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'[TECH] Connecting a TV aerial'
2008\07\17@022354 by Cedric Chang

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This aerial requires that you attach 300 ohm flat twin-lead cable.  
You can buy a transformer that converts the 300 ohm impedance to 75  
ohms.  The 75 ohm end of the transformer will have a connector for coax.

Some examples:
hhttp://www.radioshack.com/product/index.jsp?
productId=2062049&cp=&sr=1&origkw=75+300+ohm+transformer&kw=75+300+ohm
+transformer&parentPage=search
http://www.amazon.com/Recoton-V348-300-Ohm-Signal-Combiner/dp/B00006HOJL

300 ohm cable
http://www.radioshack.com/product/index.jsp?
productId=2049618&cp=&sr=1&origkw=300+ohm+cable&kw=300+ohm
+cable&parentPage=search


00000000000000000000000000000000000000000000000000
http://www.mindspring.com/~brucec/fmtrans.htm
Matching



Antennas, transmission lines, and tuner inputs all have  
characteristic impedances. You absolutely MUST match these, or losses  
will occur. Fortunately, there are only two impedances commonly used  
in home equipment: 300 ohm and 75 ohm. Antennas are almost always 300  
ohm, and tuners are becoming almost universally 75 ohm. Older tuners  
had two screws for 300 ohm, however. The flat wire with two  
conductors separated by about 3/8 inch is 300 ohm. Cable TV wire is  
75 ohm.

If you have a 300 ohm antenna, 300 ohm lead-in wire, and a 300 ohm  
input on your tuner, you can just hook it all up directly, and the  
only losses you will have are the losses in the wire. The trade-off  
here is that the wire will tend to pick up interference (and stations  
out of the beamwidth of the antenna). If noise and / or interference  
is a problem, you are going to have to use sheilded lead-in. 300 ohm  
sheilded lead in was available in past years, but I don't know if it  
can be found now. This means you are going to have to consider the  
use of 75 ohm cable, and inserting two matching transformers into the  
"chain" - one at the antenna and one at the receiver.



If you have a 300 ohm antenna, and 75 ohm tuner, you must use at  
least one matching transformer. You might as well take advantage of  
75 ohm cable in the process. It has lower losses than 300 ohm  
twinlead, and has better performance when it gets wet. There is  
little to be gained by using 300 ohm twinlead from the antenna to the  
tuner, and converting it at the back of the tuner.



2008\07\17@024244 by Cedric Chang

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Here is an example of a 300 ohm antenna
http://www.radioshack.com/product/index.jsp?
productId=2103085&cp=&sr=1&origkw=300+ohm+antenna&kw=300+ohm
+antenna&parentPage=search

300 ohm antennas are designed to match the impedance of free space  
which is ~ 376.730313 ohms .
Here is more than you will ever want to know about free space impedance.
http://en.wikipedia.org/wiki/Impedance_of_free_space


cc

2008\07\17@090213 by Harold Hallikainen

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Speaking of the impedance of free space, I wrote an article on shielding
that used the concept back in 1991. See
http://www.hallikainen.org/rw/insite/ and
http://www.hallikainen.org/rw/insite/WaveZ.pdf .

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2008\07\17@105859 by Don Kovalchik - W8DPK

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The 300 ohm twinlead has much lower loss than the 75 ohm coaxial cable,
especially at UHF frequencies.

For example, at 500 Mhz (US TV Channel 19), good quality 75 ohm coax has
a loss
of about 7 dB/100 ft, whereas 300 ohm twinlead has a loss of about 3 dB
/ 100 ft.
At higher frequencies the difference is even more pronounced.

Properly installed twinlead can perform very well.  In very weak signal
areas
it can make a huge improvement in picture quality. For proper operation,
the
twinlead must be kept away from nearby conductive objects.

However, if the signal strength is adequate, the extra loss of the coax
can be
tolerated and the advantages of coax makes a much simpler installation.

--Don--

Cedric Chang wrote:

{Quote hidden}

2008\07\17@112229 by Sean Breheny

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I deleted the message so I don't remember who said it but someone
commented that TV antennas have an impedance of 300 ohms because it is
close to the free-space impedance. I don't think this is quite true.
They are 300 ohms because the driven elements are usually some kind of
folded dipoles. I suspect that this is to make them an easy match to
balanced transmission line, which is cheaper than coax and can be less
lossy.

Of course there is a relationship between antenna feedpoint impedance
and free space impedance. However, it is not the case that an antenna
will work "better" if its feedpoint impedance is closer to the free
space characteristic impedance.

Sean


On Thu, Jul 17, 2008 at 11:00 AM, Don Kovalchik - W8DPK <spam_OUTw8dpkTakeThisOuTspamarrl.net> wrote:
{Quote hidden}

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