Question about t.v. antenna wire

I have two questions:

The quoted impedance for t.v. antenna/aerial wire, 75ohms,
is it just 75ohms or is it 75ohms per meter. What is the
significance of this parameter?

2. Why do we use mixer boxes when we want to combine
different vhf t.v. aerials into one wire, why can't we
simply combine/bundle them all together, i.e. directly
connect all the earths and similarly combine all the center
wires, and then feed this to the t.v.?

tia

 

Google is your friend:
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q="transmission+line+fundamentals"
http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q="impedance+matching"

Cheers!
Rich

 

sorry, no comprendo!

 

Simply 70 Ohms, regardless of length.

The antenna, coax and TV input should all be the same impedance.
If they do not all match, the signal will not be efficiently transferred
from the antenna to the TV. The mismatch can create signals to be reflected
up and down the coax, causing standing waves. In bad cases, this may even
show up as "ghosting" - which is rather like double vision.

As stated above, impedances must match.
If you connect two TVs to one antenna, the impedance will not match. Look at
it from one of the TVs... it sees the 75 Ohm antenna in parallel with the
other 75 Ohm TV, making just 37.5 Ohms. This is the same looking from any
direction, including the antenna seeing 2 TVs in parallel - 37.5 Ohms.
So a splitter will include resistors so that each TV sees a 75 Ohm
impedance.

Naturally, with a splitter, the signal will be divided equally between the
receivers (with some signal lost in the splitter). So, if the signal becomes
too weak for the TVs, an amplifier is added.

 

Excellent!!
Thank you very much!!!!!!!

 

Nominal impedance for the "FLAT TWIN LEAD" was 300 ohm, Coaxial Cables are
manufactured with varied impedance determined by it's presumed usage. Some
Coaxial Cable is 50 ohm, some 70 ohm, some 75 ohm etc. Most Coaxial Cable
used for Home Antennae Services is 75 ohm impedance, normal types are the
RG-59 and RG-6, specific parameters, regarding loss at specific frequency
bands, etc also vary with the different manufactured items. You have
already been pointed in the right direction with reference to the "Goggle"
sites to garner a plethora of data.

 

well no, I was not offered any particlar usefull link, I was given a list of
a
thousand links, of which none are really answering my question
in any immediate way.

 

and nor does your reply either

 

I don't believe you looked too hard. The first link from Rich Grise
had, as entry 1, a link to Agilent. This one.
http://www.educatorscorner.com/index.cgi?CONTENT_ID=2553
It is a short course on transmission lines. It is definitely worth
your time.
It was certainly worth mine.
GG

 

I don't believe you looked too hard. The first link from Rich Grise

This is a pointless conversation hence this is my last reply on this
issue. There was a good explanation from the 3rd poster. This
is good for me, and all those who don't agree can follow the
links posted. Thank you.

 

Sure, but the OP refrred to 75 Ohm coax for television.

I have a list of coax types in one of my books here.

For reception it is. For transceivers, 50 Ohm is common... RG8, RG58, RG213,
etc.

I just spotted this typing error... I should have typed 75 Ohms, as the OP
had. It appeared correctly further down my post.
Sorry about that.

 

Please note that my reference to 70 Ohms was a typing error... I should have
typed 75 Ohms.
The stated impedance of any coax refers to any length of that coax type.

If creating a splitter box with resistors...

__/\/\/\__ TV1
|
antenna __/\/\/\__|__/\/\/\__ TV2
|
|__/\/\/\__ TV3

All resistors are the same value, worked out by:

N-1
R = ----- x Zo
N+1
Where
R = resistance in Ohms.
N = number of TVs connected.
Zo = TV impedance (75 Ohms)

You can do this for any number of TVs from 2 upwards. All the TV outputs
should either be connected, or go to a 75 Ohm load. So only add as many TV
outputs as you need, and work it out for that number.
If you later add another output, you need to change all resistors to the new
value.
If signal is weak, add an amplifier before the splitter.

 

I can spot a problem with this reasoning. When you say all outputs of the
splitter
must be connected to a 75ohm load I suspect you mean that the coax cables
must
be plugged into the televisions. Now, what happens if one neighbour
(assuming
a central antenna, splitted into several flats) unplugs the coax from
his/her t.v. ,
will that cause a mismatch in the splitter?

 

B.t.w. I had a look at that tutorial and it is rather good. It gives
a good intuitive explanation and avoids the high level maths
which is required for a complete explanation, i.e.Wave
Equations (2nd order differentioal eaquations), but yes,
it is really good. I was a bit hesitant at first as it is distributed
as en exe and exe files can be worms/viruses, but I took
the risk after I read your post. Thanks!!!

 

Actually, it does. There is a thing called a "terminator," which is
just a dummy load, which you're supposed to put on the unused outputs
of a splitter. I bought up a splitter once, and I'm pretty sure it
came with a couple of terminators - I don't remember if I had to
buy them separately.

Cheers!
Rich

 

A TV is one type of load, but a 75 Ohm resistor will present the same
load. Resistive loads are commonly used for testing transmitting
equipment, as you do not want to actually transmit and not using
a load could damage the transmitter.

With no "load" inserted in it's place, yes.
There can be problems with splitting. Separate antennas is the
best option, but not always practical.

 

I disagree with some of the other poster's comments. If it is a good
(2-way) splitter, you can leave one output un-connected without
seriously compromising the quality of the signal to the other output.
This effect comes from a property called isolation. The circuit in the
splitter uses a ferrite transformer to couple the input to two
separate outputs while achieving isolation between the two outputs.

If you look inside a 75-ohm splitter, you will see a 150-ohm resistor
connected between the two outputs. When both outputs are loaded, the
resistor absorbs no power. If one output is unloaded, the resistor
prevents a severe mismatch in the output that is left connected. IOW,
the signal stays clean and free of "ghosts".

A 2-way resistive splitter will lose twice as much signal as a ferrite
splitter. The resistive version will also have less isolation.

Besides the mismatch and ghosting reasons mentioned already, poor
output isolation can cause interference from one TV to another. TV's
generate radio signals via an internal oscillator that can interfere
with another television tuned at about 90 MHz offset (in North
America).

Some inexpensive splitters do not maintain good isolation for all
loads at any frequency. YGWYPF. In this case, it is best to use
terminators as the others suggested.

Frank Raffaeli
http://www.aomwireless.com/

 

my splitter is a 4-way one, but you did not explain how the transformer
works. Is it a step up voltage or step down. Also, I thought a transformer
only isolates DC, why would it isolate 200MHz ?