EME Antenna Performance versus Resonant Frequency
(January 14th, 2015)
After having invested a lot of money and efforts into putting together a good antenna array for moonbounce communication for operation at the lower portion of the 2m band (144 MHZ), the newbees EME practioners may be tempted to go for a compromised solution and "tune" the antennas to resonate at a higher portion of the 2m band (e.g. 146 MHZ and above) in order to use other modes of communication such as voice, packet, repeaters, etc. In the EME world, what will be the cost of such adventure...?
As a case example, see below the EZNEC radiation pattern of my 2X14 RPOL "tuned" at difference resonant frequencies, but showing the effective pattern at the center frequency of 144.130 MHZ for EME operation. The first picture shows the composite of all the resonant frequencies, and the following pictures below show the individual pattern and performance data for each plot (forward gain, front/back, etc.).
The table below summarizes the results of the performance data for various Resonant Frequencies:
The results are absolutely shocking! Although the deterioration in performance may not be very significant for ordinary mode of communications (e.g. voice, repeaters, packet, etc.), where 1 dB equals ~ 1/6 of an S-Unit, in the EME world, such performance loss may be a "killer", especially for small antenna arrays where every dB counts and there is no dB's to spare...
Not only the loss in "Forward Gain" becomes very significant as the antennas resonant frequency differs more and more from the EME center frequency, but the extreme deterioration in the Front/Back and creation of massive side lobes may translate into extreme surrounding noise pickup. The combination of both negative effects can result into many dB's of performance loss. For small EME arrays, such performance loss is most likely going to be extremely detrimental, especially if there is a lot of local noise as the sub-optimized antenna system will be doing a poor job at rejecting it.
Picture 1: Composite showing the patterns at various antennas resonant frequencies for EME operation at 144.130 MHZ
Picture 2: Effective Radiation Pattern at 144.130 MHZ when Antenna is "Tuned" for 144 MHZ Operation (optimized performance)
Picture 3: Effective Radiation Pattern at 144.130 MHZ when Antenna is "Tuned" for 146 MHZ Operation
Picture 4: Effective Radiation Pattern at 144.130 MHZ when Antenna is "Tuned" for 148 MHZ Operation (very bad!)