Monday, April 27, 2020

Antenna Testing with a Two-Receiver Red Pitaya 122.8-16

Once I had the second receiver running on my 122.8-16 Red Pitaya, I decided to explore methodology for comparing antennas.  I hope this may be of interest, and would welcome any critiques of my technique.

Both antennas are installed on non-conductive poles, the Mini-Whip at 8 feet above ground and the Clifton Labs active whip at about 12.  Each antenna is grounded to an 8-foot copper ground rod at its base through insulated wire taped to the pole.  My particular Mini-Whip is from a supplier in the Ukraine, Transverters-Store.  He says it follows the latest PA0NHC improvements on the original design, and provides a schematic to prove it.  The Clifton Labs active whip is a prototype produced by Jack Smith, K8ZOA (SK) about 10 years ago.  So far as I can determine, it is operating normally.

Because everyone seems to agree that noise reduction is critical to these types of antenna, I wound the feedline coax on Type 31 "big snap-on" cores at both ends of the feedline to the mini-whip, and on the shack end of the feedline for the Clifton whip (I ran out of cores).  I am sure these chokes can be improved upon by winding smaller coax or insulated wire on cores, permitting more turns and better isolation.  HPSDR suggests that noise coming in from the antennas is roughly the same, about -125 dBM.  The band noise as received on my Carolina Windom was a few dB higher, at around -122.

For purposes of this first comparison, I ran the two antennas simultaneously for about 21 hours on 80-12 meters. Results:

Number of spots: Mini-whip 2558 Clifton whip 2094

Average SNR:  Mini-whip 14.7 dB Clifton whip 13.8 dB

Why such a large difference in the number of spots, for a relatively low difference in average signal-to-noise ratio?  I thought at first that the answer would be that spots were not evenly distributed among bands, and the antennas' performance was better on some bands than others. Back to the data:

                      Spots by Band
Band    with the mini-whip,    with the Clifton whip
80         628                            649                            
40        1254                          1073
30        224                            214
20        440                            252
17        5                                0                   

                Average SNR
Band        mini-whip            Clifton whip
80            20.3                     16.9
40            13.8                     13.8
30            13.0                     13.7
20            10.3                    10.6

So that hypothesis doesn't really work out either.  More questions than answers here, I'm afraid.  The biggest difference in spots was on 20 and 40, yet the average SNR was identical on 40 and only 0.3 dB different on 20, but in the wrong direction. The mini-whip outperformed the Clifton whip on 20 by 188 spots, despite being 0.3 dB inferior in average SNR.

My full-sized Carolina Windom out-performs both smaller antennas by a large margin.  But if you can't manage a large antenna, the mini-whip is an acceptable and very low-cost option.

Comments welcome!

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