[Lowfer] My e-probe attempt..... ( & MP audible on 137 this evening)

[Michael Sapp]

Hi All: First a quick note that MP was audible tonite (7-6-13 0000Z)  on both my NE EWE antenna and also on an e-probe


I've been trying to finish an e-probe since early June, too many other things to attend to.  I decided to try W1VD's modified Amrad design with parallel J310s.  I liked the isolation transformer on the DC injector and the  38 turn T75 bifilar toroid, as my impression was it would have good common mode rejection---that turned out to be the case.  Whipped up some basic artwork for a PCB and made some boards with Press & Peel media & laser printer.  Press & Peel works OK with a laser printer and the XYL's iron. My Harbor Freight IR temp gun helped determine the needed heat range....

http://i1296.photobucket.com/albums/ag19/wa3tts/IMG_2985_zps056a9847.jpg

Bifilar toroid a bit larger than necessary, but that was the T-75 core I had on hand.  I was going to use that second 10T bifilar toroid in the DC injector, but went with a BN-73-202 core and wound that for 50 to 75 ohm vs 1:1 as I had excess RG6 to use as a feed line.  Initial test was on my HP-141T and signals from DC to 20 MHz or so came up 30dB or more when clipping a test lead to the input.  I matched up a pair of J310s by measuring reverse junction resistances from a dozen fets purchased online.  Mostly junk box parts, a trip to Radio Shack for 10M-ohm resistors. Rad Shack had the 100K trimmer so I used that and a smaller limiting resistor versus the 50K trimmer and 47K limiter on Jay's web site schematic.  Looked at the data sheet and was trying to decide if I should run 10ma per device or 20ma.  I decided to set the bias closer to 20ma per device (40ma total) to be well into Class A service.  I have a 50KW 1020kHz station 4.5 miles to the east, which was the motivation for using parallel fets and higher idle current.  I thought about a whip but decided to try a 12 inch blade antenna first. It's a piece of K&S brass stock 12 inches long and 1 inch wide. Since I have 10W ERP beacons running 24/7 on my roof top for 144 and 432 MHz, part of the motivation for a shorter blade antenna was to not have any coincidental resonance issues at VHF.

I took the e-probe to my back yard where the local noise envelope is much reduced and it seemed to do well only a few feet off the ground. All the local NDBs were audible. It was Field Day so I had plenty of HF signals to listen to and compare.  At one point I connected the feedline outer jacket to my EWE ground system and no change in signal levels or background noise was apparent by ear.  I tried 3 or 4 places closer to my house and the noise increased some (as expected), but no where near as bad as how my ICF-2001 reacts (which I refer to as my common mode test receiver)....

Keep in mind my primary intention for this antenna was for feeding several boat anchor receivers in my basement game room versus for high-performance DXing.  I chose a location only about 20ft away from my house and installed the e-probe up about 15 feet on a TV mast and bamboo pole....

http://i1296.photobucket.com/albums/ag19/wa3tts/epant_zps624cf357.jpg


The roof in the photo is my neighbor's house, photo taken from my front porch level looking west...so I have 40~50ft of RG6 back to the boat anchor radios in the basement.  Output level from the e-probe is high enough that I have to use 10 dB attenuator on a few radios that have RF pre-amp front ends (Hallicrafters S-51, S-22, Drake SW4A, etc.).  The extra gain really brings my S19R to life with just a mixer-osc first stage....

I was concerned about transmit power levels driving the J310s too high but that was not the case. E-probe is about 100ft from my MFJ-1793 40/80/160m GP vertical monopole.  With my Boonton CA-91 RF Voltmeter on the e-probe output, 10W output on 40 meters into the GP vertical yielded 90,000uV. Going to 50W tx on 40m yielded 180,000uV on the e-probe.  100W on 40m tx to the GP vertical yielded 250,000uV on the e-probe.  In dBm those numbers are -8, -2, and +1, so the 10 dB linear range was maintained and the J310s not overdriven. The protection diodes are not even near clipping potential.

Mid day total RF output on the e-probe from all on air signals was in the 48,000 to 54,000 uV range. At night, with the AM stations reducing power, the e-probe shows 40,000 to 46,000uV on the Boonton RF voltmeter.   At night, I was hearing DDP in Puerto Rico well on my S-51 and SW4A as well as CLB and others. Works better than I would expect for being close to my house and neighbor's EMI fields....

Ran some RG-58 double shield over to my shack and copied MP on 137.78 kHz.  Here is a comparison on ARGO, darker background is my NE EWE. e-probe shows several dB more peak noise.  Hears better than I expected given it is inside the local noise envelope....

http://i1296.photobucket.com/albums/ag19/wa3tts/eprb1_zps8efd722e.jpg

I had to pad down the e-probe 10dB to drive my modified HD-1420 VLF converter. With the 10dB pad in line on the e-probe, MP's signal level was essentially identical to the EWE antenna.  Now I can hear NDBs like CLB in Carolina Beach and RYS in Detroit in mid day with my EWE. Those signals are not very strong but they are audible and copyable by earon teh EWE---perhaps 6dB above the noise. Where the e-probe is located, I am not hearing CLB or RYS in daylight groundwave, but I did not really expect to.  I went down to 24Khz and heard NAA just fine as well as WWVB and others, which was interesting.  I do have some loud signals at 19 kHz that appear to be EMI.   All & all a worthwhile project.

One other note, I use the doublet input on my boat anchor receivers and a 10:1 matching transformer to get into the 500 ohm range. The old tube radios love to be voltage driven and are nice and quiet on the balanced antenna input terminals....

http://i1296.photobucket.com/albums/ag19/wa3tts/IMG_3023_zps338d9851.jpg

Thanks for the bandwidth. 73,  Mike, wa3tts