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Old 04-30-2008, 10:02 AM
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Originally Posted by tblue01 View Post

Thanks for the info ... very interesting and educational (for me anyway). I have a question regarding how to calculate the peak frequency. Running the numbers thru the formula, the 1/2 wave for channel 66 (the peak for a CM4221) is 7.5", not the 8" one would expect. 8" would equate to channel 58. Or am I looking at this the wrong way? I have a CM4228 and I noticed that the whiskers are spread approx. 5-1/2". And if I measure horizontally from the point of the whisker to the outer edge, the horizontal measurement is 7.5".

So my question, does it make sense that the peak corresponds to the horizontal measurement of the whisker out from the bend point?

There are a bunch of things that go into the calculation as all the wires (elements) interact with each other. Diameter of the element,spacing,ect..

The spacing of the elements to each other (bay to bay) and reflector spacing along with element length all factor in. Changing any one of those factors will change the frequency responce.
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Old 04-30-2008, 02:09 PM
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When doing an NEC modeling exercise, the size and location of every bit of metal has to be included. However, once that's done, there is a much simpler option that retains a high degree of accuracy over a limited range - scaling.

In my case, I started with a CM4221, which when properly measured has certain relationships between physical size and frequency response. Peak gain is at a frequency whose wavelength equals the width of the bowtie when wiskers are spread. This is an observation only, but it fits with how EM radiation interacts with matter.

If one changes all dimensions when making a copy, you get a scale model of the original, and in antennas, that means you can scale the frequency axis. If it's twice the size, just divide each frequency in the gain table by 2 to get the new gain curve. (But 2x is a BIG change! Scaling may not apply this far out.)

Conversely, if you know an antenna's peak gain frequency and you want to move it, your scale factor becomes the frequency ratio. In this case, a CM-4221 peak gain is Ch 66, 785 MHz, and you want to move it to Ch 51, 695 MHz. The ratio is 785/695=1.129x, which is real close to 1 1/8 (1.125).

The key is that ALL DIMENSIONS have to change by this amount. Some things, like wire diameter, have smaller effects, but basic building blocks like element size and location have to all change by the same factor to maintain any degree of predictability.

Frank
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Old 04-30-2008, 05:25 PM
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Originally Posted by fbov View Post

When doing an NEC modeling exercise, the size and location of every bit of metal has to be included. However, once that's done, there is a much simpler option that retains a high degree of accuracy over a limited range - scaling.

In my case, I started with a CM4221, which when properly measured has certain relationships between physical size and frequency response. Peak gain is at a frequency whose wavelength equals the width of the bowtie when wiskers are spread. This is an observation only, but it fits with how EM radiation interacts with matter.

If one changes all dimensions when making a copy, you get a scale model of the original, and in antennas, that means you can scale the frequency axis. If it's twice the size, just divide each frequency in the gain table by 2 to get the new gain curve. (But 2x is a BIG change! Scaling may not apply this far out.)

Conversely, if you know an antenna's peak gain frequency and you want to move it, your scale factor becomes the frequency ratio. In this case, a CM-4221 peak gain is Ch 66, 785 MHz, and you want to move it to Ch 51, 695 MHz. The ratio is 785/695=1.129x, which is real close to 1 1/8 (1.125).

The key is that ALL DIMENSIONS have to change by this amount. Some things, like wire diameter, have smaller effects, but basic building blocks like element size and location have to all change by the same factor to maintain any degree of predictability.

Frank

I scaled a four element quad from ARRL Antenna Handbook specd for 145 MHZ
I made a Channel 46 scaled antenna from its dimensions.. It worked pretty darned good... I agree Scaling is a nice concept and Does work and ALL dimensions need to be scaled wire diameter needs to be scaled but it isnt as critical. ....................

Johnie
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Old 05-05-2008, 10:24 PM
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Attached is a picture of a 1-bay UHF bow-tie that I constructed. I made the elements 10" long to optimize UHF reception under channel 35 and to attempt to improve upper VHF reception on a UHF antenna. My initial tests are very positive. Has anyone else tried 10" element lengths on a bow-tie?

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.
LL
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Old 05-05-2008, 11:09 PM
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Originally Posted by Falcon_77 View Post

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.

When I dis assembled my Youtube with 8" whiskers, I too was surprised when I could still receive my 3 strongest stations with only 2 bays remaining. RF 56,49,38
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Old 05-06-2008, 07:46 AM
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Boy, I can see another set of experiments in minimalist antennae - how low can you go? The large size of these UHF-low centered bowties will compensate to some extent for their reduced number.

Bob, did you remove the end elements, or the bottom half? The former looks like the way commercial 2-bays are ganged.

I need a free night without t-storms to do some playing ...
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Old 05-06-2008, 08:17 AM
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Originally Posted by Falcon_77 View Post

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.

I have a similar result here. A 4221 performed worse than a 4220 (both non-DIY), which in turn performed worse than a Radio Shack 15-234 bowtie. I may have a very unusual situation, but this is what works.

The only DIY part of the setup made a great difference: I removed the twin-lead wires from the 15-234, chopped an inch or so off of a RS 15-1140 transformer and soldered the resulting leads directly to the bowtie ends where the twin-lead connected before. This greatly improved reception on digital channels. I verified this by comparing to an unmodified bowtie I later bought.

It might be worth trying if you have strong signals before attempting to build one of these or buy the Channel Master antennas. You can use a pencil and pencil holder to keep it upright. My setup consists of a whip antenna with a magnetic base (came with a USB HD tuner) plugged into the VHF input of a CM7777, and the modified bowtie clipped onto the whip and plugged into the UHF input. With the preamp I can feed both of my TV's with a splitter and two long-ish runs, 25ft. and 50ft.

This is my most effective setup yet--gets all DC stations (~6 mi. away) and thanks to a strong reflection somewhere gets almost all Baltimore stations from ~30 mi. away. I don't have any digitals on VHF yet but will next year, but I think given the analog picture I get from the whip antenna I should be OK when that happens. This is indoors on a windowsill, much smaller than the CM4220 it replaced.
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Old 05-06-2008, 09:01 AM
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Originally Posted by fbov View Post


Bob, did you remove the end elements, or the bottom half? The former looks like the way commercial 2-bays are ganged.

Frank. I wish I would have documented with pictures and numbers.
I photshopped this just to show you what it looked like.
LL

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Old 05-06-2008, 09:24 AM
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cpldc,
Welcome to the world of multipath! If I understand you correctly, your antenna mod eliminated the twin-lead and let you run shielded coax to the tuners. In a strong signal field, that's a big change because twin-lead makes a better antenna than coax.

On the antenna forum, one poster found that his reception was virtually the same when he disconnected his attic-mounted antenna and used only the twin-lead downlead as an antenna. Reception improved markedly when he replaced the twin-lead with coax.
http://www.avsforum.com/avs-vb/showt...8#post13779298

This leads to a bunch more questions, most out of curiosity, but the bottom line is you have something that works for you, and that's what matters!

Frank
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Old 05-06-2008, 09:33 AM
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Nice photoshop job, Bob, but you made it a 1-bay! Can I assume you meant to leave a second set of wiskers at the other end, with feedlines unchanged? Separated by ~1.5 waves, these should act like separate antennas. Any chance you tried just one (or was the picture accurate)?

Dont' feel bad about the lack of pictures; I took none when I did my 10"-to-7" series, even though that's the antenna that's in the attic these days.

Frank
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Old 05-06-2008, 10:38 AM
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Originally Posted by fbov View Post

cpldc,
Welcome to the world of multipath! If I understand you correctly, your antenna mod eliminated the twin-lead and let you run shielded coax to the tuners. In a strong signal field, that's a big change because twin-lead makes a better antenna than coax.

Yep, that's exactly it. Since I can get the DC stations with a paperclip (literally) it only made sense that the cable was bringing in more signal and not in the constructive manner.

Quote:


This leads to a bunch more questions, most out of curiosity, but the bottom line is you have something that works for you, and that's what matters!
Frank

If you wish, ask ahead--I'm pretty much becoming obsessed with OTA thanks to DTV so I don't mind helping others' habits as well I enjoy getting the channels more than actually watching them.
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Old 05-06-2008, 10:51 AM
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Quote:
Originally Posted by Falcon_77 View Post

Attached is a picture of a 1-bay UHF bow-tie that I constructed. I made the elements 10" long to optimize UHF reception under channel 35 and to attempt to improve upper VHF reception on a UHF antenna. My initial tests are very positive. Has anyone else tried 10" element lengths on a bow-tie?

A 1-bay bowtie is more effective than I thought it would be and was very easy to construct. It's currently in my attic, above and in front of my 4228.

Falcon,

What all channels can you pick up with the single bowtie? How much difference from the 4228?

Thanks, Bill
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Old 05-06-2008, 12:35 PM
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Frank. The pic is accurate.I was so surprised at what was happening. While watching the strength meter, I removed the 2 middle bays, then the bottom one, leaving the long connectors as shown.
The three channels I received are at 70 degrees, 21 miles. The modified youtube was facing north, while hanging flat against a cabinet.
Oh yeah! Despite the file name, it is a one bay.
LL

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Old 05-06-2008, 01:03 PM
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Originally Posted by wblynch View Post

What all channels can you pick up with the single bowtie? How much difference from the 4228?

Attached are my test results. I can add some pictures later or send you some, if they are of interest.

The 4228 tests were from a few days earlier, but I did not notice any tropo events on any of these tests.

I'm thinking it is time to get that Leader signal meter or something like that. These tests are on my Sony TV, which show SNR and AGC. The downstairs results for the 1-bay are w/o a pre-amp. In the attic, I substituted it for the 4228 into the 7777.

Channel 42 had a problem when the 1-bay was up against the window. Moving it a bit solved the problem as I discovered after the test.

The 1st 1-bay column are the upper VHF 30" bow-tie results.
LL
LL
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Old 05-06-2008, 02:09 PM
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Originally Posted by Falcon_77 View Post

Attached are my test results. I can add some pictures later or send you some, if they are of interest.

Cool check list. You're really getting into this hobby! Thanks.
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Old 05-07-2008, 12:04 PM
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I need to do more tests with the 1-bay. It's not doing nearly as well as the office where the 2-bay 4220 performed very well (for UHF). I have not tried the 4228 at the office, but then, getting it out of the attic is probably not going to happen.

The 1-bay was only about 2' from the 4228 in the attic, but I mounted the 4220 in its place to see how it does in the exact spot. Location makes such a difference... and maybe the 4228 isn't in the best spot?
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Old 05-07-2008, 01:12 PM
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Falcon_77,
There are two numbers recorded out of the three reported on my Sony (I have an 34XRB970). Is it a signal range, or are you recording AGC or S/N as well?
Frank
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Old 05-07-2008, 01:22 PM
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[quote=spokybob;13807532]Frank. The pic is accurate. ...
[quote]

I think we're having calibration issues. Antenna changes are 2x factors in an arena where 10x factors are common. A 4-bay should have 6dB more gain than a 1-bay, but signal strength varies by 50+dB from one station to another, local terrain is a 20-30dB factor, and there's another 20-30 dB in reception probabilities between 50% and 99%.

6dB is 4x
20 dB is 100x
50 dB is 100,000x

Simply put, antennas may be under our control, but they aren't the big drivers of reception that one might expect when all factors are considered.

Frank
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Old 05-07-2008, 02:29 PM
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[quote=fbov;13816174][quote=spokybob;13807532]Frank. The pic is accurate. ...
Quote:



A 4-bay should have 6dB more gain than a 1-bay

Frank

Let me think about this statement. Looking at my TVFOOL. Forget RF41. It is not on the air.
RF38= -47
RF56= -48
RF49= -49
RF23= -59
RF58= -60
RF45= -67
I receive all these stations reliably with the 4 bay. Then with the 1 bay I receive only the strongest three. Are my results consistent with your statement?
If that is the case, I am going to be able build a smaller antenna for those folks that live closer to the towers.
I am very interested in Mike's test
LL

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Old 05-07-2008, 02:58 PM
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Bob,
Go up a bit to post #96 where holl_ands nicely summarizes what TVFool is predicting - worst case signal levels that might be 20-30dB weaker than typical, but with no accommodation for a bunch of very real signal attenuators.

That was my point; there are a lot of factors involved that swamp antenna gain differences. You may get three stations with the 1-bay because you're sitting at the threshold today, but will get all six of them tomorrow or next week. In your case, I'd err to the 4-bay side unless you were very close (or the VHF bowtie work pans out).

Frank
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Old 05-07-2008, 03:26 PM
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Thought I would chime in... stacked phased antennas have a narrower beamwidth imagine the flashlight beam being narrower in height and width... thats where the gain comes from... in physics there is no free lunch :P
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Old 05-07-2008, 03:30 PM
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Quote:
Originally Posted by fbov View Post

Bob,
Go up a bit to post #96 where holl_ands nicely summarizes what TVFool is predicting - worst case signal levels that might be 20-30dB weaker than typical, but with no accommodation for a bunch of very real signal attenuators.

That was my point; there are a lot of factors involved that swamp antenna gain differences. You may get three stations with the 1-bay because you're sitting at the threshold today, but will get all six of them tomorrow or next week. In your case, I'd err to the 4-bay side unless you were very close (or the VHF bowtie work pans out).

Frank

I just read holl ands report again & my skull is just as thick as it was before. Back to my previous post, if -50 is the threshold for a one bay, (per my results) would I be fairly safe in building it if someone's TV Fool showed everything better than -45? I know. I know. The bigger the better.

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Old 05-07-2008, 06:36 PM
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Quote:
Originally Posted by fbov View Post

Falcon_77,
There are two numbers recorded out of the three reported on my Sony (I have an 34XRB970). Is it a signal range, or are you recording AGC or S/N as well?
Frank

I'm recording SNR/AGC on the spreadsheet. I don't have a 3rd number to record, but am considering a more informative testing device.

As for signal strengths, at my house I have no problem receiving those down into the -90's, regardless of what I'm using.

Adding the 30dB attenuator tends to split the strong stations from the weak a bit more, but KTLA has to be about 40dB stronger than KCOP, which would indicate that my actual signal strengths at home are probably between -65dBm and -105dBm and not -88 to -102 as the TV Fool plot shows. Attic reception probably shouldn't be so easy at these signal levels, but not having any trees in the line of sight for half a mile probably helps, even though I'm at 2 edge diffraction.

Moving an antenna around can easily make a difference of 6+dB, which makes it clear that location is more important than antenna size, especially indoors.
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Old 05-07-2008, 08:35 PM
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Originally Posted by Falcon_77 View Post

Moving an antenna around can easily make a difference of 6+dB, which makes it clear that location is more important than antenna size, especially indoors.

Thanks Mike, I understand finding the sweet spot. Now the 6+db is something I can not seem to understand. I am running tests tonight. My highest reading on my CECB in the bedroom with the CM4220 happens to be 76 on the meter. Placing the youtube in the same spot shows that channel at 50. How many db would that be? The channel I speak of is RF56.
My dropout number on this box is about 16.

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Old 05-07-2008, 09:54 PM
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I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.
LL

 

May 7 test1.txt 0.498046875k . file
Attached Files
File Type: txt May 7 test1.txt (510 Bytes, 24 views)

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Old 05-08-2008, 08:18 AM
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Now the 6+db is something I can not seem to understand.

I just meant that it's quite possible to find spots that are weaker or stronger by a factor of 4. Actually, a factor of 10 difference is probably easily achieved as well, especially in an attic. One spot could be 10dB weaker than one a few feet away, etc.
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Old 05-08-2008, 08:23 AM
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Quote:
Originally Posted by spokybob View Post

I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.

As for these test results, I don't know how to assign dB differences to the different signal strengths.

On my Sony, which has an AGC figure, strong signals are between 39 and 44, but weaker signals will go from 50 to 100 with relatively little change. Using stock attenuators can shed a little light on the difference, but exact measurements will remain in the realm of much more expensive instruments.
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Old 05-08-2008, 10:03 AM
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Bob,
With the single assumption that your 4220 has it's reflector, your results line up perfectly: 4220 > 4-bay > 2-bay, as one might expect. Add a reflector to the 4-bay and I expect it'll beat out the 4220, but you clearly don't need it!

And Falcon_77's right; no correlation between signal readings and dB's when using consumer products. Test equipment and professional products are another story, and another zero on the price!

Falcon_77,
Positional differences are far more likely in our diffracted signal fields than in the LOS fields Bob is working in. He may not see a positional effect at all!

Thanks for clarifying your data; it really helps to know if better is up or down.

Frank
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Old 05-08-2008, 10:07 AM
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Quote:
Originally Posted by spokybob View Post

I did my testing tonight in the bedroom with my RCA CECB and the CM4220, 4 bay and a modified 2 bay. All channels lock in.

I noticed your chart showed that the cm4220 was the strongest (if higher numbers mean better)

I also noticed that the cm4220 was pointed in a different direction than the other 2.

Were the directions that you had listed where you got the best signal?
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Old 05-08-2008, 10:24 AM
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Yes. On this particular session, I was able to rotate for best reception, without moving the antenna to various locations. 24.1 is usually the weakest, but I have to be check to see if 4.1 is still OK.

Bob 61231
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