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Reflected power on 250 watt ERP non-DA AM when it rains

B

Brian Bowers

I have a shunt fed 250 watt ERP non-DA AM that when it rains, or has wet snow on the tower, reflects back sometimes up to 10 watts. Normally during dry conditions, the reflected power is under 1 watt. When it rains, the BE AM 500 doesn't show a high VSWR or start folding back power, but I always wondered about the reflected power, and wondered if that is normal or typical. This 410' tower also has a 6 bay FM on it.
 
Somewhere the distance from your skirt wires to the tower is changing when they get wet. Also the ground conductivity can change with a good rain storm. Since you said snow, I would have a crew check your skirt wires and the connection to the tower 1/4 wave up. 10 watts reflected isn't going to do much to change the signal, but since it changes from dry to wet, it could be the warning sign of a bigger impending problem.
 
I had a very similar problem with a shunt fed tower but bad enough it would cause my Nautel to fold back. I had a consulting engineer broad band the matching network and the problem went away.
 
Since we're on the subject of AM's, is it necessary to keep brush/grass growth under control near the tower where the radials are buried? Is there a consequence to not doing that?
 
Brian Bowers said:
Since we're on the subject of AM's, is it necessary to keep brush/grass growth under control near the tower where the radials are buried? Is there a consequence to not doing that?
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Yes, it's necessary. You probably have a 50' x 50' copper screen buried around the tower. You don't want trees growing up through that. Anything that grows up through your ground system has the potential of changing things, and will be a problem for replacing it.
 
A shunt-fed antenna typically has a very narrow bandwidth.
Any physical change on the antenna will cause the impedance to shift at the operating frequency.
I don't think that 10 watts of reflected power should be a major concern for you.
Adding an RF network to improve the bandwidth will be expensive.
 
And certainly do an inspection of insulators, johnny-balls, feed-through bowls, etc. that could upset things if it's covered in gunk or cracked, then got wet.

Good time of year to inspect the inside of the ATU as well, make sure all is clean, pest-free and undamaged.

I care for a shunt-fed tower the corporate chief built 30 years ago on a existing FM tower and have had pretty fair luck with it. Tower is a little more than 90 degrees, shunt wire is dialed in to 50 ohms, with just a vacuum-variable cap nulling out the +j400.
 
I had a similar problem with a tower, the VSWR would change during the summer. Traced it to the overgrowth of vegetation near the tower base. When the weeds were cleared out for about 30 feet around the tower the VSWR went to Zero. As the weeds slowly returned the VSWR slowly crept up. Crazy Huh? All the more reason to keep the site neat and clean! ;D
 
Plus one on Grich's suggestion. Check those insulators, especially. If they're either dirty or cracked and either the dirt or the crack holds water, they're not insulators anymore!
 
I should also mention that there is a 5 foot high chain link fence surrounding the tower. The fence is topped with barbed wire. The fence is 3 sided. The transmitter building is on the 4th side, with the tower in the middle. On one side, the fence is about 2 feet from the tower, on another it is 3 feet away, and on the other, it is about 6 feet away. The transmitter building is about 3 feet away from the tower. I don't believe the fence is attached to the ground system. If that is the case, would this have any effect on what I am describing? I'm assuming at some point, it would be better to connect it to the ground system, to keep everything at ground potential, yes?
 
Brian - even though your tower is shunt fed, try putting a static drain at the output of your ATU (you can still have static building up in the sloping wire). It might clear up some of the impulses that your transmitter is interpreting as a change in load. Wet snow, sleet and the change in atmospheric conditions that occur just before it starts raining cause the same effect here, (with series fed towers). Here, once it starts raining everything settles back to normal. Static drains on all the towers minimized the condition.
 
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