Question: Lost Cause

LoTech

DTVUSA Jr. Member
#1
Hi guys. I live in a mountainous area where reception is almost nil. Here is my TV Fool report. I can receive WYMT pretty good anywhere around my house with an old Gemini A20. I'm not really interested in WAGV, and I don't get a blip of signal on WCYB and WVVK, but I'd like to get WKHA.

With my antenna mounted in my attic I get a signal strength of 95 and signal quality of 93 (scale max - 100) on WKHA with a Coolsat 8000 HD receiver, but no signal lock. No lock on the ATSC input on my RCA TV either . My guess is multi-path interference. I have mountains on all four sides, and I'm sure that I'm getting reflected signals. Heck, I can turn my antenna 87 degrees west and get WYMT bouncing off a mountain.

My question is whether or not I can do anything about multi-path, or is it a lost cause?
 

dkreichen1968

Moderator
Staff member
#2
The conventional way of dealing with multi-path is to use a high gain antenna with a high front to back ratio and narrow beam width. On an improvised basis you may consider building a chicken wire mesh or foil barrier behind your antenna and grounding it to block signals coming from those directions.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#3
:welcome: LoTech,

Aside from mountain-caused multipath, you may have problems caused by your antennas' close proximity to furnace ducting, foil-backed insulation, etc. Have you tried your antenna outside?

Here is a link to a website showing an experimenter's extraordinary measures to block multipath. There are two pages with two examples. I have no clue why the website is titled as it is: Key in text the normal way

Jim
 

MrPogi

Moderator, , Webmaster of Cache Free TV
Staff member
#4
Praise be to Allah! I have never seen a TVfool such as this one...

Seriously...
WYMT-DT (Digital)

Channel: 12 (57.1)
Network: CBS
Maximum ERP: 50.000 kW

Ouch. Helluva hole you're in there.
Get your antenna out of that attic, and put it on the roof if its at all possible, and try several locations on the roof: vary the location and height.
If that doesn't work, try dkreichen1968's suggestions.
Failing that....
Then try Jim's suggestion. Wow, that's extreme! Jim, how would you mount that on the roof and deal with the wind load?? You'd have to come up with a framing material that can take the elements, and wood is kinda heavy.

I would you call that thing, the "TV Bazooka"? The "GhostBuster?"
 
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LoTech

DTVUSA Jr. Member
#5
Partial success. My wife was out of town today, so I had time to play with making an antenna. I decided on a placing two sets of dipoles cut to channel 16 from copper tubing into a collinear array with a wire mesh reflector.

-------||------- -------||-------

-----------------------------------

It was rough with 14-2 copper wire for the phasing lines, but it allowed me to lock WAGV with a solid signal quality/strength of about 87-90. No more wild fluctuation of the signal. This was a good sign as the array was cut for channel 16 instead of channel 51. When I tried it on WKHA (16) I got signal that would jump from 70 to about 85, fluctuate a little, and then drop back. Sometimes it would go up to 90, but no signal lock. With the fluctuation in signal quality, I guess I'm still getting interference.

When I get time I'm planning to modify this antenna or make a new one. What would be better in terms of eliminating multipath interference; to put a corner reflector on my two dipole antenna, or build a 2-high x 2-wide array with a flat reflector? If the 2x2 how would I run the phasing lines?
 
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FOX TV

Contributor
#8
It looks like a 50 foot tower with separate antennas for VHF and UHF would be in order. The UHF antenna should be a back plane reflector type such as the Clear Stream C series antennas, as that design has proven to be the best method of combating multi path for UHF TV signals.

In my area in the past, it took large towers with high gain antennas to get signals in a multi edge, low signal path such as yours, and this method still applies today. An antenna can only receive what it sees, and in the case of UHF, the signals do not traverse deeply into valleys as do longer wave VHF signals. The only way to get these signals is to get the antenna higher up into the signal stream for the UHF signals, as they simply pass over the valleys and your low mounted antennas.

This would also be one situation where an amplifier may help in reception of these signals when combined with lots of elevation, high gain antennas, and quality cabling. It may be an option to try and find someone who could perform a signal strength test before you spend big money on trying to receive these signals. Some of the higher end satellite installers may have the needed equipment to perform theses tests, or you may possibly find someone in the local Amateur Radio community with the capability to perform these tests.
 

rabbit73

DTVUSA Member
#9
You will need to use extreme measures to have any chance of success at your difficult location, which will require three outdoor antennas.
For WYMT on RF12 you will need a VHF-hi (7-13) antenna, like the C5, or 7-13 yagi, aimed at 280 degrees, or whatever azimuth the terrain dictates.
For WCYB on RF5 you will need a single channel yagi designed for channel 5 or a very high gain VHF-lo (2-6) antenna aimed at 140 degrees.
For WKHA on RF16 you will need a very high gain UHF yagi designed just for channel 16 or a high gain UHF antenna like the 91XG aimed at 279 degrees.

The channel 12 and 5 antennas should be combined with a HLSJ. The output of the HLSJ should go to the VHF input of a CM7777 preamp. The channel 16 antenna should go to the UHF input of the preamp.

When I get time I'm planning to modify this antenna or make a new one. What would be better in terms of eliminating multipath interference; to put a corner reflector on my two dipole antenna, or build a 2-high x 2-wide array with a flat reflector? If the 2x2 how would I run the phasing lines?
If you want to build your own CH16 antenna, I suggest nothing less than a 4-bay antenna with reflector screen designed just for CH16. This will require longer elements and greater vertical spacing of the collinear bays than any commercial antenna to maximize the CH16 signal. The closest commercial version is the CM4221HD.

For your DIY CH16 antenna the halfwave elements of the collinear pair, each side of center, should be about 11 inches long; the vertical spacing of the bays about the same. The formula is approx 5540 divided by the frequency in MHz, with the answer in inches. The reflector should be spaced about 5 to 6 inches behind the elements. The ends of the elements, where they connect to the vertical phasing line, should be supported on high quality insulators to reduce loss. (Actually, the best place, as far as losses go, to support the elements is only at the center of each halfwave, but this is not usually practical.)

It may be an option to try and find someone who could perform a signal strength test before you spend big money on trying to receive these signals. Some of the higher end satellite installers may have the needed equipment to perform theses tests, or you may possibly find someone in the local Amateur Radio community with the capability to perform these tests.
Good idea; I would be lost without my signal level meter (SLM).
But, the signal indicators on your equipment should help.
 

Attachments

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#11
I would try one at a time, starting with CH12, then add CH16, and finally CH5. Since your reception will be at or near the "digital cliff," don't expect it to be consistent from day-to-day. If your signals were a little stronger, the AGC would smooth out slight changes in signal strength.

It's actually the sensitivity of the tuner rather than a signal level meter that determines how well you are going to do at the "digital cliff." Because you are already getting as much as you are, the tvfool report might be a little too pessimistic. The antenna gain has to make up the difference between the signal power indicated in the tvfool report and the actual tuner sensitivity at about 16dB SNR. In your case, for CH16, there are no antennas with that much gain, except for EME (moon bounce communication); the difference between -112.7 dBm and my -86.0 dBm is 26.7 dB.

This is an extract (it doesn't look right unless you log in, because the ads on the side squeeze it out of shape) from a recent tuner sensitivity test; 4th table:
Code:
Attn    Apex   Centronics       Sony          SLM    dBmV corr  Equiv
 dB    Q    S    Q    S      S   Er   SNR     dBmV    for amp    dBm
  0   100  54   100  58     66   0    25      -6.1    -22.1    -70.9
  3   100  49   100  53     62   0    24      -9.3    -25.3    -74.1
  6   100  44   100  49     58   0    23     -12.1    -28.1    -76.9
  9   100  38   100  44     55   0    21     -14.9    -30.9    -79.7
 12    77  27    65  38     55   0    18     -18.1    -34.1    -82.9
 15    26V  0    20V 27     55   0    16     -21.2    -37.2    -86.0
 18   dropout   dropout     55 3099V  13F    -24.3    -40.3    -89.1
A "V" after a figure means that it varies; the Sony errors are updated about once every second. An "F" after the SNR means that the display is frozen at that value. Adding even more attenuation makes the Sony screen go black and say "no signal." At least 15.5 to 16 dB SNR is needed to maintain lock.

Get your antenna out of that attic, and put it on the roof if its at all possible, and try several locations on the roof: vary the location and height.
Yes, a slight change in location can make a big difference.
 
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