- Posts
Hi Everyone
What I would like to know does tower hight make a difference. I see Rangemaster says use
Hi Everyone
What I would like to know does tower hight make a difference. I see Rangemaster says use
a 30 ft Tower…Well would a 200ft tower work worse or better. I know the olde rule it the higher the better
the signal,but since AM radio works off ground wave and not sky wave. What is better a short tower or a tall
tower???? I was going to use a 200 ft tower, so does anyone have any suggestions. I really need to get distance out of this transmitter,but stay legal…SeanKW40
Total posts : 45366If you’re not careful you could accidently create an unbalanced dipole with the 3 meter section and a long lead down to ground. Perhaps if your ground lead was RF shielded using coax you might be able to do it. The unbalanced dipole problem is one of the very few things the FCC has enforced against with Part 15 AM operators.
Total posts : 45366Using the center conductor of a coaxial cable as the ground lead, and hoping that the outer conductor will serve as a “shield,” and prevent radiation, is wishful thinking.
I think that the problem is with the word “shield” itself, which is what the outer conductor of the coaxial cable is called. This word was originally applied to the protection carried by ancient warriors to deflect the blows of swords and other weapons. Coaxial cable shields provide no such barrier to electromagnetic fields.
The shielding effect of coaxial cable (when it occurs) is not due to a barrier set up by the copper in the outer conductor, but instead due to currents flowing in opposite directions (differential mode) in the center conductor and the outer conductor. These currents, flowing in opposite directions, produce fields that cancel each other out, and prevent radiation.
However, currents that flow in the same direction in the inner and outer conductors (common mode) produce radiation. The coaxial cable used to “shield” a ground lead is likely to have common-mode current, and radiate.
Radio engineers, and also hams, are aware that special measures have to be taken to prevent radiation from coaxial cables. For example, when connecting a coaxial cable to some types of antennas, like dipoles and beams, “Baluns” sometimes have to be used to eliminate common-mode currents that produce radiation. However, when using coax to attempt to shield a ground lead, no measures to prevent radiation are taken.
Total posts : 45366Not to put words in the mouth of the previous poster, but it might be that it really isn’t practical to attempt to put a transmitter on a high tower because a compliant installation really isn’t likely.
As I understand it, anything not buried or electrically shielded as described above is likely to radiate. As there weren’t any constructive suggestions listed in the previous post, it’s possible there is no way to do it.
Also not noted above is whether or not there is even any benefit to placing the transmitter up high, compliance problems notwithstanding. It might be that it’s not that helpful anyway.
Total posts : 45366If compliance were not a concern, it is desirable to get the antenna as high as possible. Greater height (including the ground lead) gives higher radiation resistance. Radiation resistance is the portion of the antenna resistance that is related to the radiation of RF power. The higher the radiation resistance, the greater the antenna efficiency.
When I say, “as high as possible,” I mean up to a quarter wavelength, or so.
- You must be logged in to reply to this topic.
