Author Topic: Pixelated and Perplexed.How to make a good, small-size, DIY DBT Outside Antenna.  (Read 568 times)

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Offline martys

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FAQS: (First see attach pic of my antenna)

1)I live an unknown distance, maybe just 25+ miles from the DBT channels I want to watch that are broadcast from Sweden, but alas, I am in Denmark. No one I know succeeds in receiving these channels without a big amplified antenna located on the top of their roofs.

2)I have tried the many types of consumer-grade rectangular-flat amplified DBT antennas and they don't succeed better than to pick up the local channels.

3)There is no way my apartment rules would allow me to even get to the roof, let alone allow me to install an antenna there, but I have a very small back balcony that has a 6-ft tall corrugated steel wall on one side of the balcony that I use to mount my horizontally constructed  small 15-in swept-wing design indoor UHF antenna on its top edge with the help of 2 "clothespin" clamps. My balcony looks out across a football field which is enclosed by all the four-story apartment buildings. My antenna seems to be pointing slightly above the roofs of the surrounding apt buildings and is definitely pointed towards(with no other building obstructing its view) towards Sweden.

4)Through trail and error,  I have found a quite-critical "sweet-spot" direction that I point this antenna to that allows me to sometime receive the 6 Swedish channels I want. But half of the view that antenna sees is now being obstructed by leaves of a tall tree that is now beautifully verdant, stands about 10-ft away from my balcony, and I can't see myself sneaking out in the middle of the night with a saw and climbing up this tree to cut this big signal-blinding branch off.

5)TV watching is now a sometimes perfect for hours, especially in the evening, but depending on season and atmospheric conditions, more usually these days TV viewing has become watching a picture breaking up into pixel pack boxes.
What is driving me crazy is that the signal seems so close to adequate. I am not getting a completely "no-signal" dark TV screen as often as I am getting a pixel-packet parade.

6)The antenna I am using was originally made for indoor table-top use, but I connect to it with a 1/4-wavelength piece of high-quality 75-ohm coax cable to an antenna 12db VHF/UHF booster amp and then feed the signal from the amp output to my Dig TV, about 6-ft away. At the other end of the cable, I have a female F-connector mounted on a small square of  double-sided PCB stock and I feed the 12-V power to booster through the 75-ohm cable using a 20-uH choke(+12 V side bypassed) and use a good quality .1uF film cap paralleled with a COG 10nF SMD cap to block DC to the TV, and I have made sure I am keeping leads as short as possible.

7) The TV channels I want are on TV bands and within UHF channel 37-44 freqs. with the channels I want most around 600MHz.

8)I have tried inserting an additional 12-db UHF/VHF antenna inline-amp, but it just seems to be amplifying noise or oscillates and so this idea doesn't work.

Does anyone know how to make a better, yet small-sized DBT-TV antenna ?
« Last Edit: June 10, 2018, 02:46:37 am by martys »
 

Offline Wirehead

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Maybe you're just overloading the receiver - or have a strong nearby signal causing overload. Have you looked at the signal via something like an RTL-SDR stick? That might give a good clue..

I suppose we're talking about something around 800MHz? edit: just saw it - 600 MHz.  I've had good results with a simple patch antenna, and it was a much larger distance.
"to remain static is to lose ground"
 
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Online PA0PBZ

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Looks like you are using a log-periodic antenna, this gives a wide band but less gain. If you are just interested in something at 600MHz a Yagi for that frequency should perform better. Also, are you sure that the polarization is correct, did you try to rotate the antenna? And, the best place for an amplifier is as close as you can get to the antenna, the best place would be IN the antenna but I'm not sure if that is commercially available.


Keyboard error: Press F1 to continue.
 
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Offline Ian.M

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You are in a fringe reception area so line of sight is critical - but that's not an optical line of sight as UHF band signals will go through a moderate amount of foliage without major losses.   A couple of trees - no problem - a whole forest *BIG* problem.   A tiled roof will cause significant losses, but if it is lined with foil coated insulation, it could wipe out the signal.   Thick reinforced concrete is also a major problem - you can usually get a signal through ONE wall, but the reflections and scattering between multiple walls can wipe out any hope of getting a usable signal.

First you should determine the direction the antennae is pointing in as accurately as possible and check if it actually corresponds to the transmitter's bearing from your location.   Odds are it doesn't, and you are relying on the signal bending round or reflecting off conductive obstacles, so the loss of signal may be seasonally due to leaves in a forest on a ridge between your town and the transmitter, or due to new construction  raising or altering rooflines anywhere in your town along or close to a line between you and the transmitter, or due to a building it was previously reflecting off being re-clad in something that absorbs more signal, so even a good antenna may be ineffective.   

It may be worth trying putting a full size high gain highly directional rooftop DBT antenna on a magnetic base mount attached to the steel wall (I assume you aren't allowed a permanently mounted antennae there), but if there isn't a line of sight to the transmitter that's a gamble.   Amplifying a small antenna is a poor substitute and is usually unsatisfactory in an urban area - it boosts the noise as well and like for like, a smaller antenna is less directional so the signal to noise ratio is poorer  to start with.
 
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Offline martys

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Thanks Everyone for your Antenna Input.

The antenna shown must be very sharply pointed in the right direction to work at all.
Small adjustments so that it points more up or down make a big difference.
Small adjustments turning the pointing angle even a few degrees makes a big difference.
There doesn't seem to exist a better place within two feet along the corrugated wall it is perching upon of the present spot that works better or works at all.

I have no room for a multi-bowtie type of antenna vertically facing the signal path. I did construct a 3-bowtie one of my own, but it failed miserably. and mounting this contraption on the balcony would be an eyesore.

See the attached drawing I made that shows the construction of the horizontal polarized wing-shaped antenna I am now using.

Notice that it is made of two aluminum stampings, each piece is stamped from a thin piece of aluminum sheet and the two pieces are vertically stacked on a 1/2-in rectangular piece of plastic that is the boom.

The top and bottom pieces are mirror images of each other(In my drawing, the red and the black). 

I didn't complete the red mirror image, but one easily visualize it is just the same shape with complete mirrored symmetry, and they are separated by the 1/2-in rectangular plastic boom.


How it even works at all is a complete mystery, the fact that it outperforms any planar flat vertically polarized rectangular antenna I've tried is impressive.

I am getting standing waves. Literally, if I stand at the side of my flatscreen TV at a 90-deg angle(standing at the side facing the balcony, I can, by just standing there at this position, improve the signal strength from a breaking-up pixel square salad to a clear picture!

FAQs
1) The feed impedance of a dipole antenna is around 75-ohms.
2) I am using high quality 75-ohm foam coax and I am connecting the coax directly to the short most-front mirror-imaged elements. I feed the coax through the square plastic boom and connect to the input of my VHF/UHF antenna pre-amp as close as possible to coax exit point coming out of the rectangular plastic boom at the back of the antenna.
3)I tried to make the antenna-connect to the antenna pre-amp 1/4-wavelength to ensure the best impedance match, but I the boom is a bit too long for this so my impedance-matching coax cable is optimized in length for a much lower frequency.
4) I thought maybe use a balun, but the impedance is already matched at 75-ohms,  Not good, I am feeding a balanced antenna into unbalanced coax.
5)I tried making a coax balun from a winding it upon a 1/4-in toroid core salvaged out of a 75-ohm coax signal splitter, but it I could barely get a signal. I think I think the problem might have been that I made a 4:1 balun.
6)I tried instead to connect the 75-ohm coax directly(as the feedpoint) to the rearmost elements..didn't work at all.
7)The length of the elements and their spacing on this gizmo antenna are not close the 600-MHz resonant length desired,  except for the rear-most element This means I might do better if I construct an antenna with all elements close to a 600-MHz 1/4 wavelength.  (How does this thing work so well at all!)
8)I should be able to do better than this!

But how?
« Last Edit: June 15, 2018, 12:12:17 pm by martys »
 

Offline Howardlong

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Thanks Everyone for your Antenna Input.

The antenna shown must be very sharply pointed in the right direction to work at all.
Small adjustments so that it points more up or down make a big difference.
Small adjustments turning the pointing angle even a few degrees makes a big difference.


Hmm... the half power beamwidth of that antenna will be in the region of 80 degrees or so, so I don't think the directionality is your problem.

From your symptoms, multipath is though, e.g. reflections and other copies of the signal at similar signal strengths causing destructive and constructive interference.

By far you best bet at UHF is to get your antenna outside and above local building height, in an attempt to mitigate against multipath.

Also, check your polarisation matches that of the transmitter, easiest way to do that is look how other antennas in your locale are oriented.
 

Offline martys

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Howardlong:
Can't locate the antenna anywhere else, and I must not create eyesores on my balcony or face eviction. Many people have opted to go the satellite way, and there are many balconies adorned with small microwave dishes. I don't want to pay for this kind of TV reception when the free channels I want from Sweden just a nerd's imagination away from being receivable.

The antenna I am using, and I can assure you, through much futsing-around, has shown to be very directional.

From what I see all over the city, rooftop antennas are of the horizontally polarized type for  the most part. I have seen only a few bowtie array types installed on roofs.

Almost all of the indoor amplified antennas sold here are of the common black plastic-enclosed, think thin rectangular shapes,  and all are vertically-polarized types. People just won't buy a horizontally polarized antenna to put on their interior walls or tables, and it is simply in very bad taste to decorate any part of your living room in that nerdy way.

My antenna is outdoors on the balcony and even if pointed in a horizontal direction toward the transmitter, yet rotated 90-deg to sit in a vertical plane, so as to be vertically polarized, doesn't quite work as well.
« Last Edit: June 14, 2018, 09:26:49 pm by martys »
 

Offline Howardlong

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It only seems to be very directional, but what you’re experiencing is destructive and constructive interference as you re-orient it into and out of the right spot. If it was in free space, you wouldn’t see these effects anywhere near as much.
 

Offline martys

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It is not important what governs how my old antenna sees a weak channel.

My post is not to praise my old antenna, but to bury it.
« Last Edit: June 15, 2018, 11:59:18 am by martys »
 

Offline videobruce

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Aren't there allowances for erecting a outdoor TV antenna in the country you are living in now?

If it were me, I would move to somewhere more 'television' friendly .  ;)
 


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