Why we use a Base Matching Unit (BMU) with End/Base-Fed Verticals and Inverted-Ls

In this article we look at the reduction in feedline loss when we use a BMU to mitigate impedance extremes at the base feedpoint of a Vertical or Inverted-L antenna.  Naturally, these antennas are worked against a proper counterpoise radial system for decent efficiency, especially on the low bands.

We look at the cases of a 42 ft antenna, and use an average of the measured and modeled feedpoint impedance to calculate feedline losses* for 50 ft of RG8x and for 100 ft LMR400.

BMUs Bottom View Portable QRP and Med Power

The BMU referred to here is:

A 9:1 UNUN for the mid and high bands
A Loading Coil, switched in instead of the UNUN, for the low bands

Note that feedline losses are shown for a 42 ft EF-Inverted-L or Vertical. This represents a worst case configuration.  Feedline losses are a bit lower for longer EF-Inverted-Ls and Verticals, such as the typical 43 ft vertical and 45 ft Inverted-Ls.

 

42 FT VERTICAL OR INVERTED-L, EXPECTED WORST-CASE TRANSMISSION LINE LOSS FOR 50 FT RG8x

BMU vs Direct Feed – Cable loss with 50 ft RG8x feedline. Losses are greatly reduced throughout the frequency range using a BMU with a switched-in Loading Coil on the lowest band ===>

FIGURE 13 - 42 Foot - RG8x-Direct-vs-BMU - 12

BMU vs 9:1 Unun – Cable loss with 50 ft RG8x feedline. Losses are greatly reduced at the lower end of the frequency range using a BMU with a switched-in Loading Coil on the lowest band, as opposed to a simple Unun ===>

42-rg8x-with-BMU-vs-9_1 - 2

 

42 FT VERTICAL OR INVERTED-L, EXPECTED WORST-CASE TRANSMISSION LINE LOSS FOR 100 FT LMR400

BMU vs Direct Feed – Cable loss with 100 ft LMR400 feedline. Losses are greatly reduced throughout the frequency range using a BMU with a switched-in Loading Coil on the lowest band ===>

FIGURE 14 - 42 Foot - LMR400-Direct-vs-BMU - 12

BMU vs 9:1 Unun – Cable loss with 100 ft LMR400 feedline. Losses are greatly reduced at the lower end of the frequency range using a BMU with a switched-in Loading Coil on the lowest band, as opposed to a simple Unun ===>

42-lmr400-with-BMU-vs-9_1 - 2

 

Dave Benzel – KD6RF – 2017 Jan 5

 

* Thanks to Dan AC6LA, author of several antenna related programs, for TLDetails which accurately calculates feedline loss ===> http://ac6la.com/tldetails.html
tldetailsv20

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