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Old 23-Aug-2013, 8:40 PM   #61
tripelo
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Multipath: Antenna Elevation Pattern Discrimination

Elevation angle discrimination is a possible mechanism allowing this vertical stack to reduce frequency dependent nulls, presumably a result of multipath. Elevation angle discrimination depends on the elevation beam width of the combined antennas. The antenna’s elevation pattern also depends on height above ground.

Elevation angles of reception have importance in that signals from distances beyond the horizon can arrive at different angles with respect to the horizon. Communications research indicates, in general, lower angles near the horizon are favored for long distance reception. This may particularly apply to long distance TV reception because broadcast transmit antennas are designed to ensure maximum radiation is below the horizon (negative elevation angles). In general, full-power broadcast antennas have a narrow vertical beam width and also incorporate downward beam-tilt. With narrow vertical beam patterns and beam-tilt, the transmitted signal strength decreases rapidly for angles above the horizon.

At the receiving antenna, the effects of the ground reflections cause great attenuation at an angle of zero elevation degrees. This is due to the reflected signal being 180 out-of-phase with respect to the direct signal, and cancellation occurs.

Two conditions are necessary for a vertical stack of receiving antennas to achieve both a low angle and narrow elevation beam width:

A. Height above ground of several wavelengths
B. Large stacking separation

Computer simulations with 4NEC2 software provides for graphing the vertical beam pattern of antennas. The horizon is shown on the 4NEC2 graphs as 90 degrees. The image below shows on the left, the upper half-pattern in free space of a single long-Yagi; overlaid is a stack at 89 inches.

The second pattern, on the right side, shows the response of one long-Yagi at 64 feet that is overlaid with a stack at 60.5 feet (60.5 feet is the average height of the stack). This pattern shows multiple nulls or minimas, these arise from ground reflections being out of phase with the direct signals. The presence of ground also produces additional ~6dB antenna gain at vertical angles where the reflection adds in phase with the direct signal.




It can be seen that the antenna stack has increased gain at lower angles. This could help the problem of frequency selective fade. The rationale is; for both tropospheric scatter (tropo) signals as well as 2-Edge signals (if the nearest edge is relatively far away), the major signals arrive at lower angles. Since the troposphere reaches to ~6 miles in height, tropo can have many paths, with some signals arriving at higher angles. For higher angles, the signal must travel greater distances and may be out of phase with the low-angle & earlier signals. The stacked antennas provide some angle-of-arrival discrimination for elevation angles greater than about 1.2 degrees (88.8 degrees on the graph).

To estimate how much rejection may be required to improve a deep signal fade (null). The sum of two signals can be calculated using vector addition. It can be visualized that the sum two equal vectors at, or near, 180 degrees out of phase with respect to each other, add to near zero.

Suppose the desired, or direct, signal had amplitude of +1 Volt, and a multipath signal had equal or lesser value of amplitude, but was 180 degrees out of phase (-1) with the desired signal.

As examples, to the direct signal add decreasing magnitudes of a multipath signal:

+1 added to –1 = 0
+1 added to –0.95 = 0.05 (26 dB below a Voltage of 1 Volt)
+1 added to –0.90 = 0.10 (20 dB below a Voltage of 1 Volt)

In above, dB=20 x [Log10(Volts)]

Changing the multipath signal magnitude from 0.95 to 0.90 results in a ~6 dB improvement in the combination. In terms of dB rejection relative to desired:

Magnitudes of Multipath and dB Rejection

0.95 = 0.45 dB (relative to desired),
0.90 = 0.92 dB (relative to desired)

Difference = -0.47 dB

A multipath reduction of 0.47 dB, relative to desired signal, improved combined signal ~6 dB (26 dB to 20 dB).

As the amplitude of one of the vectors becomes smaller than the other, then the magnitude of their combination rapidly increases. The image below graphically shows the result of combining two signals arriving; one of them 180 degrees out-of-phase with respect to the other.




In the graph, as an example observe the curve at 25 dB. A 25 dB null can occur if the reflected signal has amplitude that is only ~0.5 dB lower than the direct signal. The greater the difference in amplitudes of the two signals, then the shallower the null will be. For example if the multipath signal is about ~10 dB weaker than the direct signal, then the combination may be reduced about 3 dB.

It can be seen that the deep nulls (say 25 dB) are sensitive to small changes in amplitude of the direct and multipath signals. The image below shows sensitivity to multipath signal changes of 0.25 dB relative to the direct signal.





In the chart immediately above, for an initial ~ 25 dB combined cancellation or null, if one could decrease the multipath signal by 0.25 dB, there could be an improvement in the combined signal of ~3 dB. If the initial combined null depth is ~10 dB, this improvement diminishes to ~0.5 dB improvement for a 0.25 dB multipath amplitude reduction. That is more than a 2 for 1 reduction in destructive interference for every 0.25 dB rejection of the out-of phase (multipath) signals. So, it appears that relatively modest amounts of signal discrimination can result is appreciable improvement for the combined (desired) signal.

In this case, the low angles are of most interest. The image below shows an expanded view of the elevation patterns shown in the first image above. The stacked antennas have increased gain, but in order to compare vertical patterns responses, the single antenna and the stacked antennas are shown with both patterns normalized. For convenience, the vertical angle is shown as degrees above the horizon.




It can be seen that for angles above about 2 degrees, the stack has at least 0.5 dB discrimination compared to the single antenna. On average, higher angle-of-arrival signals above 2 degrees are rapidly decreased due to the narrow elevation response of the stack.

Summary

If an appreciable fraction of multipath signals arrive at higher angles than the desired (often most direct path) then a relatively small rejection of multipath can yield an appreciable improvement in multipath null depth. It follows that vertically stacked antennas at sufficient height (AGL) and relatively wide spacing could be advantageous for rejection of multipath signals.

.
Attached Images
File Type: gif Vert Patterns (Space & Gnd).gif (25.2 KB, 4253 views)
File Type: gif Multipath+Direct= Null.gif (8.4 KB, 3765 views)
File Type: gif dB Improve per 0.25 dB Reject.gif (10.0 KB, 3632 views)
File Type: gif Compare Low Vertical Angle.gif (10.4 KB, 3853 views)

Last edited by tripelo; 23-Aug-2013 at 9:45 PM. Reason: Link images
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Old 24-Aug-2013, 1:58 AM   #62
Pete Higgins
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tripelo,

WOW! I don’t even want to know how long it took you to pull all this all together! I just hope you’re not going to bill me for it later?

This is great work and way beyond my ability to provide for the community; essential information for anybody considering multiple, same band, antennas. In addition to the expected <3 dB combining gain you’ve illustrated the importance antenna spacing can play. Again, clear concise and to the point.

I have a pretty good handle on LOS propagation and refraction extending the range to the “radio horizon”, multipath and even the “wavelet” concept that explains why we can receive diffracted signals that simple insight would suggest should be completely blocked. I even have a handle on how differences in the troposphere (temperature, water vapor, barometric pressure etc.) can cause tropospheric scattering. What I have trouble rapping my head around is which mechanism is responsible for what I see as channels fade both short term (multi-path?) & for hours at a time (tropospheric absorption?). I’m sure it has to be the “vector sum” of all these simultaneous mechanisms. I’m tempted to get a decent rotor and weld a plate into the top of my tower to support a “long mast“ and another 91XG. Other than it should work better though, I’m not really sure what to expect without trying it.

Those two RCA pre-amps. that I reported on worked out so well that my wife got after me to order two more. Ordered on Monday and received today. Since Time Warner blacked out CBS & CBS blocked Time Warner customers from downloading content (series episodes) from their website, my wife & I’ve been able to watch them from either San Diego or LA. I have also been able to let her watch the LA CBS evening news which she really prefers. Since I turned 70 last month, I had to go to the CA DMV and retake my driver’s license and motorcycle operator’s tests this afternoon so still need to bench check them tonight or tomorrow. I must have ordered two of the last ones because when I went back to the site they no longer list them.

Looking forward to your next post.
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Old 24-Aug-2013, 2:59 PM   #63
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Quote:
Originally Posted by Pete Higgins View Post

...What I have trouble rapping my head around is which mechanism is responsible for what I see as channels fade both short term (multi-path?) & for hours at a time (tropospheric absorption?)...
Multipath is difficult to counter because of the variety of sources that can cause it. Multipath is the main reason that mobile TV is not very practical for 8VSB (the US DTV standard).

It can be difficult to distinguish between multipath and direct signal fade.

There can be two classes of multipath:

1. Static Multipath (resulting from fixed objects)
2. Dynamic Multipath (resulting from moving objects)

Static or long term signal attenuation can result from multipath, when the multipath sources are stable (fixed). Examples could be reflections off a building or a fixed object that partially cancel the direct or desired signal. One prominent contributor is the surface the earth. As can be seen in the previous vertical antenna patterns, for specific angles-of-arrival, a ground reflection can cause a static fade.

Dynamic of fast fading (in minutes or seconds) is usually the result of multipath (Doppler effects). For Doppler to occur, something has to be moving such as antenna moving (Mobile TV).

For fixed antennas the something could be;

- Traffic in a city,
- An airplane flying over,
- Trees moving in the wind,
- Atmospheric motion (tropo scatter), or
- Something else.

For long distance reception, the atmosphere is the most likely ‘something’ that causes Doppler.

---------------------------------------------
Daytime Signal Fade (Independent of Multipath)

Gradual bending allows radio signals to somewhat follow the earth’s curvature for some distance beyond normal line-of-sight. This gradual bending is somewhat independent of phenomenon like ‘tropo scatter’. As you know, long distance TV signals can fade (reduce in strength) in daylight hours. This can be due to smaller temperature gradients that result when the sun uniformly heats the air in the lower atmosphere during much of daytime hours. Smaller temperature gradients result in less bending of the signals back towards the surface, thus the daytime fade.

Note: Temperature is not the end cause of the bending of RF signals. Temperature is a measure of one particular driving factor (thermal energy) that can decrease atmospheric density, thus affecting the density of everything contained in the atmosphere (water vapor, etc).

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

Quote:
… RCA pre-amps...have ordered two of the last ones because when I went back to the site they no longer list them.
Maybe your good reports caused a run on the RCA preamplifiers. Well, it was/is petty good deal.

Quote:
... I just hope you’re not going to bill me for it later?
Thanks for your comments about the elevation beamwidth and multipath.

No charge.

.

Last edited by tripelo; 24-Aug-2013 at 7:45 PM. Reason: Add note & ground comment
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Old 24-Aug-2013, 7:53 PM   #64
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tripelo,

I liked your explanation of how uniform daytime heating can produce smaller temperature gradients, limiting how far out a refracted signal travels. I know it happens because I frequently see it but I hadn’t thought it through to that point. Thanks.

Well, I configured my two new amps. for separate inputs & FM trap IN (default) and got outside before it got too hot (only 82 deg. when I finished) to check them. The first one I tried, I lost all the of VHF signals but UHF was fine. I thought I might be experiencing a temporary fade. I pulled it down and tried the second one. VHF & UHF worked great.

Back to the roof to swap them out again & high VHF was gone. I left it in –line and went back up and switched it to “combined”. To my surprise high VHF was restored. Unfortunately, the UHF antenna is my CM-4228 which has been shown to have a fairly good high VHF response. In any case, signal to noise ratio was restored to the same level as with the other two amps. so if I were to guess it was seeing the signal from my Winegard YA1713. Need to explore further to determine if the VHF port is left connected when the amp. is switched to the combined position. Finally, I tried switching back to “separate” and left the switch ~1/16” from the end of its travel. VHF & UHF signals were both restored to the SNR levels I saw with the other two amps. Presumably, this amp has a bad switch. It has enough friction that I think it will hold where I need to set it so I left it there. If it doesn’t hold I think it would be easier to replace the switch with jumpers than to try and send it back. I have a soldering station with a vacuum pump so pulling the switch is easy. I guess I should be happy that only one out of four had a minor problem?
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Old 25-Aug-2013, 7:45 AM   #65
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Pete,
I would also opt for jumpers.
Not sure I'd bother w/ removal of the switch, though.
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Old 30-Aug-2013, 4:31 PM   #66
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Stacked Antennas: Multipath & Diversity Gain?

Continuing with a previous topic: Multipath: Antenna Stack & Diversity Gain?

The following is a thought experiment.

Consider multipath effects to be the sum of only two signals, the desired or direct signal (D) and the multipath signal (M).

For a sum (combination) signal to experience a deep null (say 25 dB), the two signals must arrive at the antenna aperture nearly 180 degrees out of phase and nearly equal in amplitude.

Let D and M represent the frontal area of signals propagating through space.

Constraints on D and M:

1. Equal Frontal Area: Size is less than, or equal, that of a single antenna aperture.

2. M = - D, or Integrated Field Strength of Multipath = negative Direct





In the image above, for a single antenna there is one combination that yields a null. That is; Both signals arrive at the antenna. For the stack there are 3 distinct combinations adding to a null. These 3 combinations are; both signals arrive at the center of the stack, or one signal arrives at the upper antenna and the other signal arrives at the lower antenna (2 ways).

Consider larger frontal areas, with larger integrated field strength:

1. One frontal area larger than stack aperture.
A. Direct signal frontal area greater than the stack aperture.
Nulls could not occur at the stack aperture because the Multipath would not be sufficient to cancel the larger Direct signal.

If frontal area of direct signal could be larger than the stack aperture, then the inverse could occur.
B. Multipath signal frontal area greater than the stack aperture.
Nulls could not occur at the stack aperture because the Direct would not be sufficient to cancel the larger Multipath. Multipath would dominate.

2. Both Direct and Multipath frontal areas larger in area than the stack aperture.

In this latter case, the larger stack aperture could be considered as a single antenna and would have no multipath advantage over the smaller antenna aperture.

There are uncountable combinations of direct and multipath signals, large and small frontal areas with varying amplitudes and phases. Most combinations would not result in large cancellations or deep nulls. When deep nulls result from combinations, mostly they could be analyzed as described above.

In the image, the stack has twice the aperture of the single antenna. This increased aperture allows more signal combinations. If the number of signal combinations were directly proportional to the aperture size, then a stack could have twice as many combinations as a single antenna. For the stack; 2 times possible combinations there could be 3 times as many null-generating combinations. Statistically, this represents an increase in proportion of null-generating events.

Very wide spacing, with a large gap between apertures could eliminate the possibility of both Direct and Multipath combining at the center of apertures. That could reduce the possible number of null-generating combinations at the stack from 3 to 2. Thus, very wide spacing could allow stack performance to be similar to that of a single antenna.

Unless the statistics of the Direct or Multipath signals changed, then wider, or different spacing of the stack would not greatly improve the above scenario. If widely spaced locations were found for one antenna where the Direct signal had better statistics relative to Mutipath, then positioning both antennas in those regions could provide even better results. But, that would appear to be a matter of optimal antenna positioning instead of diversity gain.

Note:

With normally stacked antennas a combiner cannot take advantage, or mitigate the effects, of the differing phases of separate antenna signals. To mitigate multipath or to take full advantage of signals arriving with differing phase, a combining system must provide a means to either; synchronize the phase of separate antenna signals if combined pre-detection, or neglect phase if combined post-detection.



Summary

Considering information in the earlier post, it seems difficult to rationalize that an antenna stack provides diversity gain against multipath.

Absent a plausible mechanism, one could conclude:

Against Multipath: Stacking antennas does not provide diversity gain.

.
Attached Images
File Type: gif Single vs Stack Aperture.gif (10.5 KB, 3658 views)
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Old 6-Sep-2013, 2:14 PM   #67
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Multipath: STB-Converter Performance

The reception of WHAS-11 using the stack of long-Yagis was evaluated through the end of 2010 and into 2011.

Most of the time the signal strength seemed sufficient. It appeared that multipath was the main cause of occasional loss-of-lock in the daytime hours. Since multipath seemed likely as the cause, there was a possibility that differences in a DTV receiver’s equalizer could affect the situation.

Previous testing and TV viewing utilized a Channel Master CM-7000 set-top box. A comparison between two available converter boxes was made. The two other converters were Apex DT-250A and Magnavox TB110MWG. Each converter was connected to the antenna/TV combination and WHAS-11 was viewed for several minutes, then the CM-7000 was installed. This process was repeated multiple times over a period of two days. The visual display of loss-of-lock and picture pixelation was different, but there was no clear difference in overall performance.

Later in 2011, two other converters were tested. These were Digital Stream DTX-9900 and Zentih/Insignia NS/DXA-1. This time the tests were conducted in parallel, using a Holland GHS-2 splitter, both converters feeding separate monitors were operated from the same antenna. There was very little difference in visual performance. When a dropout occurred with one converter, nearly simultaneous a dropout appeared on the other converter. If there was any advantage, it went to the CM-7000. With the CM-7000, there seemed to be a very slight audio advantage, providing maybe a syllable more, either before dropout, or recovering early sometimes.

A search of literature, as to whether different converter boxes had appreciably different approaches to equalization, yielded the following report:

NAB/MSTV Digital Converter Box Evaluation – December 2008

http://www.nabfastroad.org/NABSTVDig...report.doc.pdf

In the above report, the performance of some set top boxes were evaluated. The Sansonic FT-300A appeared better able to handle both the longest delayed echos and the most advanced echos (+50 uS & - 50 uS).

Also, the “Pilot-nulled multipath threshold of visible (TOV)” test showed the Sansonic FT-300A to be 0.5 dB better than any of the other 6 converters tested.

The following image was drawn from data available in the above referenced report.




That 0.5 dB differential (left portion of above image, W Acq) for pilot nulling might not seem like much improvement. But if the pilot signal is lost, the system will lose lock. The pilot is located at the low frequency end of the 6 MHz DTV spectrum and is a fundamental synchronizing signal. A multipath signal could cause a frequency dependent null to severely attenuate the pilot. Recall, that 0.5 dB rejection of multipath can improve null depth up to about 6 dB. This means that the Sansonic unit was probably handling a worse pilot degradation than indicated by the 0.5 dB difference in applied signal.

A Sansonic FT-300A unit was purchased.

A simultaneous parallel comparison to the CM-7000 was arranged as described above. The CM-7000 lost lock a few times over the test period covering parts of several days, but the Sansonic unit provided solid reception.

The report above provided the identification of tuners and demodulators in the converters used in the test. Sansonic used a Microtune MT2131F tuner and an Auvitek AU8515AA demodulator. These components are different than any of the other test converters, they are also different than those used in the CM-7000.

Other testing confirmed that when a low noise moderate-to-high gain preamplifier is used, the tuner sensitivity is not a crucial factor in reception. So, the Sansonic tuner was probably not the critical part that contributed to better handling of multipath. The equalizer function is the likely reason for good performance and it is undoubtedly associated with the AU8515AA demodulator.

A search for more a commonly available converter that included the AU8515AA demodulator turned up Zinwell. Several Zinwell units were purchased including: ZAT-950A, ZAT-970A, newer versions labeled ZAT-970A on outside with ZAT-950 components inside. A visual inspection revealed all Zinwell units (made at that time) had the AU8515AA demodulator. Of the Zinwell units, some had can tuners (Sanyo) and some with silicon tuners (Microtune).

Simultaneous parallel tests of the Zinwell’s against the CM-7000 and the Sansonic were arranged as described above. The CM-7000 lost lock several times over the test period (covering parts of several days). The Sansonic unit and all the Zinwell units provided solid reception. Then, the Sansonic was compared against the Zinwell units, performance was identical.




A Zinwell unit was placed in service with the stack of 2 long Yagis.

Reception of WHAS-11 and WBNA-8 has been very satisfactory.

.
Attached Images
File Type: gif STB Compare WHAS-11.gif (3.7 KB, 3596 views)
File Type: gif Converter Pilot Null Test.gif (8.2 KB, 3629 views)
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Old 7-Sep-2013, 5:42 AM   #68
Pete Higgins
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tripelo,

Two thought provoking posts in a row. Today’s almost sounds like a conclusion. I’ve been looking forward to these every Friday.

I bought one of Winegards new LNA-200’s. It didn’t work out nearly as well as I had hoped. You can check out my findings @:

http://www.digitalhome.ca/forum/showthread.php?p=1782361#post1782361

Post 3688
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Old 7-Sep-2013, 12:54 PM   #69
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Quote:
Originally Posted by Pete Higgins View Post
Two thought provoking posts in a row... I’ve been looking forward to these every Friday.
Thanks Pete. Glad you like.

Quote:
...Today’s almost sounds like a conclusion...
Maybe close to a conclusion for the long Yagis. Additional work was done for reliability reasons and reduction of losses. Earlier briefly mentioned homebrew: ferrite core baluns, combiner, and preamp.

Probably change topics to some UHF related.

Will soon take a break for a few weeks to go to KY.

Quote:
I bought one of Winegards new LNA-200’s. It didn’t work out nearly as well as I had hoped.
Thanks for your testing the Winegard LNA-200.

Sometimes there are many variables at a particular test location, then combined with equipment variables it takes some time to sort it out.

Have you opened the case of the LNA-200?

Could you take some detailed photos of the innards?
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Old 7-Sep-2013, 2:54 PM   #70
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Tripelo,

Quote:
Sometimes there are many variables at a particular test location, then combined with equipment variables it takes some time to sort it out.
For my first test, I substituted the LNA-200’s power inserter in the garage for the RCA’s and pulled two cables off the HDB8X RCA amp and connected them to the LNA-200 to minimize variables. Of course, with a single input, for the array test I had to add a UVSJ and an additional cable at the amp but used the original array lead-in RG-6.

Quote:
Have you opened the case of the LNA-200?
No, but I will. The overnight low here was 78 deg. & it’s expected to climb to 103 deg. by mid-day. I start to get dizzy if I do a lot of climbing and spend too much time out in that kind of heat. It’s safer for me to wait for this heat wave to break. (I’d probably be OK but my wife would kill me)

Quote:
Could you take some detailed photos of the innards?
Will try to take & post. Probably next week.
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Old 9-Sep-2013, 11:54 PM   #71
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LNA-200 findings

tripelo,

Here is a link to my LNA-200 findings -with pictures.

http://forum.tvfool.com/showthread.php?t=13636
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Old 13-Sep-2013, 1:10 PM   #72
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Televes DAT-75 vs. Antennas Direct 91XG

A pair of UHF antennas were compared:

Antennas Direct 91XG and Televes DAT-75

Both antennas have built-in baluns.

Equipment used in the tests:

- Push-up sectioned mast with two ~5 ft. extensions
- Fifty feet RG6 quad shield coaxial cable (CATV grade)
- Sencore SLM1456CM (Digital signal level meter)

The Sencore 1456 can scan channels, after each channel is locked-on and signal strength and quality measured, it tunes to the next channel and repeats until all channels have been analyzed. The 1456 saves the results as normal computer files, they can be downloaded for later analysis.

Signal sources: Multiple DTV stations in the Dallas/Fort Worth area. All stations generally located at the same bearing with LOS at ~29 miles.

The center boom of each antenna was raised to a height of 25 feet above ground (AGL).

The following is an image of the antennas in the test positions:



Test Sequence:

The tests were performed in mid-morning during a 2-day period. For each antenna test, the Sencore 1456 was allowed to scan available UHF stations three times. The first day there were 3 scans for each antenna for a total of 6 scans. On the 2nd day, the tests were repeated for 12 more scans.

A recording (run) consists of the SLM1456CM scanning the UHF band and recording the average signal strength (dBmV) for each channel.

Day 1: 3 consecutive runs with 91XG followed by 3 runs of DAT-75
Day 2: 3 consecutive runs with 91XG followed by 3 runs of DAT-75, then 3 consecutive runs with DAT-75 followed by 3 runs of xg-91.

In all, there were 3 sets of 3 scans for each antenna. Each scan took ~2-3 minutes. So the measurements were no more than 2-3 minutes apart

The results were downloaded and graphed in Excel. All 9 runs for each antenna were averaged together to yield a composite, shown here:




Observing that the 91XG provided the overall highest received signal power, the difference was computed and graphed in the image below:




The 91XG had about 1 dB more gain for the mid portion of the UHF band.
The two antennas were nearly equal at the lower channels.

These results can be rationalized as follows:

The DAT-75

- Large reflector & essentially a stacked pair of driven elements provide lower frequency gain.

- The 3-stack spacing becomes wider (in terms of wavelength) at upper frequencies, increasing gain.

The 91XG


- Colinear directors (X directors) and long boom length provide high gain thru the band.

.
Attached Images
File Type: jpg 91xg and DAT-75.jpg (110.5 KB, 4523 views)
File Type: gif DAT75 vs 91xg Signal Levels.gif (11.6 KB, 3629 views)
File Type: gif Gain Delta 91xg-DAT75.gif (10.0 KB, 3721 views)
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Old 13-Sep-2013, 4:53 PM   #73
Pete Higgins
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tripelo,

I have never heard of the DAT-75 although if the 91XG held its own or performed better I guess it doesn’t matter. I compared my new 91XG to my ~40 year old CM-4228 last year and with two exceptions couldn’t tell any real difference. The 91XG did real well with CBS channel 43 (2.1) which TV Fool lists @ -112.4 dBm but had trouble with NBC channel 36 (4.1) which TV Fool lists @ -107.4 dBm. My CM-4228, and now my HDB-8X as well, almost always get channel 36 (4.1) but very rarely can get channel 43 (2.1). I even tried swapping the 91XG from the tower to the pushup mast and putting the CM-4228 on the tower with the same result. One of the reasons I tried the HDB-8X was to see if the differences in design would improve reception of channel 43. Other than the two channels mentioned I haven’t been able to find any significant difference between the three UHF antennas in terms of SNR’s delivered to my tuners. I don’t have a way to measure and graph signal strengths. My Samsung TV has a signal strength indicating bar graph, but sometimes one bar can give me a solid signal lock to watch and at others 3-4 bars can result in a picture that pixelates and freezes (i.e. one bar with a moderate SNR works better than 4 bars with a poor SNR).
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Old 6-Oct-2013, 11:46 PM   #74
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Satellite dish mount on tower leg

tripelo,

I finally modified the satellite dish mount that I removed from one of my rentals and attached it to the west facing leg of my aluminum tower. Attached my one remaining RCA 5-wire rotor and with a short piece of schedule 80 PVC, my HDB8X 8-Bay Bow Tie. I angled it so that I can rotate the antenna between 169 deg. (the San Diego UHF stations) and 292 deg. (the LA UHF stations). Moving it ~15 feet north and being able to rotate it for maximum SNR gave me a 2-3 dB improvement in signal strength even though its physically 6-10 feet closer to the ground.



The HDB8X still won’t get channel 43 (CBS 2.1) but has channel 36 (NBC 4.1 & .2) as well as channel 31 (CW 5.1, .2 & .3) booming in (SNR’s ~ 28.5-.7). My 91XG on the tower is getting channel 43 with an SNR of 19.9 – 20 .5 but has channel 36 dropping in & out and jumping between 0 and 17.1 with lots of correctable errors. It does a little better with channel 31, holding a steady 21.1 with no errors. TV Fool lists channel 31 @ 2-Edge -103.8, channel 36 @ 1-Edge -107.7 and channel 43 @ 2-Edge -112.9.

The LNA-200 is wedged in place, mid-way between the horizontal cross braces, by the right panel's reflector end cap. Seems much more secure than using the supplied black tie-wrap! Mounted this way I'm not seeing the overload I saw when it was mounted on the mast, below the antenna.
Attached Images
File Type: jpg HDB8X on Tower 3.JPG (49.7 KB, 3774 views)

Last edited by Pete Higgins; 7-Oct-2013 at 2:26 AM. Reason: Add LNA-200 info.
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Old 8-Oct-2013, 6:56 AM   #75
GroundUrMast
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Hi Pete,


Have you considered mounting the LNA-200 inside the tower leg? (Hoping/testing for some shielding effect.)
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Old 9-Oct-2013, 2:46 AM   #76
Pete Higgins
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GroundUrMast,

No, I haven’t. Wedging it between the PVC mast and the plastic end cap was a convenience to keep me from climbing off the patio roof and running to the garage to get another tie-wrap. Success was accidental!

I have thought about mounting it in a metal box to see if that would help but the RCA amps I bought seem to work as good (maybe even better), support dual inputs and cost a lot less, so I’m not sure it would be worth the effort. I can’t fathom how sticking it in front of a piece of PVC and partially behind a reflector rod could provide much shielding. Other than relocating the antenna & the amp., everything else stayed the same (panel cables, combiner, combiner to amp. cable and cable to garage).

I’m trying to get all the window screens vacuumed this morning. We have a whole house fan that draws a lot of dust through them and I want to get that off before it starts to rain tomorrow & Thursday. If I don’t wear myself out on that project I plan to replace the PVC mast with a metal one this afternoon. It got pretty windy yesterday and the HDB8X was bouncing all over the place. Too much flex in the PVC mast.

********************************

The TV Fool website went down while I was typing this. Finished the screens washed the windows & swapped out the mast. I still had two real heavy Channel Master 5½‘ sections circa 1962.
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Old 13-Nov-2013, 4:01 AM   #77
Gil
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Hi Tripelo,

Congratulations for the nice publication usefull for guideline and advise for good tv reception.

The conclusions that you posted involved a lot of work and spertise.

Regards

Gil
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Old 14-Sep-2014, 6:47 PM   #78
tripelo
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Thanks Gil, for your kind remarks.
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Old 14-Sep-2014, 7:30 PM   #79
tripelo
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UHF Signal Strength vs Antenna Height (AGL)

In May 2012, Bob Nelson (forum member re_nelson) and I performed some experiments to observe the effects (in a suburban area) of UHF signal strength versus height of an antenna.

Signal strength measurements of 27 DTV stations versus receiver antenna height were recorded. On average, signal strength increased with height increase. Individual stations showed variability of signal strength (in a repeating pattern), this could indicate the presence of signal layering. Signal layering can result from reflections (ground or other) that either reinforce or reduce signal strength at various receiver antenna heights.

Location: Garland, TX

Signal Path: Line-of-Sight, about 28 miles from TV stations located at Cedar Hill, TX.

Local Clutter: Single story residential homes with some trees (approximately 30 –45 feet tall). The LOS path was essentially between any tall trees.

Signal Sources:

At that time there were 27 DTV stations in the same general azimuth direction that could be received at this location. These 27 stations served as stable signal sources to compare strengths at various antenna heights.

Equipment:

Antenna: Terk HDTVi mounted to a push-up mast. The HDTVi was chosen because of it’s small aperture (allowing better height resolution), small size, and relatively flat response across the band. The HDTVi is shown in image below.

Mast: A push-up mast capable of being quickly lowered or elevated up to about 45 feet. The mast had numerical marks on it every foot for quick determination of height.

Signal Analyzer: Sencore 1456, capable of scanning the UHF TV band for signals and measuring and recording the amplitude and MER (quality).

Coaxial Cable: Fifty feet of commercial quad shield RG-6 with custom connectors.





Methodology:

Beginning at 15 feet AGL, measurements of all measurable* signals were taken at close intervals up to a height of 33 feet AGL. Most measurements were made at 1 foot intervals, between 18 and 24 feet, the measurements were made at 2 feet intervals.

* All stations had adequate signal strength, but some were low power (LP). For a few reasons, LP stations among several full power stations are difficult for an analyzer (tuner) to reliably decode. However signal strength measurements were feasible. The entire UHF TV band (Channel 14-51) is well represented by measurable signals.





Below is a chart showing the average signal level of 27 UHF channels versus height.





Each point in the graph represents that average signal strength of 27 DTV stations (RF channels) at that particular height (AGL). The averaging process takes out a lot of information, but does help provide an overall view.

The graph shows there is a signal strength plateau around 24 feet. But, increases resume at heights greater than 30 feet. Interestingly, the FCC uses 30 feet AGL for outside antenna in many propagation/contour analyses.

Much appreciation to re_nelson for his assistance, equipment, and support.

Later, the data illustrating signal strength versus height for individual stations can be graphed and posted, in a few weeks maybe sooner.

.
Attached Images
File Type: jpg HDTVi _on Mast.jpg (37.4 KB, 2777 views)
File Type: jpg HDTVi _32 feet.jpg (193.6 KB, 2823 views)
File Type: gif UHF Signal Strength vs Height.gif (8.9 KB, 3125 views)

Last edited by tripelo; 20-Sep-2014 at 2:04 PM. Reason: typo
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Old 14-Sep-2014, 8:49 PM   #80
Pete Higgins
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tripelo,

Interesting exercise.

I experienced a similar phenomenon when I moved my single HDB8-X from my aluminum HAM tower leg (@ ~19’ AGL) to my TV tower mast (@ ~40’ AGL). Most stations got stronger (marginally), some stayed about the same, I lost one and picked up a couple of others. When I added a second HDB8-X to my TV tower array most channels signal strength improved 1-2 dB but at least one actually appeared to get weaker? The two HDB8-X’s are vertically stacked.

[IMG]http://forum.tvfool.com/attachment.php?attachmentid=799&d=1410727208[/IMG]

The HDB8-X I ordered to form my 16 bay array arrived pretty damaged (kinked reflector rods & 3 of the 4 bow-ties on one panel loose). Solid Signal replaced it for me and I installed the replacement on the TV tower. I straightened the bunged up antenna as best I could and hung it from the aluminum tower leg. Once again I started to receive the channel lost by moving to the TV tower. (If only I spoke Spanish it would do me some good)
Attached Images
File Type: jpg Dual HDB8-X 10 July 2014 Small.jpg (196.5 KB, 5418 views)
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