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'[TECH] Connecting a TV aerial'
|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.
300 ohm cable
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
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.
Don Kovalchik - W8DPK
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
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
it can make a huge improvement in picture quality. For proper operation,
twinlead must be kept away from nearby conductive objects.
However, if the signal strength is adequate, the extra loss of the coax
tolerated and the advantages of coax makes a much simpler installation.
Cedric Chang wrote:
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
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.
On Thu, Jul 17, 2008 at 11:00 AM, Don Kovalchik - W8DPK <arrl.net> wrote: w8dpk
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