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Discussion Starter #1 (Edited)
I have a question about the impedance of GH antenna...

Why this antenna, in all variations, does it have to be designed for 300 ohms use, even in 75ohm world? Why it can't be for 75 ohms?

Thanks!
 

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Because all the baluns sold locally everywhere like in hardware stores,department stores, online, Lowes, Walmart, Kmart etc are 4:1 baluns, matching 300ohms to 75ohms.

1:1 baluns have to be custom made or custom ordered.

Its all a matter of standards, set in the very beginning of TV history.

I cant find the link to the history of the 300 ohm standard, but FWIW, the RG in coax like RG6 or RG59 stands for "Radio, General ground use".

http://www.therealcableguy.com/catv-cables-and-components.htm
 

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Discussion Starter #3 (Edited)
Thanks! now i got it bit clearer

But suppose in the case of a GH antenna designed for 75 ohms.... Without using balun, can I just directly connect a 75 ohm coax to the feedpoints (antenna terminals)?
 

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Nope, that would not be a good idea. The GH is a dipole, balanced antenna (with 2 active sides). The tuner, on the other hand, has a 75Ω unbalanced input, which is suitable only for a monopole antenna. So, even if the GH had an inherent impedence of 75Ω it would be balanced, so to convert it to unbalanced the resulting impedence would be 18.75Ω if a common store-bought 4:1 balun was used.

Keep in mind that in actuality an antenna can have certain impedence values at certain frequencies, resulting in certain standing waves occurring, so in electronics theory this topic is quite involved.
 

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Another reason for using 300ohms is that the characteristic impedance of a tuned folded dipole is 300 ohms. At one time, almost all TV antennas were folded dipoles so it became a de facto standard. 300ohm twin-lead, not coax, was once standard for TV antennas. Coax is now used due to it's high resistance from interference compared to twin-lead. 75ohm coax became a de facto standard because that's the characteristic impedance for monopole antennas (once almost exclusively used for short distance two way radio.)
 

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Without using balun, can I just directly connect a 75 ohm coax to the feedpoints (antenna terminals)?
Nope, the antenna is BALanced and the coax is UNbalanced. Thats why you need a balun (a balanced to unbalanced transformer).

You could use 300ohm twin lead if you can still get it, which is balanced, and then use an indoor balun on the TV sets F connector.
 

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mangostain;

You could connect 75 ohm coax cable directly to a 75 ohm antenna, but you still might want to use a balun, but not a 4:1 balun, rather, a 1:1 balun *should* be used.

BalUn comes from the contraction BALanced to UNbalanced. Most 2-pole antennas, like the Hoverman or bowtie antenna, are balanced, but coax cable is unbalanced. Indeed, you should actually ground the shield of coax for lightning protection, which will certainly make it unbalanced.

There's various flavours of 1:1 baluns. It could be an air-wound solenoid, or coax passed through some sort of ferrite materiel. I tried an LPDA using an infinite balun. http://www.qsl.net/va3rr/hdtv/HDTV.htm

In practice, you can probably get away without the 1:1 balun.
 

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old sparks, thats a pretty cleaver LPDA design - inter spacing the short elements back in with the long elements making for a shorter boom. :p

But that NEC file doesnt seem to correspond to what you built ???
 

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Because I'm using a parallel transmission line, the elements are offset to achieve the correct phasing.

I got the idea from this website: http://www.vk2zay.net/article/43, but the impedance of vk2zay's PCB boom/transmission line wouldn't work with my design, so I fabbed my own using brass tubing.

It appears in the picture that the elements don't correspond to the NEC file, but it's because half of the elements are closer to the camera. When viewing the antenna from the top, you could see that the elements form an 'arrow' shape.

Apparently, Mathias Katzar had the same idea. Check out:

http://www.vk2zay.net/article/213
 

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It appears in the picture that the elements don't correspond to the NEC file, but it's because half of the elements are closer to the camera. When viewing the antenna from the top, you could see that the elements form an 'arrow' shape.
Oh, so its an optical illusion, heh.



I wonder if it would work as the optical illusion implies ?
 

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Discussion Starter #11
Many thanks to everybody... The problem was me, I didnt know what balun do (balanced to unbalanced function) and I didn't considered that...

Thanks once again DHC forum.
 

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Another reason for using 300ohms is that the characteristic impedance of a tuned folded dipole is 300 ohms. At one time, almost all TV antennas were folded dipoles so it became a de facto standard.
Yeah, heres something on that.

Which Impedance and Why
Constructing an open wire transmission line is a balance of two factors: the optimal impedance for the line and the ease of construction. Let's spend a moment on the first of these factors.

Besides being cheaper to manufacture, TV-type transmission line uses a 300-Ohm impedance for convenience. Remember that the 300-Ohm figure is "nominal," meaning it is an approximation and may vary by 10% to 20%, depending upon the quality control of the manufacturing process. Its convenience stems from the emerging television industry in the post-WWII era. Folded dipole elements provided a rough match for the line, and TV sets were engineered to have 300-Ohm inputs. Outside of this industry, the line was used for its price and handiness, with little regard to matching. Amateurs used an antenna tuner (ATU) to compensate for reactance at the shack end of the line and to change the impedance to the emerging 50-Ohm coaxial cable standard. However, except for some briefly available transmitting versions of the line, the typical 300-Ohm line available today is composed of thin wires closely space (under 3/8").
http://www.qsl.net/co8tw/openline.htm

But I do remember reading an article on the history of television where they said there was a big debate on whether to set a 300 ohm or 450 ohm or 600 ohm standard, the latter two being easier to manufacture to tolerance. :p
 
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