the antenna which created a sweet spot for channel 50. You might only need a "very small" height adjustment to get 50. The signal strength is probably sufficient, but because of UHF scatter, your antenna is not seeing it. I've seen this happen many times.

Originally Posted by Digital Rules /forum/post/15465404

By standing behind the antenna, your body reflected the signal towards the antenna which created a sweet spot for channel 50. You might only need a "very small" height adjustment to get 50. The signal strength is probably sufficient, but because of UHF scatter, your antenna is not seeing it. I've seen this happen many times.

Yes, Digital Rules, you nailed that right on the head....
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