Building a feedline HF choke

My current HF antenna is a long wire (around 15 or 20m) connected to an MFJ-993BRT outdoor automatic antenna tuner. The tuner is fed with around 25m of M&P Airborne 10 coaxial cable which runs into the shack. When I installed this antenna, I suffered from high RF currents on the outside of the coax shield when transmitting. These currents go into the shack trying to find a path to earth, since this kind of antenna needs good grounding. Also, while receiving, the coax carried lots of interference into the antenna, especially in the lower bands.

I tried to mitigate this problem by installing a ground rod besides the tuner. This is 2m a copper tube with 50cm buried in the ground. The top of the tube is connected to the tuner ground with a short cable. After installing the ground rod, approximately half of the RF current flowed into the ground rod and the remaining half kept flowing into the shack via the coax shield.

To measure RF current, I have been using a clamp on meter. My design is similar to the design by Ian GM3SEK, but I measure voltage across the output capacitor with a multimeter instead of using a resistor and ammeter coil.

Now I have built and installed a feedline choke following the design of the mid-bands choke by GM3SEK. I use 4 turns of M&P Airborne 5 coax through 3 Fair Rite 2643167851 material 43 cores, wound as an 85mm coil. The finished choke can be seen below.

HF feedline choke

I have measured the performance of the choke using my Hermes-Lite2 beta2 in VNA mode, as I already did with my mains choke. The results are shown below.

The performance seen in these graphs matches the performance measured by GM3SEK in his document. The choke has a resistance of over 1000 ohms on most of the Amateur HF bands, and up to 5000 ohms in the middle bands.

I have installed the choke directly on the input of the tuner. The RF current flowing on the outside of the coax shield has now decreased to around 2% in several cases and 10% in the worst case. The interference received in the lower bands has also decreased noticeably.

2 Replies to “Building a feedline HF choke”

  1. I have a fan dipole for 160,75,40 meters the lengthy of the coax is new it is 95′ of LMR-400 I took off 81′ of the LMR-400 the SWR on 75 meter reads 1.5 SWR and it works fine with my linear amp. on it is the antenna is fed direct on 160 meters the SWR is 1.2 it works fine with 100 watts direct feed to the antenna but when I turn the linear amp. on the SWR is 4.9 or 5.3 if I tune it with my AT5K antenna tuner it works okay on 75 meter but it don’t work right on 160 meter with the amplifier on unless the amp.is not on if so it has to be tuner up.to work let me know what I should do I never had this problem until I changed to me and a little longer coax maybe I can put a choke from the output of my linear amp.to the input of my antenna tuner let me know please.. Paul Wozniak…KB9VWD
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    1. Hi Paul,

      That’s rather strange. In theory the SWR of the antenna system shouldn’t change depending on whether you put the linear amplifier or not. Unless the output of the amplifier is reactive. I would try to find what is causing this. Maybe you could start by measuring the antenna impedance with a VNA.

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