The contest club I am part of uses 2M Loop Fed Array (LFA) yagis for their QRO station. It’s always been advised to use a choke of some sort as close to the feed point as possible to prevent stray RF traveling back down the outer of the coax and into the station. The cheapest way of doing this is by creating a coax choke using 2 to 3 turns of coax around 2″ in diameter but this is also quite messy and risks fracturing the center conductor or foil in the feeder.
There are several commercial ferrite chokes available which replace the messy coax choke and one of our members has a large stock of unknown mix ferrite beads (which can be purchased in a pack of 5 for under £2) so we decided to test how well the unknown mix ferrites worked.
I can test for current flowing down the outer braid of a coax cable using a G4HUP RF Current Meter and a 2M handheld radio as an RF source. It’s not a very technical measurement but I hope it will prove useful and show how results can be obtained with cheap equipment. The 2M handheld is set to 144.300MHz with 5W of output in FM mode.
The first test shows the current being measured on the coax with no choke.
We can see that around 40mA is flowing back down the coax braid.
I created a simple coax choke using 2 turns of coax on a 2′ former.
An interesting and unexpected result for me. I thought a coax choke would help but was not expecting such an effective result. We can see that the coax choke is a substantial improvement with only 2mA now showing on the coax braid.
The coax choke was removed and I slid 4 ferrite beads over the coax.
Another interesting result. Using only 4 of the unknown mix ferrite beads bring the current flowing on the coax braid down to just 1mA. I have no doubt this would come down to 0mA with the addition of another 1 or 2 beads but unfortunately I only have 4 to test with.
I’m impressed by the performance of the cheap ferrite beads and will be using them on the 9 element LFA that I am currently building. They can be secured with heat shrink and look much better than a coax choke whilst also reducing the chance of failure in the coax.
The ferrite beads I used for this test can be purchased from CPC (https://cpc.farnell.com/unbranded/559-568/emi-sleeves-10-7-x-28-5mm/dp/CBBR6937?fbclid=IwAR2xQRr9uZi7sIFYZyfQuUNuKe1SFQQ–k3lUWYfsuY6fLBBn78fdivq2bI) and are a good fit on RG213 sized coax.