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Discussion Starter · #1 ·
As I posted in another post . . I am 65+ miles from Baltimore. I just bought a 4228A. Miraculously I can just barely get Baltimore stations with the antenna propped up on my deck and a Spartan 3 amp. I managed to lock 3 digital station, well before night fall, but only for a brief period of time.

My questions:

Should I go ahead and mount the antenna on a 30 ft mast and see what I get?

Should I get a better amp? I bought the "lesser" amp because I was afraid that the better amp would clip when I got local stations, the spartan does not seem to be doing that.

Should I go with a more exotic set-up, such as a stacked pair of CM4248s or a 4251?


I really want to get CBS and ABC which are not available in my local market yet!


Best Regards,


SM

 

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See my reply to your other post.


Here is my recommended order of events.


1. Mount the antenna on the 30 foot mast.

2. Evaluate the antenna with and without the amplifier. This may involve a bit of work since a proper amplifier will be mast mounted at the antenna to get the best signal to noise ratio.

3. If you can receive the signals with a single antenna but have some signal strength problems then you can consider stacking a pair of antennas. Keeping in mind that stacking will give you at best a 3 dB improvement (doubling of signal strength) and coupling of the 2 antennas is non trivial.


I don't remember you mentioning which channel numbers the stations were that you are chasing. If they are very close in frequency you could consider ordering an antenna cut specifically for that channel. Instead of working over the normal range of Channel 14-83, which is a very wide range, you would have an antenna with gain optimized for a single channel. That is the way the CATV companies do it. They use multiple antennas cut for individual channels and then couple them together.


..Doyle (KG6MY)
 

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Discussion Starter · #3 ·
Thanks Doyle. I was posting this as you were answering my other post! The two distant channels I want are 52 and 48. The others are local and strong.

I guess it is possible to trim the di-poles on the 4228, to tune them to the shorter wavelength.


Thanks for you help. Finding someone to install an antenna is my next goal. Most installers want to do simply dish installs!


Thanks again.


Best Regards,

SM


[This message has been edited by Swampfox (edited 05-01-2001).]
 

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Doesn't trimming the dipoles allow the antenna to get 69-83? I think that would hurt performance on 14-69?


I do like the idea of custom cut antennas. Lets say I got 6 single channel antennas, how can I connect them so as not to lose a lot of gain? Are there products made for combining mutiple antennas? The channels I have the most trouble with are 59, 60, 65. How could I modify a 4248 to tune in the higher band channels, shorten the dipole, move the reflectors...etc..?


Glenn
 

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Blake Aerials make narrow band antennas which are spec'ed for about 2db higher gain than their wideband counterparts.


This may be an easier way to get more gain than stacking antennas and putting up with the losses of combining the signals.


An alternative to combining signals from multiple antennas is to use a selector switch like Radio Shack # 15-1232, 15-1247, 15-1248, or 15-1249. (all on page 169 of the 2001 catalog).


As far as amplifiers are concerned, if you have a UHF only antenna then an overload input signal to the preamplifiershould, theoretically, not cause any signal degradation because the UHF band is less than an octave wide and all harmonics and intermodulation products arising from nonlinearities induced by a signal overload will be out of this band.
 

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Quote:
Originally posted by Swampfox:
My questions:Should I go ahead and mount the antenna on a 30 ft mast and see what I get? Should I get a better amp?
SF, Yes, mount the antenna on a pole (might be wise to install a rotor). If your pre-amp has problems, then try the Winegard AP-4700. For more antenna and pre-amp info, see Antenna Test .

Quote:
Originally posted by cymro:
not cause any signal degradation because the UHF band is less than an octave wide and all harmonics and intermodulation products arising from

nonlinearities induced by a signal overload will be out of this band.
Overload of the pre-amp will seriously degrade the sensitivity of same!!


------------------

Wendell

Technical Services Supervisor

MAETV
 

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Before I would start hacking an expensive antenna, I would try to determine where it is centered. By measuring the element lengths you can get a pretty good idea of this. The tip to tip of the diagonal on the bow ties should be pretty close to one half wavelength. Using the formula


Length in Feet = 468/Frequency in MHz or

Length in Inches = 5616/Frequency in MHz


you can determine where the antenna is centered. Here is a link to the ARRL TV Frequencies chart. Be sure to use the broadcast or Air number and not the CATV numbers.

http://www.arrl.org/tis/info/catv-ch.html


For example, Channel 50 which is 687 MHz would give 8.17 inches. It would be interesting to see what numbers you measure. Are all of the bow ties the same length or are they doing some "broadbanding" by cutting some to different lengths. Another factor is the diameter of the element itself. The larger the diameter of the element, the wider its bandwidth (within reason) Conversely, very narrow elements can have sharper resonance peaks and thus higher gain at a specific frequency but the gain tends to fall off as you get off of that peak. Let us know what you find.


..Doyle


Edit Note: Glen is correct, There was a typo on the inch formula. It should read 5616 instead of 516. (468*12) Sorry for the typo.


[This message has been edited by DoyleS (edited 05-02-2001).]
 

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Doyle,


The 4228 bowties are all the same length, they have a groove near the end to allow you to cut them about 3/4" shorter. The instruction say this is for tuning in 69-83. I would think this will hurt the lower bandwidth? You could determine the length, as you say, for the channel you want and optimze the antenna by cutting it for that channel. I will have to check the length to see what channel it is optimzed for from the factory.


I think the length in inches should read 5,616/Frequency in MHz.


The 4248 only has one pair of elements, a oval loop made from a strip of aluminum. Would I measure the length of each loop circumference?


Thanks

Glenn
 

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Discussion Starter · #9 ·
The array is somewhat interesting. The top bow and bottom bow are connected out of phase with the two in the middle. The diagonal of each bow is 17 inches. The "bows" are actually dipoles, folded in half. I guess it can be described as "cat whiskers ( >< ), if you can picture that.

If you measure just one of the wires in the bow, it is 8". Using that and looking at the gain chart, my guess is that the antenna is tuned somewhere near channel 50 UHF. Which is very strange, because I am never that lucky!

Anyone know?


SM
 

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Another thing that is interesting is the distance to the screen. Each whisker is treaked so that one wire is closer to the screen than the other, what would this do?


Also, one website shows moving the reflector on a yagi closer to the pick-up element, what effect will that have on the antenna?


Glenn
 

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OK, I had to go look at the catalog to make sure I get the product numbers straight on these Channel Master antennas. The 4248 is the one that looks like a bunch of bow ties mounted on a reflecting screen. The actual antenna name is a Collinear array. Each single element is 1/2 wavelength long. The center of the elements are at 0 voltage and the maximums are at the ends. That is why you see them fed from the ends. The Collinear antenna is by design a broadband antenna. A Yagi (CM-4228) is a narrow band antenna. The Yagi has a narrower beamwidth. The Collinear is very directional because it cannot receive signals from behind the reflecting screen. The reflecting screen is normally placed about 0.2 wavelengths from the driven elements to optimize gain. Forward or backward of that tends to trade off gain for bandwidth. It sounds like your antenna is centered on Channel 50 so the spacing for the driven element to the reflector would be 17" * 0.2 = 3.4" . I guess based on that I wouldn't mess with it since it is dead in the middle of your desired band. I think we started the discussion with it on your deck so I would get that puppy up in the air and see how it performs. These are actually quite popular antennas in the Amateur Radio area for picking up signals from space because of their ease of construction, good directionality and broadband gain.

Regarding the cross coupling of the elements. Basically you will have current induced into the element that is next to and parallel to any element. That current will be 180 degrees out of phase with the current flowing in the original element. Very similiar to your 110 volt house current. So by cross coupling you maximize that effect to your advantage. Each element is picking up a signal from the air and inducing an out of phase signal into its adjacent neighbor. Conversely if you did not cross couple, then the multiple elements would all be working against each other. Hope that was clear.


Glenn,

With regard to that 4248 which is a Yagi. That loop is essentially a folded dipole so you can take the circumference and divide it by 2 before using the equations posted earlier to get the center frequency. (ps: thanks for the correction on the formula) The equations are based on a half wavelength antenna.


..Doyle


[This message has been edited by DoyleS (edited 05-02-2001).]
 

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Hi Wendell,


You are correct to point out that overloading reduces the gain of the preamp.


I was thinking that such a gain loss is not relevant if the desired signal is large enough to overload the preamp in the first place.


Of course, if a desired signal on a particular channel is small and the preamp is overloaded by another channel in the UHF band, then the loss of preamp gain is indeed detrimental.


Thanks for your always useful comments
 

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Doyle,


Thanks for the info.

Here are the specs for 4228 (8-bay bowtie):


dipole length = 15.50", wire is bent in vee shape, each leg = 7.75"

dipole wire dia = 0.115"

vertical spacing of each bowtie = 8.00"

reflector spacing = 4.50"

boom spacing (horz. stacked) = 20.00"


It seems the reflector spacing is to high. Does a wider space increase or decrease gain?


wavelength formula conflicts with formula on this post:
http://www.avsforum.com/ubb/Forum11/HTML/013615.html

which one correct? any links?


My goal is to build a precision single channel antenna for each channel that I want to receive. Then run a seperate cable down for each antenna and use a multi-selector switch to choose the antenna. Will the switch give me the highest gain? Combining would not give much benifits over a broadband antenna.


Here is a fitting page for this thread http://www.avsforum.com/ubb/smile.gif :

How To Become An Antenna Guru... http://www.borg.com/~warrend/guru.html


Glenn

 

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I live in West Michigan and I live in the center of a triangle formed by the three main transmitter towers. Problem is that for NTSC you either needed a rotator(which I use) or a tri level antenna with each pointing at the correct tower to get any signal and the quality is less than optimal (the word crap comes to mind.)


Is 8VSB more capable using a single antenna solution than NTSC?


The antenna selector site always selects the same antenna and I've entered addresses all over the contry and they must have some deal with the manufacturer. Techs in my area have said that for NTSC there is no one antenna that is enough for this area.


Currently I only have one station broadcasting so my antenna is tuned to the strongest signal for WOODTV 8, but I'd like to plan ahead for the others.


Ben
 

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Glenn,


The formulas I have always used are the ones in the Radio Amateur's Handbook and the ARRL Antenna book. I couldn't bring up the link you posted but I remember reading the thread. It referenced the speed of light and was given in metric terms. They should all be essentially the same. Elements are normally 1/2 wavelength in length. At 7.75" your get 724 MHz and that looks to me like channel 56 at 723.25 MHz. As far as the reflector spacing, I gave you the normal spacing for driven element to reflector for a Yagi Antenna. The book also mentions 0.2 wavelength spacing for collinear arrays. Most of these antennas are a combination of various design techniques. The 4228 looks to have both collinear and broadside elements but these are only terms that an antenna guy would use to describe his antenna. Increasing the distance to the reflector helps the channels that are down below channel 50 because the effects of a reflector being too close are going to be typically worse than one that is a little far back. I would leave the reflector where it is. If you are trying to lower the center frequency, you could possibly find some small metal sleeves to slip over the ends of the element wires to slightly lengthen them. Most times these antennas are designed with the physical element support in the center of the element since that is a voltage minimum and placing a support there will not detune the element. That is why you can have directors riveted to the boom which appears to be shorting out all of the elements. It appears the elements on the 4228 are supported strictly at the V-joint which means you could slightly add length or take it away if needed. I would recommend having a way to add length prior to removing length so you can get back to the original length if you do not see the performance increase you want. I am assuming the elements are aluminum wire so remember that aluminum oxide is an insulator and you may want to go to an electrical store and pick up some AL-OX or No-Ox which is a gritty conductive compound that helps ensure you have a good electrical connection when joining aluminum. If your weather situation is severe, you may need to periodically bring the antenna down and "clean it up" .

All this too watch TV....No wonder our wives look at us strangely.


..Doyle
 

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Doyle,


I want to build or modify a couple antennas to optimize them for a certain channel. One for ch. 59,60,65, and one for ch 31,36. In your previous post you said "The reflecting screen is normally placed about 0.2 wavelengths from the driven elements to optimize gain. Forward or backward of that tends to trade off gain for bandwidth." What I want to do is maximize the gain and minimize the bandwidth. Would placing the reflector closer maximize the gain, at 0.2 the distance would be 3.1"? You also said that the smaller the dia of the element the higher the gain. I am going to try some smaller dia wire to improve the gain. I'm thinking of drilling out the rivets and use screws to attach the element. Then I can change them out easily with different length wires. That gives me the option of using any kind of material for the wires. What material would be best, aluminum, steel, copper? I have a full fabrication/machine shop in my home garage so I can build pretty much anything I want, like offroad race cars and mountain bikes.


Glenn
 

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I ran some side-by side tests on four Channel Master antennas last June. These were the 3021 4-bowtie screen antenna, the 3028 8-bay bowtie, the 3022 suburban yagi, and the 3023 fringe yagi. These numbers correspond to the 4221, 4228, 4308, and 4248.


The antennas were tested for gain on channels 26, 42, 55, 64, and 67.


On channel 26, the 3028 8-bay had the highest gain by .8 dB. On channel 42, the winner was the 3023 by 3 dB over the 3028. On Channel 55, the 3023 once again was the best antenna at 3 dB over the 3021. On Channel 64, the 3021 took top honors by .1 dB over the much larger 3023, and on Channel 67, the 3021 also had the edge by 1 dB over the 3023.


This test wil be repeated in June with lots more antennas.


Channel Master's own test data verify the CM 3021 has the most gain above channel 60.


KC
 

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I am baffled by the following: I live in No. Va area and to get DC station my antenna aims thru a large stand of trees approx. 100 yds away. During the winter with no leaves on the trees I could not lock onto the NBC (48-48) at all - signal all over the place from 0 to 50. All the leaves are out - have done nothing to antenna - signal locked in mid to upper 80's which is what I get with CBS & ABC. Fox is in mid 70's and PBS is in low 70's with occasional drop outs. I could probably cure PBS by rotating antenna a few deg. to the east as their digital broadcast is about 20 deg east of the DC stations. I wonder if the leaves solved some sort of multi-path problem.


Using a Winegard VHF / UHF yagi with preamp to a DTC 100.


Interested in any comments.


Thanks - Bruce
 

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Glenn,


Boy you are biting off alot. Antennas can become a hobby unto themselves and like Home Theater you can always make improvements. My wife and the neighbors were quite happy when we put a new roof on last year and I took down the 2 sections of Rohn Tower that were on top of my 2 story house along with the large beam antenna that had a 32 foot 4 inch diameter boom and 6 elements that were each 25 feet long. Plus assorted other UHF and VHF antennas ( A regular antenna farm) I now have a Radio Shack long boom UHF yagi for HDTV and that is it.


I assume you are going to work with the 4228 8 bay bow tie antenna. I think your idea of adding the screws for easy changing is a good one as long as you don't create too large of a discontinuity at the connection point. At these frequencies things more than a 1/4 inch make a difference. I would leave the element diameter the same. On Yagis you see larger diameter elements to get a little more bandwidth. Looking at the picture, the element size is already at the minimum you should be working with. For experimental reasons I would use copper Wire or brazing rod for the elements. It is easy to work with, can be made straight and is an excellent conductor without oxidation problems. Once you have a design that works, you can change it to a more rigid aluminum since copper bends pretty easily and will not handle birds or wind.

With regard to the reflector spacing, I would optimize the antenna elements first, leaving the reflector where it is and then once you have that optimized, you can play with the reflector spacing. I don't expect that you will see a big difference with that term. There is a program called Yagi Optimizer that runs on the PC that I used a number of years ago. It would plot the response pattern of the antenna and allowed you to tweek various dimensions such as reflector spacing to optimize your antenna. I am sure there are much better programs out there now. You can do a "google search" for Antenna design software and get a lot of hits. I would also recommend that you stop by your local Amateur Radio Store and pickup a copy of the ARRL Antenna Handbook. There are just too many items to do something like this by remote control.


..Doyle
 
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