How to build a UHF antenna... - Page 7 - AVS Forum
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post #181 of 4829 Old 05-22-2008, 06:48 AM
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Quote:
Originally Posted by Don Bouldrey View Post

Thanks mclapp. This was a quick and dirty experiment to check the potential success of either building a more proper and exact 4-bay out of quality parts or ordering in a real 4221. The reflector is spaced only by the thickness of the 2x4. I based it on some of the pictures in this thread.

I tested it with the butt of the 8' 2x4 on the ground and compared results against the folded dipoles set at 10' AGL. It wasn't even close.

I hooked it up again this morning in the fog. It did better and even did a decent job pulling in VHF 9. Go figure.

I guess it's actually 10" on 9" spacing. Is this or the tight reflector spacing factors that break the deal? How important is precision in the bends of the phasing lines and elements? Your pdf's indicate you're quite precise here.

You've got a couple of bad things going on with that, as best as I can tell from the picture. The reflector being that close will throw things off alot and the way your wires are attached to the elements are not good. The phasing wires should only touch the V elements at the screw attachment and no where else. Be sure that where they touch the metal is clean, without any coatings or corrosion.

The precision of the bends and phasing lines are not super critical but the better you do it the better the end result, plus or minus up to 1/2" on some things would still get good results. Try to keep the crossovers at least 1/2" apart.

If you are looking to get channel 9 with this as well then go to a 36 x 36" reflector.

If you go to the larger reflector try bending the outer 8" 2" towards the bowties it has a slight corner reflector effect. Check out the pictures of the ones I built to see wht I mean.
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post #182 of 4829 Old 05-22-2008, 08:45 AM
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I just finished trimming, shaping and spacing the elements to your dimensions. I also installed the phasing lines properly. It's showing some promise now. It's almost on par with my dipoles set at around 20' or so. I'll work on the reflector next, then give it a bit of altitude. As it is, I'm getting steady 77% signal on VHF 9 at 16.6 miles.

Does the reflector need to be spaced that far back from the phasing lines (if insulated by wood) or only from the element tips?

Edit: I put 4" of spacing in between the phasing lines and the reflector. It's working. VHF 9 went up by 10% and my two most difficult UHF channels are at 74 and 85 without any tweaking or orientation. Thank you. Maybe now I can get rid of my floppity dipoles.
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post #183 of 4829 Old 05-22-2008, 09:56 AM
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Quote:
Originally Posted by Falcon_77 View Post

1/4 is for a dipole or 1/2 for both sides.

Bow-ties are 1/2 wavelength on each element (each side) or a full wavelength for both sides.

Are you sure? I think for an open-ended bowtie it should also be 1/4 wavelength elements, or 1/2 wavelength for the full array (approximately). I haven't found a great reference for this, but there is this one at cebik.com (registration required, I can't post the URL because I haven't made enough posts here, search for "bow tie").

I've built a small DB2 with 4.5" elements that pulls in all the stations that my 7" DB4 does (nothing below channel 24). I had thought that people used 7" or longer elements for response to lower channels (VHF high?).

I would love to see the modeled frequency response for this is someone has done it, or has a reference. My understanding, though, is that if you are only trying for higher UHF channels, you can go smaller than most people seem to be building. Please correct me if I''m wrong about that, preferably with data.
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post #184 of 4829 Old 05-22-2008, 11:39 AM
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Quote:
Originally Posted by olaf44 View Post
Are you sure? I think for an open-ended bowtie it should also be 1/4 wavelength elements, or 1/2 wavelength for the full array (approximately). I haven't found a great reference for this, but there is this one at cebik.com (registration required, I can't post the URL because I haven't made enough posts here, search for "bow tie").

I've built a small DB2 with 4.5" elements that pulls in all the stations that my 7" DB4 does (nothing below channel 24). I had thought that people used 7" or longer elements for response to lower channels (VHF high?).

I would love to see the modeled frequency response for this is someone has done it, or has a reference. My understanding, though, is that if you are only trying for higher UHF channels, you can go smaller than most people seem to be building. Please correct me if I''m wrong about that, preferably with data.
These 4 bay antennas are collinear arrays not dipole arrays so the element length is 1/2 wave instead of 1/4 as with a dipole. Just google collinear arrays and you'll get all sorts of info.

Those closed end bowties may also be 1/2 wave elements because the closed end also acts as part of the length, think of it as the wave traveling around the outside parimeter of the whole bowtie. These aren't quite as effective because the folded part (the end) doesn't do much for horizontal radiation.

Here are the gain and SWR plots for a simple 4 bay using 9" phase lines and no reflector, which I have posted a diagram for in an earlier post.

 

4 bay 9 inch SWR plot uhf.pdf 6.4482421875k . file

 

4 bay 9 inch 2d azimuth plot uhf.pdf 21.27734375k . file
Attached Files
File Type: pdf 4 bay 9 inch 2d azimuth plot uhf.pdf (21.3 KB, 14 views)
File Type: pdf 4 bay 9 inch SWR plot uhf.pdf (6.4 KB, 23 views)
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post #185 of 4829 Old 05-22-2008, 12:50 PM
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Quote:
Originally Posted by olaf44 View Post

I had thought that people used 7" or longer elements for response to lower channels (VHF high?).

A secondary reason for my 10" bow-tie design was to improve upper VHF reception as compared to commercial designs with 8" elements. I don't think it's a coincidence that most commercial UHF antennas do better on 13 vs. 7.

A 5.5" bow-tie would be optimized for around 1000MHz, which is well above any TV stations, but it still might work depending on how strong your signals are.
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post #186 of 4829 Old 05-22-2008, 12:55 PM
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Quote:
Originally Posted by Falcon_77 View Post

A secondary reason for my 10" bow-tie design was to improve upper VHF reception as compared to commercial designs with 8" elements. I don't think it's a coincidence that most commercial UHF antennas do better on 13 vs. 7.

A 5.5" bow-tie would be optimized for around 1000MHz, which is well above any TV stations, but it still might work depending on how strong your signals are.

For an open end bowtie yes for a closed end (trianglular shaped) bowtie no

10" may actually take the VHF-hi reception alittle below 7 and the commercial version with the stock reflector actually acts a director on ch 7 but as a reflector on 9 and above
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post #187 of 4829 Old 05-22-2008, 01:13 PM
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It appears I spoke too soon. Once the low stratus deck started giving way to broken cumulus, my rebuilt 4-bay started going down the tubes. Since then I've walked about 50 miles in between the signal meter and the antenna searching for that elusive sweet spot.

Oddly, VHF 9 at 269° and 16.6 miles is still doing great. What gives me fits is 41 @ 264° and 22.3 miles and 47 @ 222° and 7.8 miles. They alternate between decent signal and utter failure, sometimes in lockstep and sometimes opposingly. Also odd, no matter how I position the antenna, 31 at the same transmitter site as 47 (222° and 7.8 miles) comes in at 95 or better all the time.

Analog reception was so simple compared to digital.
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post #188 of 4829 Old 05-22-2008, 02:42 PM
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Quote:
Originally Posted by olaf44 View Post

Are you sure? I think for an open-ended bowtie it should also be 1/4 wavelength elements, or 1/2 wavelength for the full array (approximately). I haven't found a great reference for this, but there is this one at cebik.com (registration required, I can't post the URL because I haven't made enough posts here, search for "bow tie").

Falcon_77 and mclapp are correct. The UHF bowtie is not a half-wavelength dipole, but a full-wavelength dipole, also called a collinear pair.

For references please see my post #7177 on The Official AVS Antenna Topic! thread:

http://avsforum.com/avs-vb/showthrea...0#post13734770

If you can not measure it, you can not improve it.
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www.megalithia.com/elect/aerialsite/dttpoorman.html
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post #189 of 4829 Old 05-22-2008, 03:31 PM
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Well thankyou for looking into it; Louisiana isnt the most extravagant place and Ill bet the s/n ratio etc may keep it in the green area. I have the wood ready and will try the 10" for sure since the stations are in the 17-38 range as you'd recommended. I have some reflector material as in Don Bouldry's shown reflector thats galvinized, Ive read that 1" or 1X2" mesh is good, ; Id try that kind ; I also have 2X2" chicken wire. Normally I would have seperated the crossovers but the simple tape solution seemed allright. Actually some of your designs belong at Nasa really they are fine.
I spent a while looking at engineering projects. Ive tried many of them and never found anything that could beat the bowtie; things like driven reflector quadrature dipole array, some collinear arrays cut to specification, a VHF horn antenna from chicken wire 11 feet long, rhombic antennas, UHF corner reflector, mini pyramidal UHF antenna, log periodic antennas; two stacked fan dipole; indoor horn antenna; yagi

It looks like UHF will innovated again. I can post any of these I xeroxed.
Im gonna try the 10" for 1-38 range, maybe using galvinized refelctor and try in wood a clone in copper for the attic.
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post #190 of 4829 Old 05-22-2008, 04:35 PM
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Quote:
Originally Posted by rabbit73 View Post

For references please see my post #7177 on The Official AVS Antenna Topic! thread:

http://avsforum.com/avs-vb/showthrea...0#post13734770

Nice explanation on that thread.

I was considering center mounting the elements at the low voltage point as you mentioned, but talked myself out of it figuring that if it was super important the commercal builders would mount them that way.

The end effect definitely has to be a factor so I cut my elements alittle on the short side to compensate for the loading effect.
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post #191 of 4829 Old 05-22-2008, 07:04 PM
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Now it's a 9x9 with a 24x50 reflector and 4" separation between elements and reflector.

Adding the reflector wings brought the signal for VHF 9 up to 97%. UHF 17, 27 and 31 are also at 97. 45 comes in at 92. That leaves me with the problem children. 41 and 47. I found the sweet spot to get both stable at 77... at least until the weather changes or the wind blows

Question is: can I improve UHF reception for these two PITA channels by increasing element/reflector separation another inch or two? Is the anything else I can do to get a better signal out of these two?

My towers are all between 195° and 269°. The two problem signals are at 222°/7.8 miles and 264°/22.3 miles. I'm currently aiming at 245°.
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post #192 of 4829 Old 05-22-2008, 08:33 PM
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Heres the 1955 printout for an allwave in chickenwire, even for size Im more commited to spaced elements. Im seeing that Channels 14-30 is 10 1/16";
31 - 47 is 8 7/8"; 48 - 64 is 7 5/8" and 65 - 83 is 7 1/8"


http://i180.photobucket.com/albums/x...X/Allwave1.jpg


http://i180.photobucket.com/albums/x...X/Allwave2.jpg
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post #193 of 4829 Old 05-22-2008, 10:29 PM
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Quote:
Originally Posted by Don Bouldrey View Post

Now it's a 9x9 with a 24x50 reflector and 4" separation between elements and reflector.

Adding the reflector wings brought the signal for VHF 9 up to 97%. UHF 17, 27 and 31 are also at 97. 45 comes in at 92. That leaves me with the problem children. 41 and 47. I found the sweet spot to get both stable at 77... at least until the weather changes or the wind blows

Question is: can I improve UHF reception for these two PITA channels by increasing element/reflector separation another inch or two? Is the anything else I can do to get a better signal out of these two?

My towers are all between 195° and 269°. The two problem signals are at 222°/7.8 miles and 264°/22.3 miles. I'm currently aiming at 245°.

My models show best performance with a reflector spacing at 5" also try angleing your bow tie elements forward about an 1 1/2" at the ends and bend the reflector screen forward about 2" at the ends, the bend should start about 1" in from the outer edge of the bowtie elements. See my earlier post that had the picture. http://www.avsforum.com/avs-vb/attac...5&d=1211427493 These are slight improvements but they all add up.

The bending of the reflector will make the beam width slightly narrower which may help you some with the near-by station since you should get that with a coat hanger, you may be having multipathing issues with that.

If you are getting alternating strong signals and then losing lock on one or two stations you may be having multipath issues.
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post #194 of 4829 Old 05-23-2008, 06:20 AM
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Thanks again mclapp. The nearby station confuses the bejeezus outta me since there is another station broadcasting from the exact same coordinates and I get a 97% signal without regard to orientation. The antenna can be laying face down on the ground, pointing the wrong way and I still get a 90+ signal.

There is a major difference in transmitted power between the two (418 kW vs. 226.5 kW) but I receive a 29 kW signal at 97% from 16.6 miles away. Does a lower frequency allow for better transmission at lower wattage?

I appreciate your advice. I'm working on DTV Antenna version 27 now to institute these tweaks. Gotta get it right one of these days.
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post #195 of 4829 Old 05-23-2008, 07:44 AM
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Quote:
Originally Posted by Don Bouldrey View Post

Thanks again mclapp. The nearby station confuses the bejeezus outta me since there is another station broadcasting from the exact same coordinates and I get a 97% signal without regard to orientation. The antenna can be laying face down on the ground, pointing the wrong way and I still get a 90+ signal.

There is a major difference in transmitted power between the two (418 kW vs. 226.5 kW) but I receive a 29 kW signal at 97% from 16.6 miles away. Does a lower frequency allow for better transmission at lower wattage?

I appreciate your advice. I'm working on DTV Antenna version 27 now to institute these tweaks. Gotta get it right one of these days.


Several things going on here I think:

1.Analog power is measured differently than Digital.
Analog is measured as peak Digital is measure as average.
I believe that the energy is spread out more in a digital signal
making the average power measurement more applicable to a
digital signal.

2.Higher Frequencies require more power for the same result.



3.You could have two different transmitting antennas on the same
tower at different heights.

4.You could have two different towers on the same antenna farm
with different types of antennas and their directionality..






John
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post #196 of 4829 Old 05-23-2008, 07:57 AM
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Quote:
Originally Posted by tombobiche View Post

I have some reflector material as in Don Bouldry's shown reflector thats galvinized, Ive read that 1" or 1X2" mesh is good, ; Id try that kind ; I also have 2X2" chicken wire. Normally I would have seperated the crossovers but the simple tape solution seemed allright. Actually some of your designs belong at Nasa really they are fine.

If you use chicken wire for the 4-bay the twists (and the wires that connect them) must be horizontal to be an effective reflector for the horizontal polarization of the antenna.

If you can not measure it, you can not improve it.
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www.megalithia.com/elect/aerialsite/dttpoorman.html
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post #197 of 4829 Old 05-23-2008, 09:47 AM
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Quote:
Originally Posted by johnied View Post


4.You could have two different towers on the same antenna farm
with different types of antennas and their directionality..






John

I have a question about directionality. Do most TV broadcast antenna's use a horizontal dipole, like what we use to receive the signal? If so, is it stronger perpendicular to the dipole as opposed to parallel, like a dipole is at reception?

Does the FCC store the directionality and type of antenna or is there some way to find this out? Does TVFool take this into account?
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post #198 of 4829 Old 05-23-2008, 10:41 AM
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The FCC has antenna pattern and polarization for TV transmitters in its database. I know TV Fool takes the pattern into account; I do not know if it uses total power or only power received by a horizontally polarized antenna into account, however.
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post #199 of 4829 Old 05-23-2008, 11:56 AM
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Quote:
Originally Posted by toastyfries View Post

I have a question about directionality. Do most TV broadcast antenna's use a horizontal dipole, like what we use to receive the signal? If so, is it stronger perpendicular to the dipole as opposed to parallel, like a dipole is at reception?

Does the FCC store the directionality and type of antenna or is there some way to find this out? Does TVFool take this into account?

They can do all sorts of things to change the pattern.. beam tilt for
example imagine tipping a flashlight up and the beam goes further out
but overshoots the area underneath.. just an example...



Actually if i recall on the FCC website you can do a TV station search and somewhere along the bottom when the stations info comes up there is map data not sure and like he said above tvfool has radiation patterns too.



If I find the link for the FCC i will repost it here by editing...


John
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post #200 of 4829 Old 05-23-2008, 12:14 PM
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Quote:
Originally Posted by johnied View Post

Actually if i recall on the FCC website you can do a TV station search and somewhere along the bottom when the stations info comes up there is map
data not sure and like he said above tvfool has radiation patterns too. If I find the link for the FCC i will repost it here by editing...

FCC TV Database Query:

http://www.fcc.gov/mb/video/tvq.html

If a station is directional, it will have a polar plot, for example:

http://www.fcc.gov/fcc-bin/polarplot...78&p360=0.822&

A question I have is how some stations can achieve a non-directional pattern (at least in the azimuth plane). If most signals are horizontally polarized, how can such elements radiate equally on the off-axis? I suppose they use arrays to compensate for this, but I would like to know more.
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post #201 of 4829 Old 05-23-2008, 01:57 PM
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Here are a few broadcast antenna white papers from Dielectric:
http://www.dielectric.com/broadcast/dtv.asp
You'll find that many "Omni" antennas have 1+ dB "fluctuations"....
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post #202 of 4829 Old 05-23-2008, 07:31 PM
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I built one today from steel wire and wood based entirely on mclapp's excellent pdf dimensions; The steel was fairly malleable and seems to be some sort of stainless after cleaning. . The movie I was watching on Fox "Being Julia" was pulling about 19% with the 7X7", with the new 9 1/2 " I immedialy got 87%; I dont expect to have any more lost signals! Its behind the table with no reflector as can't fit fit in that spot. Ill test it on analogue for the weakest stations; but fundamentally is a complete success by my standards, so thankyou for all the advice, training , diagrams, pictures and material, to switching to a larger antenna for my area. Hope to install a mesh eventually as the summer progresses....
LL
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post #203 of 4829 Old 05-24-2008, 05:55 AM
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Quote:
Originally Posted by tombobiche View Post

I built one today from steel wire and wood based entirely on mclapp's excellent pdf dimensions; The steel was fairly malleable and seems to be some sort of stainless after cleaning. . The movie I was watching on Fox "Being Julia" was pulling about 19% with the 7X7", with the new 9 1/2 " I immedialy got 87%; I dont expect to have any more lost signals! Its behind the table with no reflector as can't fit fit in that spot. Ill test it on analogue for the weakest stations; but fundamentally is a complete success by my standards, so thankyou for all the advice, training , diagrams, pictures and material, to switching to a larger antenna for my area. Hope to install a mesh eventually as the summer progresses....


I'm glad it worked out for you, I have had good success with these antennas and what's great is they inexpensive and fairly easy to build.

I'm now working on a new idea that was passed along to me by another forum member that looks very promising.

It has to do with the reflector spacing and my modeling shows a 2 db increase in net gain on VHF-hi with a possable slight increase in UHF as well over previous designs.

I liked the chicken wire feed horn antenna in one of your previous posts, imagine that on the roof of a condo.
I was able to read most of it, it would be interesting to know if it really works.
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post #204 of 4829 Old 05-24-2008, 06:43 AM
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I have another version of the horn thats only four feet tall that fits in an apartment attic. Since digital is different these seem the best antennas concievable, where the older ones may work best on analogue. I did try the log periodic, yagi and circular antennas without much success.
Im looking at your design now and as predicted it favors the UHF band when the smaller antenna will bypass the lower stations. Every station has a tendency to pull higher, so Im sure that ouside and even with the reflector it will even further surpass. Of course theres so many factors in material and construction vaying from your concept,and the dimensions are optimal.


Oh heres a better image of it

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post #205 of 4829 Old 05-24-2008, 10:22 AM
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post #206 of 4829 Old 05-24-2008, 11:44 AM
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Quote:
Originally Posted by n4yqt View Post

The modulation type (ie. digital, analog, etc.) doesn't make a difference to an antenna. An antenna resonants to a RF signal regardless of it's modulation type.

Well the ATSC scheme is certainly more problematic with multi path,
so the antenna could certainly make a difference from analog to digital
:P


John
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post #207 of 4829 Old 05-24-2008, 01:00 PM
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Quote:
Originally Posted by johnied View Post

Well the ATSC scheme is certainly more problematic with multi path,
so the antenna could certainly make a difference from analog to digital
:P
John

ATSC is a monumental pain in the butt when compared to NTSC. Analog was so simple. Hang a couple of dipoles up in a tree and be done with it. Digital is antenna experiment week.

John, thanks for the description of the potential problems and the info you posted to me yesterday.
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post #208 of 4829 Old 05-24-2008, 01:38 PM
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Total reflector width is now 36" with 4½" between the phasing lines and the reflector. Everything is adjusted to mclapp specs and dimensions. The best combination of direction, altitude and tilt I've found so far gives me 77-80 signal on the two problem stations and everything else is 90+. All channels are stable regardless of wind or weather. We had a brutal thunderstorm yesterday with 3¾" of rain in two hours. The antenna performed like a champ. No fallouts on any of the channels. This antenna should hold up fairly well in a hurricane.

Maybe there's some dielectric interference with the pvc frame between the reflector and elements but it's doing the job for now. A further tweak may involve getting rid of this and mounting the elements on spacers to expand reflector clearance to 5".

Edit: This thing is made from 100% barn junk and it's mounted to the remains of an ancient Primestar dish system that died sometime in the years prior to 1999 when we bought the place
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post #209 of 4829 Old 05-24-2008, 02:11 PM
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Quote:
Originally Posted by Don Bouldrey View Post

Total reflector width is now 36" with 4½" between the phasing lines and the reflector. Everything is adjusted to mclapp specs and dimensions. The best combination of direction, altitude and tilt I've found so far gives me 77-80 signal on the two problem stations and everything else is 90+. All channels are stable regardless of wind or weather. We had a brutal thunderstorm yesterday with 3¾" of rain in two hours. The antenna performed like a champ. No fallouts on any of the channels. This antenna should hold up fairly well in a hurricane.

Maybe there's some dielectric interference with the pvc frame between the reflector and elements but it's doing the job for now. A further tweak may involve getting rid of this and mounting the elements on spacers to expand reflector clearance to 5".


I wouldn't worry about the PVC frame and the reflector there shouldn't be any problem there.

On a suggestion from another AVS member I've been modeling a new reflector screen set-up. I increased the reflector screen spacing to 15" back from the phasing lines and increased the width to 44" with the screen bending in 3 1/2" at the ends. The modeling has shown a 2 db net increase over the 5" design on VHF-hi and a slight increase on UHF above channel 30. I have not tried this yet in the REAL world and it's still in the tweeking stage but may be worth trying if your looking to squeeze out all you can.
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post #210 of 4829 Old 05-24-2008, 03:59 PM
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I had read earlier about an RCA box not doing too well. I switched to the Magnovox at Wal-Mart because although slower it locks down the station better, and lets you type in the virtual station, its ok. Others like Zenith, Insignia cost $10 and dont even perform as well; but by '09 theyll be more out there as the date approaches. The RCA seems to be user friendly but missing a quality chip... heres a site for that.

So theyll be obsolete Curtis Mathes TVs for $15

http://www.wtfda.org/
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