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crowneagle 23-Apr-2011 10:20 PM

My messed up db4
I made a db4 antenna off a set of plans on You Tube. The problem is my phase line setup is completely different than all the other plans. When I first built it, I thought the crossing of the phase line was suppose make the two wires touch. When I finally realized my mistake, I corrected it only to find that my reception went down the crapper. Now, I went back to my original design. The only difference between my antenna and others of db4 design is that when my phase lines crossover, I twist them tightly around each other two times before going to the final set of wire ears. This makes my antenna violate the 1.25" law that the phase lines are suppose to be away from each other. Does anyone understand why this is happening? With my antenna and a tinfoil reflector, I'm able to get all the stations in two metropolitan areas even though I'm in a purple-pink reception area as reported by TV fool maps. It is an indoor setup at 5' off the ground. Did I stumble upon something new? Can this setup damage anything? I get twice as many stations with this modification along with a much flatter frequency response and a more forgiving directional response. :confused:

GroundUrMast 23-Apr-2011 10:47 PM

I can un-fold a paperclip and stick it in the antenna connector of my TV. I get quite a few channels...

I could damage my TV's tuner if I pushed the wire in too far. Your antenna does not sound dangerous to me.

We have both demonstrated the resilience of the ATSC signal format and the engineering behind it. I'm not rushing down to the patent office Monday morning.

I'm glad for your success though. :)

FWIW: Phasing lines should ideally be sized so that there will be minimal reflection of signals as they transition from the antenna element into the phase line and then as the signals transition from the phase line into the balun or transmission line. (I'm not aware of the 1.25" phasing line law.)

Curious; How did you measure flatness of frequency response?

crowneagle 24-Apr-2011 2:28 AM

Actually, my statement of frequency response was done unscientifically. All I know is that without the phase lines twisted together, I was having a harder time picking up channels in the 40+ range and I was having a easier time with the channels in the 15-40 range. Also i would have to move the antenna quite a bit to pick up individual channels. If you look up the home made HDFTV antennas on You Tube, there are two wires that run parallel to each other that connect to the balen. Generally speaking, they say that they should be about 1.25" from each other for the length of the antenna; crossing without ever touching each other. That is why I'm dumfounded that my antenna works even better with the two wires twisted together on either end half way between the top set of ears and the bottom set of ears. I've never seen a set of plans that do it this way. I suppose the end justifies the means. Antenna building seems to be more of an art form rather than a science.

GroundUrMast 24-Apr-2011 3:31 AM


Antenna building seems to be more of an art form rather than a science.
I can understand that it could seem that way. I assure you, it's most certainly a science. That a Youtube video fails to offer instruction in the science is not proof that antenna design is haphazard.

Here's a bit of bed-time reading on the science of antennas: A link to Edmund Laport's textbook Radio Antenna Engineering, published in 1952 is included.

The laws of physics apply, and therefor an understanding of math and physics are crucial to being able to design antennas as well as any other electronic circuit.

My experience in circuit design is fairly basic. But I have been able to design and build a few simple antennas over the years, thanks to the work of much smarter minds than mine.

John Candle 24-Apr-2011 7:07 PM

Tv antenna design
The following statements are not aimed at you to put you down. The internet is loaded with information that is false. There must be several hundred variations on the Same Basic UHF antenna design by now on the internet . Many variations are so bad that making some change might make it better in some way. Almost all of the variations origanate from Ego and Self Importance. Many are only interested in putting their Name Up In Lights for Bragging Rights.

crowneagle 25-Apr-2011 12:14 AM

The only light I'm interested in is the 20watt compact flourescent next to me on the couch. I'm just looking for someone with real technical knowledge tell me what the heck is going on with my antenna. The general rule of thumb for a db4 is to keep the phase lines away from each other; usually 1.25" according to most of the plans I've seen. Technically speaking, why do I get better reception when my wires touch when I cross them? What happens technically when they touch? Someone teach me please!

John Candle 25-Apr-2011 12:46 AM

Tv antenna design
It is the metal antenna elements phasing relationship. It has to do with wave length and fractions of wave lengths. The commonly used wave length fractions are , 2/3 wavelength , 1/2 wavelength , 1/4 wavelength. It has to do with having the metal receiving elements of the antenna phased together or not phased together. It has to do with the spacing of metal receiving elements. It has to do with the diameter of the metal receiving elements. It has to do with if a metal reflector is placed on one side of the metal receiving elements. It has to do with the spacing of the metal reflector between the metal receiving elements and the metal reflector. There is volumes of antenna design information on the internet and in several hundered books and computer design programs. Here are a places to start , , , , ,

GroundUrMast 25-Apr-2011 2:06 AM

1 Attachment(s)
Wavelength = Speed of Light / Frequency. UHF channel 30 is near the center of the UHF band has a center frequency of 569 MHz. Therefor the wavelength of UHF channel 30 is about .527 Meters or 20 3/4". This same basic formula is at the heart of calculating any antenna element lengths.

You will notice that there is about 1/2 a wavelength between each of the dipole (bow-tie) elements of common UHF panel antenna. The phasing line that connects the top element to the next element below will therefor be about 1/2 a wavelength. A UHF signal traveling the length of that 1/2 wave long line will be 180 out of phase with the signal entering the line. By twisting the phasing line one half turn (180), the signal is returned to a phase angle of zero degrees and will be in phase with the signal present at the center-point of the second element from the top. This provides a means to combine signals from each of the two elements, in phase, so they add together to provide twice the signal power to the feed line.

In the common panel antenna designs, the phasing lines also serve to transform the low impedance of paralleled elements. Two 300 ohm dipole elements in parallel have an impedance of 150 ohms. 1/{(1/300)+(1/300)} By using a 1/4 wave length section of transmission line with a characteristic impedance of 450 ohms (actually the math works out to 424.264 ohms but 450 ohm twin-lead is the closest manufactured product), one is able to transform a 300 ohm impedance to match a 600 ohm load. This is convenient because two 600 ohm impedances in parallel form a 300 ohm impedance. Hopefully you see the progression toward forming a four element panel antenna out of two two-panel sections.

The characteristic impedance of parallel lines at RF frequencies can be calculated - . You can find calculators online that will do the math for you... In the case of two #12 gauge wires spaced 1.25", the characteristic impedance is about 416 ohms. That's fairly close to the ideal 450 (424.264) ohms I mentioned above. Once you understand the math, you will see that moving the phasing line conductors together will lower the impedance, making the wire diameter smaller will increase the impedance.

It's not realistic for me to think I can evaluate your antenna... But you can read and learn for your self.

Why your crude antenna works as well as it does...? Some errors combined to cancel one another out I suspect.

John Candle 25-Apr-2011 4:34 AM

TV Antennas and Reception
99 % of home made antennas are copies of standard Tv antennas that were and are made using real science and math. What is happening with these copies is each person makes this change and that change and some other change and 98 % of the persons making the changes have no idea what they are doing , they are only interested in slamming something up on the internet for bragging rights. The whole situation is out of control.

crowneagle 25-Apr-2011 10:55 AM

Thank you Mr GroundUrMast. You have now officially created a student of the science. I obviously have much to learn and I thank you for places I can start for my studies. I'm semi-retired and I'm always looking for subjects that I can learn and understand. I appreciated your help and your patience.

As for you John Candle, I hope that your name does not equate with your passion for the human race. Happiness can be a fleeting thing sometimes; so hold on to it tightly next time you find it.

John Candle 25-Apr-2011 10:44 PM

TV Antennas and Reception
I am very happy. And I can look straight at the Truth about the humans. And I can state the truth clearly. The truth is often a unpopular subject.

GroundUrMast 26-Apr-2011 1:43 AM is a source of information I can also recommend. Navigating the site is... Well it's worth the effort IMO.

Here's a couple of many informative pages:

And yet more reading... offers a visual demonstration of how two sine waves add together when they are not in phase with each other. This is helpful when trying to understand what happens when signals from two or more sources are combined.

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