Question: The reflect effect

#1
I'm happy with the performance of my 4221HD as an indoor antenna, but I'm still doing a bunch of experiments, partly cause I'm such a curious fellow, but also trying to get the best recommendation for my neighbors in the building.

So I pulled off the rod reflectors and now I have a bidirectional antenna! It was very directional before, with only 1 to 4 out of 27 stations bleeding through from north to south or vice versa. Now I'm getting 21 stations from both directions. I definitely know the reflector does some good, cause if a Chicago station is pixelated, I can fix it just by holding the reflector in place. But just with those Chicago (south) stations, turns out that elevating 6 inches toward the ceiling has more effect than adding the reflector.

So I gotta leave it this way for a few days to see if it stays "close enough for jazz." It's fine right now -- everything missing was already iffy in the first place -- but I doubt it'll last. It'd be great if it did, since not everyone has a seating arrangement like mine, meaning they'd need a rotor and a remote-control.

Here's my question: the reflector adds tremendously to the "front to back ratio," right? And it clearly seems to take much more away from the backside than it adds to the front. But what's the proportion? I'm sure it depends on the channel, but I'm just looking for a rough guesstimate.

E.g. the 4221HD has a minimum FB ratio of 18 dB. Might the reflector add 3 dB on the front (give or take) and subtract 15 dB to the rear? Like I say, I know for sure it adds SOMEthing on the front, cause I've repeated the experiment over and over. So the purpose isn't JUST to slash noise from the rear, as some other sites imply.

Just trying to be a good little scientist. :playball:

TIA,
Rick
 
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dkreichen1968

Moderator
Staff member
#2
Like I say, I know for sure it adds SOMEthing on the front, cause I've repeated the experiment over and over. So the purpose isn't JUST to slash noise from the rear, as some other sites imply.
No, I'm fairly sure that the primary effect that the first person to put a reflector on an antenna was looking for was added gain. The way it works is that the reflector reflects signal back toward the driven elements. When the reflected signal is close to being in phase with the primary signal, it adds energy/gain to the signal in the driven elements. That is part of the reason the gain varries depending on the broadcast channel. The closer the reflected signal is to being in phase with the primary signal the more gain there will be. If the reflected signal is out of phase with the primary signal it will actually cancel it out, which is why reflector placement is crucial. The fact that the reflector helps block multipath signals is really a fringe benefit. In many cases with digital having a bi-directional antenna will outway the added gain and multi-path protection.
 
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n2rj

Moderator
Staff member
#3
Here's my question: the reflector adds tremendously to the "front to back ratio," right? And it clearly seems to take much more away from the backside than it adds to the front. But what's the proportion? I'm sure it depends on the channel, but I'm just looking for a rough guesstimate.
If you know the F/B ratio and forward gain, you can determine how effective the reflector is, no?

Let's say an antenna had 18dB of f/b and 3dB of forward gain... this would mean that the antenna is providing 15dB of rejection of signals from the rear and in a design like the bow tie with reflectors which provides identical gain in both directions without the screen, this would mean that the 15dB of rear rejection is from the screen.

It also does vary by frequency, a lot. You can't get both optimum gain and f/b so you compromise between both. For TV antennas you want as much gain as possible, but in the case of mountainous areas you want rear and side rejection as well to combat multipath.
 

nbound-au

The Graveyard Shift
#4
The 4221HD is a 4bay bowtie, or as we call them here down under a phased array. Its infact 4 very small yagi antennas that are connected so that the signals all reach your coax cabling in phase with each other [at the same time] and therefore add to each other. by removing the reflector you are turning your mini yagis into dipoles which have approximately half the gain of the mini yagis. This means what you are receiving via the front is probably going to be approximately half of what you had before.

Before:
Fowards gain: 12db
F/B ratio: 18db
Rear gain: -6db

After:
Forwards gain: 6dB
F/B ratio: 0db
Rear gain: 6db
Side gain (est): ~0dB

Also you loose most of your multipath rejection (multipath from the sides (approaching 90degrees) is still reduced as the dipoles act as reflectors for each other.). Problem is, multipath usually doesnt occur from the sides at such a sharp angle, and there is little to no gain at such extreme angles on a dipole anyway.


Someone correct these figures if Im wrong, its been a while since I did such things.
 
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#5
dkreichen1968 said:
No, I'm fairly sure that the primary effect that the first person to put a reflector on an antenna was looking for was added gain. The way it works is that the reflector reflects signal back toward the driven elements. When the reflected signal is close to being in phase with the primary signal, it adds energy/gain to the signal in the driven elements. That is part of the reason the gain varries depending on the broadcast channel. The closer the reflected signal is to being in phase with the primary signal the more gain there will be. If the reflected signal is out of phase with the primary signal it will actually cancel it out, which is why reflector placement is crucial. The fact that the reflector helps block multipath signals is really a fringe benefit. In many cases with digital having a bi-directional antenna will outway the added gain and multi-path protection.
Cool. See I was confused because of this definition of F/B Ratio:
http://www.hdtvprimer.com/ANTENNAS/glossaryA.html#fb said:
F/B (Front to Back Ratio) - This ratio tells how good the antenna is at rejecting signals from the rear. It is seldom truly important because interference seldom comes from the rear, but it can happen.
But a reflector can't add any signal strength, right? Let's assume I'm just holding up the reflector, like I used to do with my tin can on the Monoprice ant. No electrical connection whatsoever between reflector and coax output, to keep things simple. So then F/B ratio quantifies the net effect of a reflector! By saying FBR is unimportant, they seemed to be saying the reflector is unimportant. But I guess they were just trying to say the statistic itself isn't very useful.

n2rj said:
If you know the F/B ratio and forward gain, you can determine how effective the reflector is, no?
But, but how would I ever know any of that stuff? You're not going to make me buy a signal meter, RU?? :eyes:

nbound-au said:
Before:
Fowards gain: 12db
F/B ratio: 18db
Rear gain: -6db

After:
Forwards gain: 6dB
F/B ratio: 0db
Rear gain: 6db
Side gain (est): ~0dB
Wait! Wait! I have a theory. Maybe I can understand now what n2rj was saying. Here's my theory: total gain = forward + rear + all side gain = some constant for any given channel, cause the reflector can't add or subtract from total gain, based on the Rickideemus theory.

Now, CM says 13.65 dBi average gain for the 4221HD, by which I assume they mean forward gain, and 18 dB "minimum" FBR. So let's say we have 14 dBi forward gain for channel 35 and FBR = 20 dB. Put all other (side) gain, with the reflector, at -2 dBi, just because I don't know what I'm talking about. Now it becomes:

With reflector:
Forward gain: 14 dBi
F/B ratio: 20 dB
Rear gain: -6 dBi
Side gain: -2 dBi

Without reflector:
Forward gain: 3 dBi
F/B ratio: 0 dB
Rear gain: 3 dBi
Side gain: 0 dBi

On accounta 14 + (-6) + (-2) = 6, so if side gain increases to 0 without the reflector, we have 6 dBi to divide up between forward and backward.

MAN, IT WAS HARD WORK CRANKING OUT ALL THAT THEORY!! (Alright, bring out the horsey, Jim)

:flypig:
Rick
 
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nbound-au

The Graveyard Shift
#6
The quoted gain figures are the maximum forward gain only (in trade guides they actually have a map showing the gain in all directions around the aerial [aka. lobes]). All other gain is somewhere between it and zero. Reflectors do add to total gain (its on this basis a log-periodic antenna has gain) . And side gain likely decreases without the reflector to reflect signals in from large angles.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#7
... But a reflector can't add any signal strength, right?
As mentioned in an earlier post, the following applies to screen-type, Yagi and some other antennas but not for parabolic dishes. The driven element of a properly designed parabolic dish antenna is 'blind' (NOT looking at the source) to the initial signal I will refer to below: it only collects RF from reflected signals from the perfectly shaped (reflector) bowl.

Back to the specific topic, which is most easily visualized using a screen-type antenna in your mind's eye, such as a Channel Master 4221, 4228 or the even the new 'HD' versions: this is about the reflector.

The receiving element on any antenna is the 'driven' element (or a combination of elements). At a specific distance behind the driven element/s there is a mirror: the reflector.

The RF signal arrives at the driven element and that element collects some signal. At virtually the same time, the same signal passes beyond the driven element/s, strikes the mirror and reverses its direction. The reflected signal is absorbed AGAIN by the driven element/s, doubling (or more than doubling) the initially collected signal level or strength.

If the "mirror" is removed, the driven element/s only collect the first pass of the signal.

So, a reflector DOES add signal strength.

No electrical connection whatsoever between reflector and coax output, to keep things simple. (Alright, bring out the horsey, Jim)

:flypig:

Rick
This has to do with the specific engineering of the particular antenna and it can be a bit confusing. Example, in relation to antenna booms: my Project-35 Yagi has electrically bonded directors and its reflector, but my Project-38 and 48 Yagis have insolated director elements and reflectors. Although the three examples are connected to coax via a transformer (baluns) my antenna masts are grounded which grounds the boom AND the director/reflector elements on my Project-35.

You may have an inline coaxial lightning protector, again electrically grounding the coaxial shield and I have to assume the coaxial fitting on the back of your TV set is electrically 'grounded', at least to some extent via its polarized 110 VAC power plug.

Channel Master 4221s and 4228s have screens that are electrically (physically) connected to their supporting masts, thus, they are grounded if their masts are grounded. On the other hand, the Kosmic Super Quad reflector screen is insolated from its supporting mast.

Your question made me discover that and I will retest the KSQ this Spring or Summer in 'stock' configuration and then retest with the screen electrically connected to its (grounded) supporting mast, to see if there is a difference in its performance. Great Q. No horsey, but this: :thumb:

Jim
 
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nbound-au

The Graveyard Shift
#8
Antennas/tv coax arent required to be grounded here at all. Therefore the screens may be "grounded" if on a mast sunk directly into the ground, or not if mounted on a tripod on a tile roof for example. Makes no (or at least negligible) difference to the received signal strength.

On a side note that means sometime aerials can give you a little buzz (nothing to major), and they arent always connected to ground via the tv, as some tvs dont use a grounding pin here and therefore there may be a floating voltage of usually 0-40V. (insulated tools are a good start :D )
 
#9
The quoted gain figures are the maximum forward gain only (in trade guides they actually have a map showing the gain in all directions around the aerial [aka. lobes]).
But not the 13.65 dBi I quoted. CM has 11.5 "average gain" plastered all over the internet for the 4221HD. My research tells me this is in dBd, which when converted to dBi = 13.65 average gain. In another life I was a math professor, and I'm pretty sure the average is smaller than the maximum here. Actually, the maximum can be found in a chart on another board, and it's over 15 dBi.

Reflectors do add to total gain (its on this basis a log-periodic antenna has gain) .
I dunno, man. I'm reading articles that refer to antenna reflectors as "parasitic elements," specifically because they don't add to total gain, by which is meant the gain (after you add it up) coming from all directions. I know the forward gain -- normally the gain that applies to the signal displayed on the TV -- is increased by a reflector. I thought that was clear from my statement "I definitely know the reflector does some good," and from my little chart.

Rick
 
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#10
If the "mirror" is removed, the driven element/s only collect the first pass of the signal. So, a reflector DOES add signal strength.
But clearly, this only applies to signal strength coming from the front. The reflector simultaneously reflects signal AWAY from the rear. The reflector "directs the gain flow" -- that's almost an exact quote from some article I can find if you make me. ;)

If we were in some strange universe where identical signals were coming at the same time from opposite directions, exactly one-quarter wave length apart, then a reflector would be counterproductive (maybe). I'm trying to find an answer to my highly theoretical question, and you guys are just WAY too grounded in reality. :daffy:

Guess I might need to crack open a book on this stuff some day before the world comes to an end.

Rick
 
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nbound-au

The Graveyard Shift
#11
But not the 13.65 dBi I quoted. CM has 11.5 "average gain" plastered all over the internet for the 4221HD. My research tells me this is in dBd, which when converted to dBi = 13.65 average gain. In another life I was a math professor, and I'm pretty sure the average is smaller than the maximum here. Actually, the maximum can be found in a chart on another board, and it's over 15 dBi.
My mistake, i must have misread :)

I dunno, man. I'm reading articles that refer to antenna reflectors as "parasitic elements," specifically because they don't add to total gain.
In a yagi or bowtie, all elements excluding the driver itself are parasitic. See: Passive radiator - Wikipedia, the free encyclopedia
 

n2rj

Moderator
Staff member
#12
Wait! Wait! I have a theory. Maybe I can understand now what n2rj was saying. Here's my theory: total gain = forward + rear + all side gain = some constant for any given channel, cause the reflector can't add or subtract from total gain, based on the Rickideemus theory.
With any antenna, you are not going to magically add gain in all directions. If you want more gain in one direction, you take away from another. There's no such thing as a free lunch.
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#13
On a side note that means sometime aerials can give you a little buzz (nothing to major), and they arent always connected to ground via the tv, as some tvs dont use a grounding pin here and therefore there may be a floating voltage of usually 0-40V. (insulated tools are a good start :D )
nbound-au, BINGO!

Before grounding my masts I had about 40 VAC (unknown frequency or frequencies) and there is just enough current to light a standard LED to about half-brilliance between the masts and ground. The 'buzz' or 'tickle on the skin' inspired me to make that test.

Jim
 
#15
My Mistake

I messed up by confusing gain with signal strength. Hence this idiotic assertion:
Rickideemus said:
But a reflector can't add any signal strength, right?
See, where I live there's plenty of signals coming from both directions. But then it dawned on me: you professionals often deal with installations where all the signal is coming from one direction. Obviously, then, a reflector can increase signal strength for every channel on the dial! A signal from the opposite direction might be reduced from zero to zero, or from 0.001 to 0.00001, but all those reductions together could never equal the increase in the forward direction.

n2rj said:
If you want more gain in one direction, you take away from another. There's no such thing as a free lunch.
Hence the Rickideemus theory that net gain, from all round the compass, is a constant for any given channel and any given antenna. So, I guess my little chart is about right, cept I probably have the side gain going in the wrong direction.

BTW, I went by the Salvation Army the other day. Know what they were doin? Yup -- handing out free lunches. Dang! Another old saw down the tubes. :becky:

Rick
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#17
I wrote:

Before grounding my masts I had about 40 VAC (unknown frequency or frequencies) and there is just enough current to light a standard LED to about half-brilliance between the masts and ground. The 'buzz' or 'tickle on the skin' inspired me to make that test.
------------------
N2RJ wrote: I thought that was mostly with older TVs. I know some older TVs did put voltage on the aerial leads.
------------------
Ryan,

I have noticed AC that 'tickles the skin' present on antennas in my area for over 40 years including on the non-grounded half of my 15 meter horizontal dipole. I always wondered if it had something to do with all of the full power TV transmitters less than a mile away and the other services on the same towers. RF Soup?

Jim
 

nbound-au

The Graveyard Shift
#19
Its quite impossible to get 40VAC from a transmitter at that distance. Its purely to do with something being ungrounded.

If you were getting 40VAC off a vertical mast, you'd be getting something crazy off the properly cut (ie. tuned to the output frequency) horizontal elements on your aerial (bye bye tv tuner)
 

Fringe Reception

Super Moderator, Chief Content Editor
Staff member
#20
nbound-au,

I used to work in engineering for a 50 kW AM radio station and we were required to do partial antenna-proofs quarterly and full proofs annually to maintain our license. These were done by taking dozens of field strength reading using a special, calibrated receiver all over the region. One of our monitor-points was in the center of an Elementary School playground and there was over 1 volt present there at a range of somewhere around 6-8 miles. Neither the CE nor I ever proved why the extreme node was there but my theory was the (grounded) chain link fence happened to be the right length to behave as a collector/reflector.

Regarding 40 VAC here, it cannot be caused by power lines because its an underground neighborhood. RF-Soup is my guess. :bolt:

Jim
 
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