Low Frequency Ferrite Rod Tuning Coil for the R-390A

          The R-390A can be used below 500 Kcs without any major modifications please see previous pages for info. Link below.

    Additional info on using the R-390A below 500 kcs using a variometer for antenna matching.

    Shown above two different coils for matching an antenna to the R-390A below 500 kcs. On these pages we will discuss the coil on the right that is tuned with a ferrite rod.

          The coil and capacitor provide quite a tuning range for matching the antenna to the input of the mixer at E209 for R-390A reception below 500 kcs. In most of my testing just using the coil to match a random wire antenna provided a close match and provided sufficient RF gain and aided in selectivity in the 300 to 500 kcs range.

              A simple alligator clip and capacitor provide mixer input to test point E209 from the antenna coil.

     A trip to Lowe's will provide you with a ABS plastic pipe with a 1-7/8 inch diameter. Sanding the pipe will aid in holding the windings as you wind the coil.


        After winding several coils I decided that using 4-40 hardware was the best solution for tap points on the coil. The coil in front just used a twist in the wire to form a soldering loop. When winding the coil when you reach the 4-40 bolt just scrape off some of the enamel and wrap once around the bolt and continue. The neat feature of the bolts is that you can lay the project down half way through the coil because the 4-40 bolts secure the windings as you go along and you can take a break from the strenuous exercise.

   WINDING Tips: Rig a vertical holding jig for your small reel of wire as the reel must be able to rotate freely as you pull wire from the reel - you will only pull off what you need. Feed the wire through the jaws of the vice but don't clamp the wire yet. Secure one end of the winding on your coil form. Take the coil form and walk across your shop and extend about 20-30 feet of wire and tighten the vice to temporary hold the wire while you wind your first turns. Hold the coil form with two hands and slowly rotate the coil and wind while you approach the vice - keep the wire taut as you wind. When you get to the vice which is temporary holding the wire then use with one hand to keep the coil from unwinding from the coil and use the other hand to loosen the vice so that you can meter out additional wire then close the vice again. When the coil has enough windings keep the wire taut and estimate where the wire enamel needs to be removed for a connection and then use a razor blade to accomplish same. Secure the end by wrapping the bare wire around the 4-40 bolt which was previously installed or insert the bare end in the PRE drilled holes.     Its not Rocket Science - play with it.      An alternate method is to get your wife to assist you - you will be amazed at how many winding techniques she can offer and how a simple process can be turned into a lengthy complicated task.  

                 Ready for winding the coil has been sanded and is 7 1/4 inches in length.


             When you reach the bolt, strip the wire and then do a single wrap and continue the winding. If you are exhausted after the first winding take a break. But if you want to do some real radioing then man up and continue with the project.


          Go to the ARRL site and order a "Coil Winding" calculator. This will help you with "What If" questions such as change of wire size, number of windings, diameter changes etc.


           My overall plan for the coil was to utilize a ferrite rod inserted in the coil to vary the inductance. Using the coil taps and the rod provides a large tuning range.There are plenty of distributors of ferrite rod on ePay and several US distributors. The material type of rod is not that critical in this particular application. Type 61 material is OK.


    TIP: A 4 inch rod is OK as you can either use it by itself or glue another rod to the end to give you 8 inches. Also if you break the rod you can glue it back together. Mark the rod in increments for logging your tuner settings with a Brother Label maker.




          By marking the rod you can log tuning settings for future reference.

              Video of R-390A low frequency tuning via coil and rod.



       Most of my tests using several random wire antennas worked very well without using a capacitor across the coil. I do use this cap in a lot of antenna tuning situations on other bands . The alligator clip selects different values. Handy when you are doing real radioing.



                    Repair any broken rod with Super Glue. If it is a tight joint performance is not degraded.

When gluing two 4 inch rods together end to end to make a longer rod use JB Weld - sand the ends with 400 sandpaper very light for good adhesion of the epoxy.



                                  When gluing rods end to end use 6 minute JB Weld.


         When the fear of loose windings keeps you awake at night apply a little E6000 across the coil windings DO NOT cover the entire coil as it will effect the Q - - just do a couple of narrow strips across the coil 90 degrees to the windings.


                      Coil length is 7-1/4 long. Inserting the ferrite rod increases the inductance.



        TIP: Need more inductance to put in series with your tunable coil for use on a lower frequency that is out of range of the home brew coil? Use a 2.5 mH choke - you could even tap it in between the windings(see arrows) to select smaller inductance values. BUT the Q will be lower than the larger size coils which you can fabricate and beside when you wind the larger coils you are doing some real radioing.
                       RETURN to K4CHE Index