![]() So, there’s some RF on the coax shield by design. This made sense to me, since this antenna configuration relies on the shield of the coax for the counterpoise. One or two bands wouldn’t load up properly. As I suspected it might, it affected the tuning of the antenna. Not long after this post was published, I tested this 1:1 unun in line with the coax to my 30-foot wire and 9:1 unun. Since this article seems to get a lot of traffic, I figured it was time for a long-overdue update. I’ll be sure to update this post with any new insights I gain. Since the initial problems were very intermittent, only time will tell if I solved those problems or not. I want to make sure that the line isolator doesn’t affect the T1’s ability to tune my antenna. Well, this part will have to wait until I have a chance to get out for some portable operating. While that tells me nothing about how effective it is in reducing common-mode currents, I at least know I didn’t make any serious screw-ups in building it. ![]() It was basically flat from 160M through 6M. In lieu of that, I hooked it up to a 50-ohm dummy load and checked the SWR. I don’t have access to the equipment necessary to do any type of exhaustive testing of the line isolator. This foam provides a slight downward pressure on the choke to prevent it from shaking loose in the case during handling. As an additional precaution, I put a piece of packing foam on top of the choke before attaching the lid. To mechanically secure the core, I used a piece of two-sided foam mounting tape to mount the choke to the bottom of the case.I used a 5/64-inch drill bit and had to use a reamer to get the holes to the right size for the BNC connectors I used. Drill the holes for each of the BNC connectors and wired up the choke, as shown.I used a couple of small nylon tie-wraps to hold the windings in place. This makes installation in the case a little easier. Take note of how the 5th turn goes across the core. The RG-174 coax is wound on the FT-140-43 core for a total of 10 turns.Hammond Manufacturing 1591MSBK Enclosure (2.2 x 3.3 x 0.8 inches).RG-174/U coax (approximately 24 inches).(Note: Using a line isolator at the antenna end of the coax would defeat the purpose in using the coax as a counterpoise.) A quick survey of my junk box stash of parts showed I had everything I need to build a line isolator from scratch. My proposed solution for this is to use a line isolator between the tuner and the coax feeder. Since this has only happened when using the 9:1 unun, my suspicion is that common-mode RF currents on the coax shield are the culprit. ![]() (Running through the T1’s diagnostic mode always seems to restore operation to normal.) I’ve also had one of my keyers behave erratically once or twice. On a couple of occasions, my Elecraft T1 auto tuner began to act up, refusing to load up on one or more bands. Most of the time, this has worked well for me with no issues with stray RF getting back into the equipment. This type of antenna generally the uses coax feeder as a counterpoise, since the 9:1 unun configuration provides no line isolation. One of my favorite portable antennas is a 30-ft wire fed through a 9:1 unun. ![]()
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