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

The unit on the left below is the medium power 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 - MedPwr

This medium power unit can sustain CW 75 Watt operation (85 degree ambient temperature), and should easily operate digital modes at 100 Watts, although not extensively tested at this level.  (The medium power T-Tuner portion of this project is shown here ===> http://vtenn.com/Blog/?p=1558)

The circuitry is simple and consists of a 10.5 uH inductor as a loading coil for the 80 M / 75 M band, and is switched over to a 9:1 UNUN for all other bands from 60 M on up to 6 M.

FIGURE 15 - BMU Med Pwr Schematic - 21
The loading coil for 80 M / 75 M is 28 turns of #18 wire on a T106-2 core.  A powdered iron core is used here as a ferrite core of the same size was not able to sustain full power operation without going into thermal runaway.  Core loss was a low 0.14 dB.  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 middle 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 brown insulated wire left side below:

BMU Bottom View Med PwrThe UNUN is 8 turns wrapped trifilar on an FT114-43 core.  High permeability ferrite is used in this location as low shunt inductance is required to achieve a faithful impedance transformation.  8 turns trifilar was found to provide 380 uH of shunt inductance on the secondary side, and was sufficient for 75 Watts continuous operation.  (Fewer turns resulted in melted tie-wraps and a shattered core !!!)

BNC 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 Med Pwr


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 inductor and UNUN 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:  -1.10 dB
40 M:  -0.31 dB
30 M:  -0.63 dB
20 M:  -1.10 dB
17 M:  -0.69 dB

15 M:  -0.61 dB
12 M:  -0.69 dB
10 M:  -0.84 dB
8 M:  -1.3 dB
6 M:  -1.0 dB


Copyright Dave Benzel- KD6RF – 2017 Mar 1


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