Antenna cable impedance matching

[jack wall]

Can any one tell me how to match a 2000 foot 9914F coaxial cable (50 ohm) to a 75 ohm RG-11 cable. The use of these cables are to send TV signals from the nearest hilltop to a home on the cayon floor. The hilltop will have both a UHF and a VHF antennas to feed the signals. The 2000 feet of 9914F is already in condut in the ground. The ends need to be mad up.
Thanks
Jack

 

[flatulent]

You can buy transformers for this. Minicircuits.com will have some.

 

[Miguel Gaspar]

They are named baluns.

 

[E-design]

You will need to boost the signal otherwise your losses over that distance will be high

 

[flatulent]

You will need to boost the signal otherwise your losses over that distance will be high

This is a most important item. You will have to put amplifiers in the line at the input from the antenna and at the juncture between the two cables.

I am wondering if you might be better off in using a method discussed on the board several years ago. In this method the mountain top had one high gain antenna pointed at the transmitters and one high gain antenna pointed at your receiver with an amplifier between them. Your receive is connected to a high gain antenna pointed at the mountain top.

In many situations the attenuation of free space is less than cables.

 

[saubhik]

For this you need a BALUN - Balanced Unbalanced Microwave device , or otherwise you can do it by attaching a transformer.

For BALUN the impedance should be Z = SquareRoot of (50 *75) =near about of 62 Ohms.

 

[Kral]

jack wall,
A balun (contraction of "balanced-unbalanced"), is used to convert a balanced source to an unbalance load, or vice versa. Since the cable is coax, it is unbalanced. If the 75 Ohm load is also unbalanced, then an ordinary transformer will do. A balun is not necessary or appropriate. A transformer with a turns ratio of Sqrt(75/50) = 1.225 will work.
Regards,
Kral

 

[davidkho]

Hi, I'm fresh in electronic
Just wonder why we need to match the cable and what does balance-unbalance mean
Tx

 

[Kral]

davidkho,
A balanced signal/cable is one that supplies a voltage with respect to a common point (ground). A balanced cable is a 2 wire cable that supplies a differential voltage, i.e., the voltage is applied between the two conductors, without a common (reference) point such as ground. Ballanced cables have the advantage that spurious noise pickup to to capacitive or magnetic coupling is applied equally to both conductors, so that the net effect on the differential voltage is zero. Unbalanced cables are usually shielded to avoid the effects of noise pickup. A common example of a balanced signal is the 300 Ohm twin lead cable used by older tv antennas. A common example of an unbalanced cable is coaxial cable, in which the shield serves as the 2nd conductor.
.
Matching is important to avoid ringing in response to step inputs, and to provide maximum power transfer. from the source to the load. The characteristic impedance of a cable can be very closely approximated by the following equation: Zo = SQRT(Ln/Cn), where Ln is the inductance per unit length of the cable, and Cn is the capacitance per unit length of the cable.
.
A balun serves two purposes: It has a balanced(unbalanced) input and an unbalanced(balanced) otuput winding, and it provides a turns ratio that provides impedance matching as well.
Regards,
Kral

 

[davidkho]

Thanks

 

[mhtplsh]

wht do all this headche.
There is better method.
why not use 75 ohm vhf/UHF antennas. This will solve the problem of matching.
Further u can also retransmission from the hill if law in ur country permits. It is more simple. less maintainance.