![]() ![]() 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. See Mix Selection for other applications.įor extra large cables use the FB400-31 which has an inside diameter of 3 inches (76mm).One of my favorite portable antennas is a 30-ft wire fed through a 9:1 unun. Use Mix 61 for VHF and above for RFI/EMI suppression and below 30 Mhz for multi-ratio impedance transformers (baluns/ununs). USE FB102-31 for 4 turns of RG-8X for 16X increase in Z per bead – see pictures below for examples). If you need more Z, just use more beads in series on the cable or use more windings thru a larger ID bead (e.g. Mix 31 has better response below 10 Mhz but above 10 Mhz mix 31 and mix 43 are very close. For 500 ohms we would need 5 mix 31 beads or 6 mix 43 beads to have greater than 500 ohms. For example if you have a 50 ohm RG-8 coax cable and want a choking impedance of 500 ohms at 10 Mhz, you can check the FB102 column (since FB102 will pass RG-8 through its center ID) at 10Mhz and find that Mix 31 has a Z of 108 ohms/bead and Mix 43 has a Z of 91 ohms/bead. Application Note: Use impedances for each bead at your desired frequency to select number of beads needed for desired Z.
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