indoor flat antennas

[crp]

part of the instructions for flat antennas is that the cable from the "wires" drop straight down.

well, they really do mean it.

if you are having reception problems with one of these, ensure the drop is totally down, no up curl at the end. if you need a longer cable to reach your tv from the bottom of the cable, do so.

[ross549]

Are you talking about the mohu leaf antenna?

[crp]

Are you talking about the mohu leaf antenna?

among others, yes.

[ross549]

Hmm. Coax is, by definition, shielded..... so I wonder how the position of the tail would even matter.......

 

Adam

[Capt.Crow]

There are different ratings of coax . TV coax is not so good for transceiver signal . etc...I forget the values just now .

[ross549]

If you are referring to RG-58, 59, or 6, then they do have different frequency ratings. Most TV antennas are rated RG-6 these days, replacing RG-59.

 

Adam

[Cluttermagnet]

OK, can't resist, have to weigh in here. First, the wire Temmu describes in post #8 is known as open wire line. It can work perfectly fine under the right conditions. It is not necessarily as 'unshielded' as it looks at first glance. But let's leave that detail for later...

 

Coax is interesting stuff. Guys, at the frequencies we are talking about, there is a lot of 'skin effect'. This means that the current the shield is carrying bunches up in a thin, thin layer near the surface of the shield. This leads to the not so obvious fact that in a lot of radio antenna systems, the coax can be carrying very different currents on the inside surface of the shield vs. the outside surface. The inner current is referred to as differential mode. It is the current carried between the center conductor and the shield. This is also, BTW, the desired mode. But there can also exist a separate current on the outside of the shield called common mode current. Common mode can result from pickup by the coax of nearby transmitted signals. There is an entire industry of devices that do fix or purport to fix common mode current on the outside of coaxial cables, but again, this is a subject for a different discussion.

 

What you guys are really addressing here could well be called antenna-feedline interaction or antenna-feedline coupling. This coupling is almost always undesired and causes an antenna to turn in a less than optimum performance. You can see spurious modes arise in the antenna's overall pattern as seen in 3D plots (drawings), and the antenna departs, sometimes greatly, from the classical pattern which would be expected of a given antenna type. For example, the antenna could lose gain or develop nulls in the directions you want to receive from, and develop useless high angle modes. There are no TV transmitters above us, by and large. They are always horizontally placed from the receiver, and their signals come from paths at very low angles with respect to the earth. So an antenna which looks 'up' or performs best at angles well above the horizon is useless compared to one that responds best at low horizontal angles.

 

This is a classical problem and concern in ham radio circles- how does one optimally orient the feedline with respect to the antenna to minimize antenna-feedline interaction? It makes intuitive sense to me that the designers of that mohu antenna want the feedline contained within the plane of the antenna. This would be true whether coax or open wire were being used, BTW. This is to miminize that coupling. The further away you get from the antenna, the less likely that coupling will be severe. So if the end of that feedline makes a turn to connect to the set, that would generally be OK. But bending the feedline away from the plane of the antenna right at the antenna would likely produce severe coupling and pattern distortion.

Edited July 28, 2014 by Cluttermagnet

[ross549]

Thank you for that explanation!