[Lowfer] W1VD my attempt at an e-probe

[Michael Sapp]

Jay & All:  

>Wonder if you could take a signal strength measurement (output from the e probe ... >preferably in dBm) at 1020 and 2040? I'm interested in both the signal level at 1020 and how far >down the IP2 product is. At this location the strongest signal is a 50kW 1080 about 11 miles away. The signal at 1080 is -50 dBm and the 2160 is -140 dBm (limit of Perseus SDR >sensitivity).

>Thanks in advance for your time on this.

>Jay W1VD  WD2XNS  WE2XGR/2

    OK. My HP141T has the 8555 RF Deck 10MHz~18GHZ head, so it's not real useful at 1 ~ 2 MHz.

     What I did was use my FT-847, preamp off, AGC fast, 300Hz InRad CW filter.  Fifty feet of of double-shielded RG-58 from the e-probe injector to my Telonic 0 to 50 dB step attenuator, 10db per step. A double shielded rg-58 jumper to the FT-847 with ten Type31 beads on the jumper for common mode isolation.  Handful of 10, 6, 3, and 1 dB stick attenuators. Zero beat the AM carriers on 1020 and 2040 kHz to be certain the BFO offset was at 650.0Hz with ARGO....

The Technical Supplement for the FT-847 states that receiver sensitivity is 20uV for 500 kHz to 1.8 MHz and 2uV for 1.8 to 28MHz for AM, so it essentially has a 20 dB attenuator in-line in the AM band. Taking that into consideration, the e-probe output at 1020KHz just pins the S-meter scale at +60.  I need 55 ~ 56 dB of step attenuation to get to S7 range with one S-meter display segment of modulation wiggle.  Now when I go to 2040 kHz, the signal is also S7 range with one S-meter display segment of modulation wiggle.  I re-tested with an additional 10dB attenuator in line and the relative S-meter measurement differences repeated at 55 ~ 56 dB down. 

Keeping in mind the FT-847 adds 20dB of attenuation in the AM band, I would have to say the 2040 kHz signal is 75 ~ 76 dB down. 

Voltage at the battery powering the two J310s in the e-probe 12.5VDC and 37.6 ma total current draw. The J310s run just above body temperature at that bias setting, which avoids having to heat sink them.....(I decided that would be a "safe" starting point for initial e-probe testing).

Using my old Wavetek 132 as a signal source, 6000uV (-31.5 dBm) pins the S-meter on the FT-847 in the AM mode at 1020 kHz with the preamp off. 5,500uV just comes off the top FS level.  I have to run 15~16 dB of attenuation between the e-probe output and my FT-847 to be just at full scale S-meter level for the on-air 1020 kHz signal in the AM mode.

Unfortuneately, I dont have a narrowband generator.  I have to rely on my attenuators for referencing the output of my (checked) HP-435K-05 dual power 0dBm 50MHz power reference source to calibrate the Boonton CA-91 RF voltmeter.  That said, I could be a dB or two off, but close enough for amateur radio purposes......

Powering the e-probe with a 24V battery pair and (a well bypassed) LM317 adjustable regulator would likely be worthwhile to take a few 2IP measurements at different voltages at some point.  But I think the next few nights I will just sip some coffee and spin the knobs on my old boat anchor receivers now that I have something better than the (noisy) rain gutter and downspout on my front porch as an rx antenna......:)

Regards,  Mike wa3tts


 Mike,  wa3tts