what resistance (ohms) should a TV aerial measure?

Question

We live in an old house. There's a socket in the wall to plug a tv cable into. The signal is not very good. I put a multimeter across the terminals in the socket and it is reading infinite resistance. This rings alarm bells because in my understanding the aerial is actually a bunch of wires connected to itself. So if there's no resistance it implies there's a break in the cable somewhere.

More details: We run a cable from our TV across the room and into the socket. The socket in theory runs a cable inside the wall cavity up quite a distance through the wall onto the roof and to a high antenna attached to the top of a chimeney. It's not easy to get up there and it would be quite a job to take it all apart so if I can figure out where approximately the fault is first it would save a lot of time.

Answer 1

I agree with you that a short circuit is what I expect for DC resistnce. TV antennas can have a folded dipole as the driven element which gets the impedance into a better range, or a split dipole or several different antennas mixed on the same pole. This link shows both types.

http://www.yellowpages.com.au/articles/television-antenna-services-27308/how-high-should-i-mount-my-tv-antenna-20121017t120043n00f0032



The basic configuration of the split type is open circuit, and the folded type (with the elongated loop) is a short circuit. These will very likely be connected through a broadband transformer (balun) and then the coaxial cable down to the wall socket, which is an unbalanced cable. A short circuit would be a few ohms from the room end, and is what I would expect to see. The usual fault is corrosion around the terminals at the antenna or balun transformer connection so it could well be open circuit. Try when wet on the highest ohms scale? If it changes it sort of shows the cable is ok.



If there is a masthead amplifier it needs to be powered from the bottom somehow. Here is a balun transformer, connected right at the antenna to match it to 75 ohm coaxial cable..

http://bourneelectronics.com/Universal-TV-Antenna-Balun-P1483329.aspx



If you have a capacitance meter (on the multimeter) you can get some idea of the distance to the open circuit if you know the cable type. e.g. 75 ohm polyethylene foam TV cable (e.g. RG6) is something like 50pF/meter or 16pF per foot. This can be confused by a break in the outer sheath or wire though. The other idea is look with binoculars.



You could always call a TV antenna person. With changeover to digital the antenna itself might need attention.

Answer 2

I think infinite is correct - there is no connection at the upper end - the "link" is the EM waves in the atmosphere across the various elements.

While your cable might have deteriorated, it is far more likely that the connection up at the antenna out in the weather has gotten corroded or loose or the antenna no longer points in the direction of the strongest signal.

Unfortunately, the only real test is go up on the roof after bridging the connection inside and measure the resistance back down when you undo the connections to the antenna and then put things back and see what is broken on the antenna, if anything. Cell phones make it a lot easier than the good old days to talk to someone in the house as you adjust the antenna.

Answer 3

Tv Aerial Connections

Answer 4

It depends on what type of TV antenna. A folded dipole has a close loop that giving close to few ohms reading. But a 1/4 length dipole has infinitive resistance reading. Or if an TV antenna amplifier was installed in between, then the reading could be a few hundred ohms. Or if some more TV signal splitters were installed to share the signal to every room, then it might read a few ohm or infinitive ohm depending on the device design.

Most antenna junction joints break down and rusty after years of service, that reduce TV signal strength.

Answer 5

If the downlead is a coaxial cable, I think you should see effectively a short circuit, as at the antenna there is a BALUN to translate between the balanced elements and the unbalanced down cable. Usually, it is also a matching balun from 300 to 75 ohms. I think you have an open circuit, as you suggest.

The other possibility I can think of is if there is a masthead amplifier fitted, there will be an isolation capacitor down at the power supply end, which will look like an open circuit if you check with an ohmmeter. Is there a power supply near the outlet somewhere?

Answer 6

Tv Aerial Cable

Answer 7

It should be infinite resistance. The only way I can think of is a TDR, time domain reflectometer. It sends a pulse down the cable which reflects back at the end of an open cable a certain time later, and the time is directly related to distance. You can build your own if you have an oscilloscope, it is just a square wave generator with the scope connected to show the reflection. See source. I built one, it works pretty well.

Answer 8

A Tv antenna is a DC open circuit. The impedance of it will be 75 ohms inductive and the actual resistance of the antenna is 377 ohms, the imedance of free space. Your meter will measure infinity or open.

Answer 9

A DC resistance of infinity is correct, if the source is AC coupled or a TV antenna. You need more specialized equipment to troubleshoot this issue.