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WHAT THE RMU DOES:
The RMU modifies 80m resonance of the 807-xx.
By using different values of capacitors inside of the RMU, we are able to place or change the resonance on 80m anywhere we wish, WITHOUT affecting resonance on other bands.
WHY IS THIS IMPORTANT ?
The 80/75m band is unique in that it is impossible to have a low SWR across the entire band, due to something called Fractional Bandwidth. Therefore the user must choose his/her favorite resonante frequency and adjust the antenna for that. On normal dipoles, "adjust" means adjust the length.
BUT... with an OCFD antenna, adjusting the antenna's length on 80m ultimately affects its resonance on ALL higher harmonic bands, often moving resonance upwards and outside of the ham band. So length adjustment is a bad idea! DO NOT CHANGE THE LENGTH OF ANY AERIAL-51 OCFD ANTENNA!
By moving 80m resonance with an RMU, we are able to adjust 80m resonance without affecting resonance on the higher bands. In theory, we can place the frequency of minimum SWR (SWRmin) anywhere we wish . . . by adjusting the value of the capacitors inside of the RMU.
In addition, by swapping the RMU for one with a different value, users are able to change the SWRmin as desired (i.e., use an RMU-CW for a CW contest and an RMU-R1 or RMU-R2 for an SSB contest; and if 75m is your favorite band segment, us the RMU-75).
THIS IS A UNIQUE FEATURE OF THE RMU!
and the Aerial-51 807-xx antennas.
As seen in the graph on the right, the frequency of minimum SWR is just below 3.600 MHz. This is preferred by hams who operate 'Mainly CW' and/or Digital Modes.
Note: if desired, the frequency may be set a little lower (on special order and with longer lead time).
In the meantime we have added more capacitance to the RMU-CW, moving SWRmin to about 3.560 MHz.
NOTE: The 'Frequency of Minimum SWR' shown for each RMU above is only the approximately frequency. The actual frequency may vary by +/- 25 kHz due to manufacturing tolerance of the capacitors, height of antenna above ground, and consistancy of the earth below it, which varies from one QTH to the next.