Install an antenna at ground level is better? - Canadian TV, Computing and Home Theatre Forums

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post #1 of 15 (permalink) Old 2019-04-23, 11:33 AM Thread Starter
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Install an antenna at ground level is better?

Hi everyone,

I've read the Knowledge Base & FAQ and looked through the info for an answer, but I didn't find advice on where to put an antenna in this particular situation.

I try to help a friend who wants to install an antenna to capture the TV. She lives in St-Michel-des-Saints (Quebec). All signals in this area are very weak and 2e edge type.

The reports of TV Fool are confusing: the higher the position of the antenna, the less the signal is good. Here's TV Fool TV Signal Analysis Reports for an antenna installed at 1 foot from the ground and the other at 15 feet:

At 1 foot: TV Fool 1'

At 15 feet’s: TV Fool 15'

For example, in the case of CBFT-DT channel 19 (Montréal), the NM dB results are: 1’=-4.8; 15’=-11.8 (at 200’, it's even worse:-15.4).

What can explain these results?

Even if for me, this defies logic, is it better to position the antenna at ground level?
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post #2 of 15 (permalink) Old 2019-04-23, 04:37 PM
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Quote:
Originally Posted by Roch Lafrance View Post
What can explain these results?
Even if for me, this defies logic, is it better to position the antenna at ground level?
Hello, Roch Lafrance; welcome to the forum.

I have seen this before. The signal is stronger with the antenna lower because the reflection of the signal off the ground in front of the antenna is adding to the direct signal.

https://www.digitalhome.ca/forum/186...ml#post1334335

WVNY NM:
5 ft, +0.6
6 ft, +6.4
7 ft, +6.1
8 ft, +5.8
9 ft, +5.6
10 ft, +5.3
13 ft, +4.8
15 ft, +4.5
20 ft, +4.0
30 ft, +3.2
40 ft, +2.8
50 ft, +2.4
100 ft, +1.7

However, the software that produces the signal reports is much less accurate with 2Edge signals. You must make some tests to confirm the report results. The last time I saw the test results agree with the report was when there was water in front of the antenna that reflected the signal.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 2019-04-23 at 08:02 PM.
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post #3 of 15 (permalink) Old 2019-04-23, 06:04 PM
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Here is the terrain profile for CBFT done by TV Fool. If you click on the callsign in your report, you will see the profile. It shows terrain interference to the signal:


If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #4 of 15 (permalink) Old 2019-04-23, 07:08 PM Thread Starter
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Thank you very much rabbit73 for your answer.

I had seen the terrain profile before and I thought it would not be easy. You would have to have a tower 1000 feet high to get an LOS signal in this place.

However, we see that there is a very weak signal, but a signal is existing. Do you think this is a usable signal or does this reflection make it unusable? In other words, is it worth the energy or is it lost?

And if it's worth a try, is it better to have a low antenna, to benefit of a possible ground reflection, or the highest possible?
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post #5 of 15 (permalink) Old 2019-04-23, 07:37 PM
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Originally Posted by Roch Lafrance View Post
I had seen the terrain profile before and I thought it would not be easy. You would have to have a tower 1000 feet high to get an LOS signal in this place.
Here is another terrain profile using different software:



Quote:
However, we see that there is a very weak signal, but a signal is existing. Do you think this is a usable signal or does this reflection make it unusable? In other words, is it worth the energy or is it lost?
Yes, it is a very weak barely usable signal. The reflection can add to the direct signal, making the sum greater than the direct signal alone. I couldn't find a coverage map for CBFT, but I did find one for CFTM, which should be similar:





Quote:
And if it's worth a try, is it better to have a low antenna, to benefit of a possible ground reflection, or the highest possible?
Try it low first, then raise it. There must be no objects in front of the antenna that would block the signal. You will need a high gain UHF antenna for CBFT like the Antennas Direct 91XG or the Solid Signal HDB91X and a preamp. You will also need a high gain VHF-High antenna for 10 and 12, like the Stellar Labs 30-2476.

I think the chance is good that it will work, but I can't give you a guarantee.

If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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post #6 of 15 (permalink) Old 2019-04-24, 08:42 AM Thread Starter
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If it's a reflection of the signal, it will not be obvious. There is nothing there: it is at the end of a dirt road and surrounded by a forest. How to capture this signal?

What I understand from all of this is that under certain circumstances:

1: it may be better installing a antenna as low as possible when there is reflection of the signal;

2: the angle of the antenna should be adjusted according to the signal reflection. The story of Peter Putnam is very interesting about this: "Knife-edge refraction works so well at this location that I actually received all of the Albany DTV channels with the CS600 resting nose-down on the ground and its rear elements tilted up at a 45-degree angle against the mast!"

Quote:
You will need a high gain UHF antenna for CBFT like the Antennas Direct 91XG or the Solid Signal HDB91X and a preamp. You will also need a high gain VHF-High antenna for 10 and 12, like the Stellar Labs 30-2476.
I will do some experiments at the end of May. I will focus on two UHF channels (CBFT and CBMT) with respectively -4.8 and -5.3 dB NM. I think that an Antenna Direct DB4e (12dB gain at these frequencies) with an amplifier should be enough for testing.
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post #7 of 15 (permalink) Old 2019-04-24, 01:59 PM
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Quote:
Originally Posted by Roch Lafrance View Post
If it's a reflection of the signal, it will not be obvious. There is nothing there: it is at the end of a dirt road and surrounded by a forest. How to capture this signal?
By the emprical method, AKA trial-and-error. Do you have any kind of signal strength indicator to track improvement?



Quote:
What I understand from all of this is that under certain circumstances:

1: it may be better installing a antenna as low as possible when there is reflection of the signal;

2: the angle of the antenna should be adjusted according to the signal reflection.
The angle of the antenna should be adjusted to capture the maximum signal.
Quote:
The story of Peter Putnam is very interesting about this: "Knife-edge refraction works so well at this location that I actually received all of the Albany DTV channels with the CS600 resting nose-down on the ground and its rear elements tilted up at a 45-degree angle against the mast!"
Yes, a very interesting story by Peter Putman;
I had the link before
Then I lost it
Now I have it again, thanks to you.

It is a special case of signal enhancement by ground reflection because the ground slopes down in front of the antenna. The diffracted signal, coming down from a terrain peak, arrives much like a skywave.

Les Moxon, G6XN, describes that type of enhancement in detail in his book HF Antennas for All Locations.

Quote:
I will do some experiments at the end of May. I will focus on two UHF channels (CBFT and CBMT) with respectively -4.8 and -5.3 dB NM. I think that an Antenna Direct DB4e (12dB gain at these frequencies) with an amplifier should be enough for testing.
The DB4e was rescaled down for 14 to 51, so it should do better for 19 and 21 than most other 4-bay antennas. However, it doesn't have a tilt feature; that would be a custom mod.

Ground gain and radiation angle at VHF
https://www.qsl.net/oz1rh/gndgain/gnd_gain_eme_2002.htm

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 2019-04-24 at 02:27 PM.
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post #8 of 15 (permalink) Old 2019-04-25, 09:45 AM Thread Starter
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Thank's for the illustration of the skyline multi-path effect. Choosing a mounting site page of HDTVPrimer is very interesting.

I understand that methodical work and especially patience will be determining factors ...

Quote:
Do you have any kind of signal strength indicator to track improvement?
I planning to go there with my laptop, hauppauge USB tuner and use the hauppage signal monitor. Would there be more useful software to use for signal strength indicator?
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post #9 of 15 (permalink) Old 2019-04-25, 01:29 PM
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Quote:
Originally Posted by Roch Lafrance View Post
I planning to go there with my laptop, hauppauge USB tuner and use the hauppage signal monitor. Would there be more useful software to use for signal strength indicator?


That sounds good, if it works. I couldn't get mine to behave. An alternative is the SiliconDust HDHR.

In addition to signal strength, signal quality, as defined by SNR and uncorrected errors, is also important. You want an SNR above 15 dB and no uncorrected errors.

Does your friend's TV have a signal monitor in the menu?

If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 2019-04-25 at 02:40 PM.
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post #10 of 15 (permalink) Old 2019-04-25, 01:58 PM Thread Starter
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Does your friend's TV have a signal monitor in the menu?
I dont no, but i have MediaPortal on my laptop with this feature. I will plug the laptop directly on the coax (no need to go in the house each time). If i catch something, i'll do the fine tuning on the TV set.
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post #11 of 15 (permalink) Old 2019-04-25, 09:27 PM
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Quote:
I will do some experiments at the end of May. I will focus on two UHF channels (CBFT and CBMT) with respectively -4.8 and -5.3 dB NM. I think that an Antenna Direct DB4e (12dB gain at these frequencies) with an amplifier should be enough for testing.
First post.

I found that CBFT and CBMT are rather easy to receive in Sherbrooke, QC. CBC transmitter on Mt. Royal is 363 kW omnidirectional, a lot more than CFTM at 11 kW. I can get CBMT signal with a 8-bay non-amplified antenna in the attic. Here is my TV Fool Report. I estimate CBMT should be more around 10 dB NM than -17.4 dB on the report.
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post #12 of 15 (permalink) Old 2019-04-26, 01:26 PM Thread Starter
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Welcome Cuivreux

Quote:
I found that CBFT and CBMT are rather easy to receive in Sherbrooke, QC. CBC transmitter on Mt. Royal is 363 kW omnidirectional, a lot more than CFTM at 11 kW. I can get CBMT signal with a 8-bay non-amplified antenna in the attic. Here is my TV Fool Report. I estimate CBMT should be more around 10 dB NM than -17.4 dB on the report.
No amp with a attic antenna with a TV Fool NM at -17.4 dB? Wow! Encouraging for me. And what is the situation for CIVM 26 (TQ) at -13.3 NM?
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post #13 of 15 (permalink) Old 2019-04-26, 05:06 PM
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What I'm saying is that I'm pretty sure there is an error in TV Fool database about CBFT and CBMT. In my attic I can receive any signal over a noise margin of 10 dB. CBC's transmitter is at least 30x more powerful than any other transmitters on Mt. Royal in Montreal. There is no way CBMT should be in the same power level than CFTM (TVA) or CFCF (CTV) on TV Fool Report. CBC is listed at -17.4 dB NM on my report but I would say it is closer to 10 dB in real life.

I don't get any stations from Mt. Royal other than CBC and SRC.
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post #14 of 15 (permalink) Old 2019-05-28, 09:29 PM
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It is very common for antennas to work better lower than higher.
Especially under non line of sight conditions.
And the secret to UHF reception in weak sigal areas and non line of sight arras is capture area.
And nothing has more capture area than a 4 bay or 8 bay bowtie.
Yagi designs have very small capture areas so they suffer from break up under non line of sight
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post #15 of 15 (permalink) Old 2019-05-29, 03:31 PM
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I don't think it's "Capture Area" [which is directly related to Gain] that allows 4-Bay, 6-Bay and 8-Bay Antennas to excel....when Multipath causes signal levels to vary with Height (see example in below link), then SOME of the Bowties may be in NULLS, whereas OTHER Bowties are hopefully NOT.....so it's equivalent to Vertically Stacked Space Diversity. And when Multipath causes signal levels to vary with Horizontal Distance, then ONE of the 4-Bays in an 8-Bay Antenna MAY be in a Null, whereas the OTHER 4-Bay hopefully is NOT. OTOH: Since a Yagi collects signals along it's X-Axis, it should provide at least SOME protection against Multipath that results in Nulls aligned along the X-Axis...which would be less frequent:
Siting the antenna
BTW: I intentionally designed my FF4 and FF6, as well as Vertically Stacked FF4 so that Bowties are NOT all equally spaced, thereby maximizing protection against Vertically Stacked Multipath NULLS:
https://imageevent.com/holl_ands/sta...oubleanglerefl

Fol. on-line Calculator indicates that ANY 15 dBi Gain Antenna has a Capture Area of 0.464 m^2:
https://www.everythingrf.com/rf-calc...ure-calculator

Fol. Wiki article for "Antenna Aperture" [aka Capture Area] shows that Aperture and Gain are directly related, hence any (say) 15 dBi Antenna at a particular Frequency would have the SAME Aperature....and YES, the Aperature of a Yagi Antenna is MUCH larger than the actual Frontal Area;
https://en.wikipedia.org/wiki/Antenna_aperture

Fol. W8JI article summarizes Capture Area for several simple Antennas, including the "Quad" (i.e. Square Loop)....but does NOT specifically talk about Yagis and the ~3 dB increase in Gain (and hence Doubling the Aperture) due to adding a Reflector to a Multi-Bay Antenna:
https://www.w8ji.com/capture_area_ae...e_aperture.htm

Fol. F4AZF article illustrates how Yagi Capture Area is used to determine Minimum Vertical and/or Horizontal Stacking Distances:
http://www.f4azf.com/crbst_5.html

BTW: You would think that calculating the Aperture Area and hence Gain of a Parabolic Dish would be super simple: A=pi*R^2. WRONG....by about 1-6+ dB!!!! The "Feed Antenna" ideally would illuminate the entire surface of the Dish with the SAME Signal Level....never happens in practice. There is always an Illumination Loss as the center of the Dish is typically a HOT spot and the edges of the dish have LESS Illumination....and any illumination that extends beyond the dish also contributes to Illumination Loss:
http://www.w1ghz.org/antbook/chap4.pdf

I found that the "best" Feed Antenna for either a 124-in (10.3-ft, 5 dB Illumination Efficiency Loss) or 7-ft (CM4251, 4 dB Illumination Efficiency Loss) Parabolic Dish is one that has a fairly wide Beamwidth, like the 2-Bay Triangular Bowtie with (smallish) Reflector in CM4251] so that it TRIES to Illuminate the Entire Dish [without "Too Much" blockage of the incoming & outgoing signals] ....and much higher Gain Feed Antennas didn't really help very much due to their smaller Beamwidths mostly Illuminating the central HOT spot:
https://imageevent.com/holl_ands/par...artsparabolics
https://imageevent.com/holl_ands/par...icfbmod8ellpda
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Antenna Simulations, Overload Calculations, etc: http://imageevent.com/holl_ands

Last edited by holl_ands; 2019-05-29 at 05:02 PM.
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