TECH: dual core -61 ferrite rod MW/LF external antenna for DSP ULR

Steve Ratzlaff <steveratz@...>

Taking Kevin Schanilec's dual core MW/LF FSL as a reference, I tried that concept using the Amidon -61 7.5" ferrite rod today, using it with the DSP ULR PL-380--I believe it's a success, and bench tests indicate it tunes from the bottom of the DSP ULR's LF range, 153 kHz, to the top of the MW range, 1710 kHz. A switch selects between the MW and LF coil configurations. Kevin's passive-tuned FSL used a dual section variable capacitor; mine left the capacitor out since the DSP's internal "varactor" (easier to write that) tunes the coil.
I wound two 80-turn coils using 40/44 Litz (on the new Scotch blue Painter Tape), decided what possible inductances might work for both parallel and series connections to tune MW and LF, used the DM4070 LCR meter to adjust the positions of the coils on the rod, set for 580 uH each coil. That has the coils fairly close together in the center, about 0.35" spacing, with 1.8" bare rod left on each end--which should be enough for securing the rod in a "Gary" type external plastic mount. I admit I haven't mastered how to wind the two coils "contra-wound"--even after two tries they still came out such that my connections for max inductance weren't the same as Kevin's true contra-wound coils. But no matter, just use the LCR meter to find the parallel and series connections that give the highest readings, then wire the switch for those connections. Short leads are needed to keep stray capacitance down, especially for the DSP ULR.

Using the same bench setup as before, and using the theory that the DSP IC is still tuning the coil as long as the RSSI reading is greater than zero, I got higher than zero over the full 153-1710 kHz range, which appears to indicate the new single ferrite dual coil rod tunes the whole MW/LF range. Using the radio and loopstick outdoors, I was able to hear LF beacons as well as MW stations at good level, comparable to the separate external LF and MW loopsticks I've already tried, though daytime signals are limited to much less than the full tuning range, especially for LF.

MW coil configuration (radio in MW mode)
530 kHz 35/16
1010 kHz 36/15
1710 kHz 23/10

LF coil configuration (radio in LF mode)
153 kHz 16/11
163 kHz 35/12
170 kHz 35/22
180 kHz 36/19
200 kHz 36/17
310 kHz 36/16
400 kHz 36/15
510 kHz 35/13
530 kHz 36/13 (radio in MW mode to find max tuned frequency)
900 kHz 32/10
1000 kHz 25/12
1100 kHz 22/14
1140 kHz 15/14

(I can send a picture of the finished setup, if interested.)

NE Oregon

Join to automatically receive all group messages.