Issue with Korner 9.2 FM Yagi

[mwdxer]

I use a Korner 9.2 FM Yagi (German made) on the Northern Oregon Coast. The Korner 9.2 in supposed to be very directional. F/B in the order of 30 DB. Yet mine seems to have some strange issue. The directivity is not good. I run RG6Q out about 90 feet to the Korner. I added a few chokes along the way and the 4-5 wrap coil just before the matching transformer. When the RG6Q is connected to the Sony receiver, I get a short from the dipole loop where the RG6Q is connected, to the boom. That should be totally isolated. If I disconnect the RG6Q from the Sony on either end, the isolation is normal. Of course I am sure this is messing up the directivity of the Korner. The RG6Q is top of the line and only a few months old. Has anyone ever experienced anything like this? A couple times, the directivity did improve, but just for a day or two. But the weather was bad, so I could not check anything out. After the bad weather, I dropped the Korner down, checked the connections and everything is tight. This seems strange to me. Even if the RG6Q was compromised how in the world would I get a short between the dipole loop and the boom?

[Mike-CT]

I've never used a Korner antenna so I'm just guessing. I see you use chokes and a balun plus 4-5 turns of coax, I'm not sure why you need all that. You would use either the balun or the 4 turns of coax. The four or five turns acts as a balun. If the coax is good (no short somewhere between the foil outer layer and the copper center conductor) and you are using the 4-5 turns directly connected to the dipole on the antenna, then the problem is at the antenna. If the antenna is the only possibility left, you need to get a voltmeter and do some serious resistance checks, looking for shorts or open connections somewhere. Visual inspection doesn't always find the problem. A multi-meter will. Also check any F fittings for water, rust, corrosion.

[mwdxer]

Thanks Mike. When the coax (RG6Q) is not connected to the balun, I get no short. Connecting the ohm meter across from the boom to the dipole, it is open, which it should be. Only when I hook up the coax to the radio, I get the short. That this doesn't make since, unless that wraps which is tied to the boom frame it getting some continuity some how. The reason I added a couple chokes, as I wanted to make sure the RG6Q had no affect on the directivity. Maybe a bit over kill, but even so, chokes or not, there should still be no short. I had a couple days when the directivity was normal, so something is amiss. When our weather improves, I will take the mast down. We have had so much rain and wind this Winter, getting to the Korner has not been an easy task.

[WHJ]

What is the F/B you are seeing on local stations? How high is the antenna?
Actual F/B can be reduced by other metal or wires relatively close to your antenna.
If your antenna is pointed towards a large metal structure, like a water tower, even miles away, signal reflections from it can make appear that the antenna directivity is at fault.
Other similar things can influence the measured directivity. Sometimes, the performance of antennas with high modeled directivity do not live up to those figures in real life due to factors which are hard to control.
The Korner 75-to-300-ohm balun will provide a DC short to the coax connected to it.