Shunt fed am towers

[Larry the radio guy]

Hi,

Does the FCC still allow shunt fed AM towers? I remember in years past they would not allow shunt feeding above 1000 watts. I have an AM tower that can support a new fm translator antenna but the problem is jumping the base insulator with the fm coax. I know about using a transformer across the base, feeding at a neutral point and using a skirt but it would seem a shunt feed would be the best. The station is a nond with quarter wave tower. If I remember, the shunt goes about 20% of the height from the bottom and must have a series cap but no matching network. I also remember they tend to shift impedance in some cases. Anybody have experience with this?
Thanks

 

[fm-engineer]

I'm pretty sure you can no longer use a slanted shunt feed at any power level. From the description of your tower, a simple isocoupler on the FM coax is the most cost effective means of getting the FM antenna up. If the tower is at least a 1/4 wave in height, you could put the coax on insulators, ground at the 1/4 point near the top of the tower and also ground below the base insulator. One advantage with a 1/4 stub is the tower is now ground at DC. I've found it works better for static etc compared to a static drain choke, which you probably have now inside your ATU. The cheapest route is still going to be the isocoupler.

 

[rfburns]

I'm pretty sure you can no longer use a slanted shunt feed at any power level. From the description of your tower, a simple isocoupler on the FM coax is the most cost effective means of getting the FM antenna up. If the tower is at least a 1/4 wave in height, you could put the coax on insulators, ground at the 1/4 point near the top of the tower and also ground below the base insulator. One advantage with a 1/4 stub is the tower is now ground at DC. I've found it works better for static etc compared to a static drain choke, which you probably have now inside your ATU. The cheapest route is still going to be the isocoupler.

Correct. Using a single slant wire to shunt feed a tower creates high vertical angle radiation. That's why the FCC no longer allows them. Either a folded unipole or isocoupler will be your best options. I've seen the 1/4 wave stub (or bazooka feed) used and they seemed to be high maintenance. Adding anything else to the tower is not easy with the 1/4 wave stub either.

 

[Grounded Grid]

I've never seen more than a couple. One of them (formerly in Portland) is long gone. The other one is at a daytimer I work on now and then in Omak. It's their original tower, put up in 1947.

 

[dc2bluelight]

There was one in Southern WI many decades ago, 1kW, about a 300’ self supporter at 1230. Aside from their vertical radiation angle the “base” resistance was like 208 ohms. Not real efficient. They’ve long since moved on.

 

[davideduardo]

I'm pretty sure you can no longer use a slanted shunt feed at any power level. From the description of your tower, a simple isocoupler on the FM coax is the most cost effective means of getting the FM antenna up. If the tower is at least a 1/4 wave in height, you could put the coax on insulators, ground at the 1/4 point near the top of the tower and also ground below the base insulator. One advantage with a 1/4 stub is the tower is now ground at DC. I've found it works better for static etc compared to a static drain choke, which you probably have now inside your ATU. The cheapest route is still going to be the isocoupler.

For many years, shunt fed towers were the preferred system for the higher-power stations in Mexico (50 kw and above). I have photos of a couple, particularly the XEB-1220 100 kw operation, at https://www.americanradiohistory.com/Archive-1963-Radio-Photos/Mexico-Radio-Photos.htm