80 Meters and Up – Portable Base Matching Unit for 42 to 48 Foot Verticals and Inverted-Ls – QRP Version

The unit on the right below is the QRP BMU (Base Matching Unit) intended for 42 foot to 48 foot vertical, sloper, or Inverted-L antennas.  Naturally, these antennas (like all end-fed antennas) require a decent radial system for efficient operation on the lower bands.

BMUs Top View QRP Portable and Med PwrThe purpose of a BMU is to mitigate the impedance extremes presented by the antenna and allows for longer low-loss coax cable runs if desired, and for low-loss matching that is within range of most tuners.  To this end, we use a loading coil for 80 M / 75 M, and a 9:1 UNUN for all other bands 60 M through 6 M.  More on BMUs here ===> http://vtenn.com/Blog/?p=1158.

XCVR - T-Tuner - BMU - Antenna - QRP

This QRP unit can sustain at least CW 5 Watt operation (85 degree ambient temperature), and should easily sustain 25 Watts ICAS, although not tested at this level.    (The QRP T-Tuner portion of this project is shown here ===> http://vtenn.com/Blog/?p=1552)

The circuitry is simple and consists of a tapped approximately 11 uH and 10 uH inductor as a loading coil for the 80 M and 75 M bands, and is switched over to a 9:1 UNUN for all other bands from 60 M on up to 6 M.  (A tapped inductor is really not required as a single 10.5 uH unit provides adequate matching for the entire band.)

FIGURE 17 - BMU QRP Schematic - 21
The loading coil for 80 M and 75 M is 11 and 10 turns respectively of #22 wire on a FT82-61 core.  A  ferrite core is used here and is of a sufficiently low loss, 0.23 dB at 75 degrees F,  and physical size to handle 5 Watts continuous operation.  Note:  my experience is that there is a fair amount of variation in core material, and tweaking is often required to center up the SWR minimum right in the desired portions of the 80 M / 75 M band.  Cores are available from standard parts suppliers like DigiKey or direct from Amidon at a few $ each.

Note the loading coil wrapped with red magnet wire right lower below:

BMU Bottom View QRPThe UNUN is 9 turns tapped at 3 turns for the primary on an BN43-6802 binocular core.  High permeability ferrite is used in this location as low shunt inductance is required to achieve a faithful impedance transformation.  9 turns was found to provide 640 uH shunt inductance on the secondary side, and was sufficient for 5 Watts continuous operation.  (Fewer turns resulted in unacceptable heating !!!)

SMA connector is used at the input.  Naturally, any connector of your choice would be fine.  I chose banana jacks for the antenna and radial connections.

BMU Top View QRP


The “SWR” method and “Back-to-Back” method of UNUN characterization doesn’t tell us a lot about it’s performance into real antenna impedances, so efficiency of the unit into actual antenna impedances was measured using the procedure in the  article “Z-Substitution” Method for Measuring UNUN Loss Into Real-World Load Impedances.  The loss figures below are for the average impedances presented by a 43 foot antenna:

80 M / 75 M: Loading coil used, 0.14 dB
60 M:  -0.74 dB
40 M:  -0.42 dB
30 M:  -0.66 dB
20 M:  -0.95 dB
17 M:  -0.43 dB

15 M:  -0.80 dB
12 M:  -0.80 dB
10 M:  -1.30 dB
8 M:  -0.33 dB
6 M:  -1.35 dB

Copyright Dave Benzel- KD6RF – 2017 Mar 1







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