How to build a UHF antenna... - Page 2 - AVS Forum
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post #31 of 4831 Old 03-06-2008, 10:26 AM
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Frank,

Thanks for all the good information!

Since my first post, I made the antenna as per the directions on the blogspot page. My first test was with the antenna right next to the tv, connected with a 6' cable. I was pretty impressed with the results. Then I put it up in the attic and ran a 100' coax from it to the tv. The results were far less satisfactory. So now it's back sitting next to the tv.

Once we get a little deeper into spring, I'm going to take a shot at making a weatherproof antenna using some extra backyard fence (1" x 2") that I've got.
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post #32 of 4831 Old 03-06-2008, 11:09 AM
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I wonder if it's the attic or the cable ... an easy test before the cable's run.

You lose signal either due to cable losses or attic losses. 100 ft. of cable should cost you 5-6 dB; next step is a mast amp. Why not test an amp in the attic now and possibly avoid the roof?

Get a good amp, too. Many people recommend Channel Master and Wineguard, but avoid Radio Shack

Pictures of projects are welcomed, too!
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post #33 of 4831 Old 03-07-2008, 09:29 AM
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We have UHF channels all the way up to Ch 69 so the full bandwidth is required. YMMV
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post #34 of 4831 Old 03-07-2008, 09:58 AM
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Only until next Feb. ...
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post #35 of 4831 Old 03-07-2008, 11:05 AM
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Not for Canada.
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post #36 of 4831 Old 03-07-2008, 01:39 PM
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The "700 MHz" band channels are being repurposed to new services, such as mobile TV,
including enhanced feeds targeted to handheld TVs in sports statiums--and at home.
Also transferring 1080p movies to laptops/PCs, traffic updates and other
wireless data distribution services that may become "must-haves".

You may want to maintain upper channel coverage post-Feb2009.

Qualcomm's MediaFlo service on CH55 is well on it's way to coast-to-coast service.
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post #37 of 4831 Old 03-07-2008, 04:09 PM
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Gentlemen,
I can see a variety of reasons for antenna makers to keep their products the same while claiming they're "optimized for HD/DTV." Canadian reception and speculative future services are not among them.

Nor do these two have anything to do with antenna requirements for DTV reception. This thread is about making antennas; if the market won't serve us, there are a few of us who will serve ourselves.

So, instead of telling us why we need something we can't use, how about helping us meet our needs? That would serve all concerned by allowing the builder to aim for the spectrum of interest to them, be it post-transition domestic DTV or broadcast on-demand video.
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post #38 of 4831 Old 03-08-2008, 12:18 AM
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Posted by fbov:
Quote:


Gentlemen,
I can see a variety of reasons for antenna makers to keep their products the same while claiming they're "optimized for HD/DTV." Canadian reception and speculative future services are not among them.

Nor do these two have anything to do with antenna requirements for DTV reception. This thread is about making antennas; if the market won't serve us, there are a few of us who will serve ourselves.

So, instead of telling us why we need something we can't use, how about helping us meet our needs? That would serve all concerned by allowing the builder to aim for the spectrum of interest to them, be it post-transition domestic DTV or broadcast on-demand video.

I'm not sure what point you are trying to make. Is it that UHF TV antennas that are currently made to receive channels 14-69 won't be good enough once DTV vacates channels 52-69? Don't tell the folks where I grew up in Buffalo, NY that they shouldn't have an antenna to receive Canadian stations on channels above 51. And I suppose that at some time in the future when one is interested in receiving the MediaFlo service on channel 55 that one will be expected to purchase another antenna, since they only have an antenna that will tune domestic frequencies up to channel 51?

From your post at the top of this page, you apparently believe antenna manufacturers should consider redesigning their antennas to "peak" at a channel below 51 to increase gain somewhat less than what one loses by insertion of a splitter. Probably not worth the effort.

There are issues of substance and issues that are trivial. If you want to make an antenna limited to certain frequencies, which you claim is the subject of this thread, make it as you like. But someday you may be making it again to receive the 700 MHz band.

IMO, Canadian reception and speculative future services are reasons for antenna manufacturers to keep their products the same, not redesign them for "domestic DTV", as you put it. New and improved is not always what it's hyped up to be.

And I have built my own yagi antenna which I have mounted on a 35 foot mast. And I don't care that it probably receives channels above 51, it works great for me.
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post #39 of 4831 Old 03-08-2008, 08:46 PM
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Quote:
Originally Posted by milehighmike View Post
... it works great for me.
Mike,
That's my point; what works may be different for each of us ...

I'm working on a little analysis, and if it works out, I'll post it here. I've used the actual CM-4221 GIF file used for the HDTV Primer model to grab dimensions, and the correct basis dimensionts gives an 8" bay spacing and wisker length. The wisker spread results in a dipole that's 1/2 wave at the model's peak gain. Seems like a good fit.

Since I've already got a 9" spacing with 8" wisker lengths, I'm going to try the experiment - 7" through 10" bay spacings and wisker lengths. In effect, 87.5%, 100%, 112.5% and 125% of the commercial design. Assuming no gain differences, the curves should look like the attached.

I added in the local stations and TVFool signal projections that I'll use for testing. 58 is my hardest, and should be bad only with the 125%. 16 and 28 should show a drop at whatever the TV's tuner threshold might be. It'll be interesting to see if these predictions pan out.

As to what works, that depends on your needs;
- the 125% gives low channels 2.5 to 5dB more gain in my projections, at the cost of any gain above ch 52. It gets ch 14 well, but not 64.
- somewhere between 112.5% and 125% is an optimum with maximum gain for US post-transition channels
- For my part, I'm going to lengthen my wiskers so I'm a full 112.5%, 9" bay spacing, which should give me 12 dBi gain out to ch 63 ... and down to ch 20.

It's a matter of understanding the tradeoffs. I'd like to talk about it; anyone else?

Frank

 

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post #40 of 4831 Old 03-09-2008, 10:05 PM
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Today I made and measured a series of 4-bay antennas at a range of sizes based on the CM 4221. The commercial design is an 8" element length and bay spacing. I set up a stand where I could vary the element spacing and length, and tested 7", 8", 9" and 10" antennas. This corresponds to peak gain channels of 88, 66, 51, and 40, respectively.

I tested on my local stations:
Station\tChannel\tChnl Rx
21.1 /\t16\t-70
31.1 /\t28\t-73.3
8.1 /\t45\t-72.6
10.1 /\t58\t-77.5
13.1 /\t59\t-73.5

The results are about what you'd expect.
- the lowest frequency, ch 16, is also the strongest signal; I had to remove the reflector from the 7" to get any fall-off, despite a large predicted change in gain. You can't beat a good aerial signal!
- channel 28 is noisy, but the trend is to less signal with the shorter antennas
- channel 45 shows almost no change
- the weakest station is ch 58, and its signal gets stronger as the antenna gets smaller, as expected.
- its neighbor, ch 59, shows a similar trend, but with an opposite reaction when the reflector was removed.

I will also note that results with my "good" antenna are consistently better than what's shown here, save for a couple points on 58 and 59 against the 7" antenna. It was an accident, but one I'm happy I made.

Bottom line, you can move the peak gain of an antenna to one end of the UHF band, but only at the expense of gain at the other end. That's no surprise. What's interesting is the ability to tune this type of antenna by a simple scaling of element spacing and length.

HAve fun,
Frank

 

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post #41 of 4831 Old 03-09-2008, 11:01 PM
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FYI: Dipol made a 4-Bay where they extended the length of the two TOP whiskers,
which would help the response at lower freqs (see special version 3 on pg1):
http://www.dipolnet.com/kat01.pdf
Note that they also added some (too small?) DIRECTORS in front of bow-ties...
Gain charts are for various 4-Bays with DIRECTORS, but not for extended whisker version.

Dipol's webpage home:
http://www.dipolnet.com/
BTW: World-wide TV frequencies are more or less same, but channel #'s vary country-to-country.

This opens up a series of questions as to which whiskers are best to extend.
What about extending all FOUR whiskers on the top bow-tie?
What about extending all FOUR whiskers on both top and bottom bow-ties, but not middle two?
What about extending only one set of whiskers in EACH bow-tie?
Exercise for the student using NEC simulation.....

A set of metal back-to-back "sockets" could assist in temporarily adding whiskers.
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post #42 of 4831 Old 03-10-2008, 10:54 AM
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One reason I chose the 4-bay approach was that a series/parallel combination of 4 dipoles has the exact impedence of a single dipole across the frequency range. Compare the gain curves of a 4221 and a 4228 and you'll see Channel Master did a poor job of ganging the 4-bays - the expected gain curve should look like the 4221, raised 3dB for the gang. If I ever make the 8-bay, it'll be two 4-bays with baluns feeding a combiner to get around this.

I expect wisker mods to a single bay will alter that bay's gain curve, but what will a unique element (or two) do to gain at the down lead? I avoided single-element modifications as much as possible because I expect the overall gain to drop due to the impedence mismatch from the modified elements. (Plus 9" wiskers barely fit through the access door to my garage attic.)

Unfortunately, this analysis is the extent of my analytical capabilities; I've neither model nor electronics to do more than read the TV's (or CECB's) signal meter. I'd love to see the modeling runs for the ants I made as well as some of these mods. Maybe there's something out there that's really better, not just hype.

For my part, I now have a fully 1 1/8x scale CM4221 clone and it works. I also have enough parts for a second one, but my test stand will never hold a 4228 clone. I'll need a new test stand.

Have fun,
Frank
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post #43 of 4831 Old 03-10-2008, 03:06 PM
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Any antenna with 2:1 freq coverage is going to have miserable VSWR....
so what's a few more fudges...

The fol. aren't exactly 4-Bay antennas, but it should give you an idea
wrt VSWR range for "similar" broadband fans and bowties:
http://www.cebik.com/mu/mu6a.html
http://www.cebik.com/vhf/corner5.html
VSWR gets worse beyond the limited freq range in these examples.

And here's soup-to-nuts-to-pineapples:
http://www.cebik.com/radio.html
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post #44 of 4831 Old 03-10-2008, 05:41 PM
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What distance have u been able reach and still get a solid UHF High def picture. I am refering to anyone that has built and used the antenna in the site below.

http://uhfhdtvantenna.blogspot.com
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post #45 of 4831 Old 03-11-2008, 08:32 PM
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Quote:
Originally Posted by u00gwa1 View Post

What distance have u been able reach and still get a solid UHF High def picture. I am refering to anyone that has built and used the antenna in the site below.

http://uhfhdtvantenna.blogspot.com

Funny thing is the first one I built (like the one in the link) works so well, I've given up trying to top it ....well at least in the attic. I'll probably keep using it for the OTA recorder since it' receiving distant stations (60 & 75 miles) at almost 60 degrees apart.
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post #46 of 4831 Old 03-12-2008, 08:20 AM
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Did you build the exact antenna shown, or one that's close?

What channels are you receiving?

The reason I ask is that the blogspot antenna is a 7" design, which shifts the gain response of a commercial CM-4221 (an 8" design) so it peaks at channel 88 with significant loss of gain below Ch 25. A few posts up, I talk about the effect of this basis dimension on channel gain, predicting response based on 4221 curve scaling and then making and testing the series.

It works; you can shift the channel gain.

Thus the 2-part question, what did you build and what's its channel response?
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post #47 of 4831 Old 03-17-2008, 11:10 AM
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Quote:
Originally Posted by fbov View Post

Did you build the exact antenna shown, or one that's close?

What channels are you receiving?

The reason I ask is that the blogspot antenna is a 7" design, which shifts the gain response of a commercial CM-4221 (an 8" design) so it peaks at channel 88 with significant loss of gain below Ch 25. A few posts up, I talk about the effect of this basis dimension on channel gain, predicting response based on 4221 curve scaling and then making and testing the series.

It works; you can shift the channel gain.

Thus the 2-part question, what did you build and what's its channel response?

BUMP - I'm curious as well
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post #48 of 4831 Old 03-27-2008, 09:45 PM
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I have built 3 of the YouTube 4 bays that use 7" whiskers and 5 3/4" spacing. This design works well for the garage TV using a CECB. My hair stylist is using my first antenna. The last antenna I built, I used 7" whiskers and 6 1/4 in spacing. I lost considerable gain on RF58 & a little on RF56. I showed no change on mid range, but I was surprised I lost some on RF23. I am going to experiment with more combinations. I did a reflector experiment by holding up an old truck grill to the backside. LOL. The spacing appeared to be crucial.

Bob 61231
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post #49 of 4831 Old 03-28-2008, 08:14 AM
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http://www.youtube.com/watch?v=EWQhlmJTMzw

That's the one I built, no reflector though, I'm sure one could easily be added.
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post #50 of 4831 Old 03-28-2008, 08:35 AM
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I built the blogspot antenna first, using cardboard. It works fine. Upstairs in the room facing the towers, everything is perfect. Downstairs where it has to go through multiple walls or be next to the chimney, my amplified HDTVa pulls in one more channel. I also build the YouTube one, with a 1x4. But if you want a reflector, installing the coathangers on the narrow edge of a 2x4 works better, so you get 4" spacing to the reflector.
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post #51 of 4831 Old 03-28-2008, 08:41 AM
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Quote:
Originally Posted by spokybob View Post

I have built 3 of the YouTube 4 bays that use 7" whiskers and 5 3/4" spacing. This design works well for the garage TV using a CECB. My hair stylist is using my first antenna. The last antenna I built, I used 7" whiskers and 6 1/4 in spacing. I lost considerable gain on RF58 & a little on RF56. I showed no change on mid range, but I was surprised I lost some on RF23. I am going to experiment with more combinations. I did a reflector experiment by holding up an old truck grill to the backside. LOL. The spacing appeared to be crucial.

I've got to take a look at that video (I've had miserable luck with Youtube in the past). I avoided arbitrary dimensions so I could reasonably rely on modeling predictions, so I can't comment on why you'd be losing gain at both ends by growing half an inch. I can talk a little about reflector spacing.

You tune a dipole by making it 1/2 wave long. You tune an array by making elements 1/2 wave apart (and twisting the feedline). You tune a reflector by making it 1/4 wave away, so the total path, there and back, is 1/2 wave. The reflector has a bigger effect because it drastically modifies the directional gain profile. The flattened, bi-directional toroid of a vertical dipole array becomes a lopsided single peak with side lobes, but with a whole lot more gain in the direction of that peak.

In my case, I think the reflector helps. Your mileage may vary.

Frank

PS I should mention that a channel-frequency table will tell you the frequencies of the channels you want. Those numbers, in megahertz, divided into 300 (speed of light in m/MHz) yield the wavelength in meters. As an example, 600 MHz, Ch 35, would have a 0.5m wavelength, so 1/2 wave is 9.86", but ch 35 a bit low to be practical.
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post #52 of 4831 Old 03-28-2008, 11:41 AM
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Frank. I am not good at theory, that's why I appreciate all the reports from you and others on their results. The reason I am trying to avoid a reflector is the 37 degree spread of the xmitters. Before the transition, we also have a great PBS that is 110 deg. from the antenna farm, so I have to include that one until Feb. 2009. I know a few older folks that still rely on rabbit ears & decades old antennas.
Bob ZIP 61231

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post #53 of 4831 Old 03-28-2008, 03:50 PM
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Bob,
Looking at your polar plot, you need an omni! A reflector-less 4-bay will do you well today, and may even pick up enough of Ch 4 to work next year (unlikely). It takes a bit of mental exercise to visualize these things, even with pictures ...
http://www.hdtvprimer.com/ANTENNAS/types.html

Scroll down to the dipole diagrams and see if you can see the "donut" shape of the reception pattern. The dipole sits in the hole. Scroll farther to see what stacking several dipoles does to the pattern.

You should see something like a donut squashed out of round, much less gain vertically and a pair of bigger lobes horizontally - that's the pattern you want.

If you're still into construction, I'll recommend a 4-bay with 9" wiskers and 9" element spacing. Peak gain should be around Ch 50, so you'll still get all your current UHFs. You'll actually aim the antenna elements at the gap between antenna farms, so a major portion of the gain falls toward the transmitters.

On the commercial side, you're in a great place for a smart antenna. Hooked to the proper controller, it adjusts gain on an antenna array to steer maximum gain electronically for every station. Your signals are high enough this would probably work very well! The down side is you need a tuner with smart antenna control ...

Let us know how you make out,
Frank
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post #54 of 4831 Old 03-28-2008, 10:56 PM
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Hi Frank.
I made another 4 bay using 9" spacing & 9" whiskers. I noticed a gain on RF49 just as you predicted. However, 56 & 58 dropped way off, unlocking sometimes on 58. Also, my RF45 (12.1) which is off to the side, did not pick up at all. OK. So I rotated to find the sweet spot. I could not find a spot that picked up all channels strongly. The 9" must make it too directional.
I am making these out of #12 gauge copper wire. Will a larger wire show improvement? Another question. I bought a bagful of 300/75 transformers at a buck each. Is there a way to check them? Or is it a go/nogo thing?

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post #55 of 4831 Old 03-29-2008, 10:31 AM
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I've now seen the YouTube antenna. If you're following it exactly, there's a problem with the feedlines that may help with higher frequencies.

One of the rules in RF is to keep spacing constant, because conductors respond to fields, and spacing changes field strength. Twin-lead antenna cable has great propagation because the conductor spacing is constant. That's what you're looking for in the feedlines; parallel as much as possible, with constant spacing wherever they must cross. (Twin-lead's downfall is interference, and we address that by keeping the feedlines vertical, perpendicular to the horizontally polarized wavefront.)

YouTube does not maintain spacing when the feedlines cross. The CM-4221 does:
http://www.warrenelectronics.com/antennas/4221.htm
and so I did as well (attached). CM even mentions it in one of their install sheets.

I tested 4 antennas (all on a 1x3" pine board with the same reflector) ranging from 7" to 10" and posted it here. Scroll up to March 10th (can't see post numbers in this view) and you'll see I have a Ch 58/59 pair that are my weakest signals and so show the most movement as antenna scale changes. However, I'm a single-site receiver - only one antenna farm - so directional makes sense, and perhaps explains my results for Ch 58/59.

Perhaps in your case, upper UHF is what matters!

Frank
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post #56 of 4831 Old 03-29-2008, 11:18 AM
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Quote:
Originally Posted by spokybob View Post

Hi Frank.
I made another 4 bay using 9" spacing & 9" whiskers. I noticed a gain on RF49 just as you predicted. However, 56 & 58 dropped way off, unlocking sometimes on 58. Also, my RF45 (12.1) which is off to the side, did not pick up at all. OK. So I rotated to find the sweet spot. I could not find a spot that picked up all channels strongly. The 9" must make it too directional.
I am making these out of #12 gauge copper wire. Will a larger wire show improvement? Another question. I bought a bagful of 300/75 transformers at a buck each. Is there a way to check them? Or is it a go/nogo thing?

Wire gauge and material are second order effects beyond my current knowledge; I'm using #18 galvanized ceiling wire.

Balun function is go/no go, but insertion losses can vary as can DC pass-through (important for antenna rotors). I'd pair them up and see if I saw a difference when inserted bwtween antenna and TV; a working pair with 300 ohm leads attached should only show an insertion loss in the range from 0.5 to 4 dB (10% to >50%). Less is better.

Scaling an antenna, done right, has no effect on directionality. You just move on the channel-gain curve. The 9" should have the same directional pattern as any other 4-bay bow-tie array, just at slightly different channels.

However, the gain curve drops quickly above the peak gain channel. That's why I suggested 9" as my projections indicate no more than -2dB drop from peak gain at Ch 58. It's very strange that 49 would improve but 56/58 degrade due to a gain curve shift like this. There's something else going on.

In fact, it's very odd that you have any issues with any of these stations! I've attached a TVFool plot for your zip, and your issues are with stations that rabbit ears should bring in loud and clear! Your Ch 58 is 26dB stronger than mine, but there's a simple explanation.

My Ch 58 is 25 dB stronger at the post office - local terrain degrades my signal significantly, and has a greater effect at higher channels. What does the plot for your street address look like? Are there other factors involved? Here's a bit of background on terrain effects, in case you're not really LOS.
http://www.hdtvprimer.com/ANTENNAS/siting.html

Frank
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post #57 of 4831 Old 03-29-2008, 06:38 PM
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Quote:
Originally Posted by fbov View Post

Wire gauge and material are second order effects beyond my current knowledge; I'm using #18 galvanized ceiling wire.


Scaling an antenna, done right, has no effect on directionality. You just move on the channel-gain curve. The 9" should have the same directional pattern as any other 4-bay bow-tie array, just at slightly different channels.

However, the gain curve drops quickly above the peak gain channel. That's why I suggested 9" as my projections indicate no more than -2dB drop from peak gain at Ch 58. It's very strange that 49 would improve but 56/58 degrade due to a gain curve shift like this. There's something else going on.

In fact, it's very odd that you have any issues with any of these stations!


Frank

Thanks again Frank for your ideas. Here is where I have been doing all my recent tests of antennas. I use the DISH622 box because of the easy to use strength meter.

I do have a CM4221 on the roof, but I am developing cheap antennas for the older folks around here that use rabbit ears. I want to use an antenna that hangs up on an interior wall like in the picture. I did borrow rabbit ears for testing. When placed by a window, it only receives from the antenna farm and NOT for 4.1 and 12.1. I plugged in my address to tvfool. Lat41.2 Long90.76. Results are similar to yours. TVFOOL puts ZIP at east edge of town. I am 2 miles west, on the west edge of Aledo.
Again, heeding your recommendations I went back and tried to make the connectors more parallell on the 9 incher. The crossover wires go up about an inch. This pic does not show my excellent workmanship.

LOL As you say, "there's something else going on". I hooked up a CECB on the garage TV and tested my antennae again. I came up with roughly the same results. The 9" whiskers was the worst of all that I have made.
The 7" whisker on 5 3/4" spacing with the wires being insulated but touching at the two crossover points is still the best so far. I have a CM4220 that works very well in the garage, but it has to be pointed just right to capture all stations.
I am not sure what my next attempt will be. I do not want to recommend a smart antenna because of the price. I would like to hear from someone that has one in an area like mine, where the xmitters are 20-35 miles away in more than one direction.

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post #58 of 4831 Old 03-29-2008, 07:27 PM
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My first thought was "why does he have a ship poster under his antenna?"

My second was that you've got a great idea here! If you're getting better results, go with it. I'm not to the point where I can predict with authority, and you're effectively testing to customer requirements, so you know what works.

I'd love to see a direct comparison with the 4221 and 4220, but testing's not your objective and the roof likely isn't convenient. Keep doing what works!
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post #59 of 4831 Old 03-29-2008, 07:54 PM
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Thanks for your help Frank. I did have the 4220 on the roof for a short time. 8.1 went to low power for a while & I got dropouts so up went the 4221. I still get dropouts during the day on 12.1 which is off to the side. But in the evening I get it just dandy. And I have the other PBS at 24.1 that shows the same programs during the day most of the time.

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post #60 of 4831 Old 03-29-2008, 09:00 PM
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How crucial are the reflectors?

Do I understand correctly that a reflector can actually hurt reception in some cases? Or just a poorly built reflector?
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