NJDTechnologies

Radio: it's not just a hobby, it's a way of life

Current Operating Frequency and Mode

CQ 474.5 kHz CW

The E-Probe: a compact yet very powerful receive antenna

– Posted in: 630 Meter Instructional Topics, 630 Meters

For several years I have listened to people comment about a variety of flavors of E-Probe antenna.  Some complain about it being noisy while others will try to convince you that its like having a beverage antenna.  Given that this is a 1-foot tall receive antenna used at medium wave and long wave frequencies any claims about its functionality seem far fetched…  At least on the surface!

Since there was such a large range of opinions about how the E-probe performed, I really didn’t know what to expect so I had to build one and see for myself.  I knew before I started that living in a suburban environment, the cards were probably stacked against me.  The good news is that I had all of the parts necessary to complete the project.

PA0RDT

I’ve never been a  big fan of using a Bias-T to send power down  coax to power a piece of remote electronics.  I don’t have a high level of risk tolerance for some things and that happens to be one of them.  I also worry that poor decoupling will result in unintended signal ingress and common mode issues.  That’s a problem easily solved, however.  I can use a 1:1 isolation transformer near the antenna feedpoint.  Larry, W7IUV / WH2XGP, recommends  five turns for both the primary and secondary, wound on a type 73, 75, or 77 binocular core.  The transformer can be located at ground level unless power is being sent down the coax.  In that case the transformer will have to be added to the inside of the probe such that power can be taken from the coax.  Seems like a lot of work so I decided to use a separate line for power.  I have hundreds if not thousands of feet of Bell line so it wasn’t a big deal.  That means that the schematic will need to change to break the connection between the 470 uH inductor and the main line to the coax connector.

I used a knife to section a small piece of copper clad board into a main section, a power rail and a ground rail.  Since this project was using Manhattan-style construction, I decided to use make life more simple and use “MESquares” from QRPme.  I used one square at each major intersection point, where leads would otherwise be floating in the air.  This approach made for a very clean look for the project.

S2120002

Components installation was very easy but I will give this warning:  Be sure to check the data sheet for the active components.  As it turns out, the pin-out for the J310 is really dependent on the manufacturer.  I had encountered this in the past but it had been so long that I forgot.  My first J310 did not make it as a result and had to be replaced.  Using the solder pads made this an easy process.

S2120005

Power connections are on the side rails and the center section of the board is at Vcc as well since the 2n5109 is thermally bonded to the  board and the case is at Vcc.  I was not particularly careful with the selection of component tolerances.  All of these components came from my junk box.  The signal output pad is in the left-center edge while the antenna connection is at on the right- center edge.

Unfortunately I did not get any pictures of the antenna.  I had sealed the housing by the time I made this realization.  the antenna consists of about 6 inches of wire connected to the antenna pad with a piece of double sided copper clad board, about the same size as the main board, connected to the opposite end of the wire.  Both sides of the board are electrically bonded.  I could have used a whip or only a piece of wire – there are a number of different varieties and approaches to E-Probe antennas.  One document suggests that longer antenna elements should be avoided if located in densely populated areas with broadcast stations nearby.  Longer elements will result in more signal which might result in unwanted overload and mixing within the circuit which can lead to broadcast interference.  I live in media market number five with a 100% frequency utilization by broadcast stations.  Shorter is likely better here.

The probe is housed in a PVC tube so end caps were fitted for a BNC connector and a hole for power leads.

S2120006

The weight of the board is very small so it simply floats within the tube on the stiff wire connecting the BNC connector to the RF input pad.  The antenna sits accordingly above the main board.

S2120008

A hook was added to the top cap because I wanted to use a pecan tree to support the antenna rather than a pole since I have located this antenna in my front yard.  The top cap is permanently connected to the tube using PVC glue.  The lower cap is sealed through the liberal use of DAP.  All of the inputs and outputs through the PVC are sealed with DAP to ensure they are water tight.

S2120009

Coax is connected to the BNC connector and vinyl tape provides water seal.  Very small rope, like that used for guying my transmit vertical, is used to ensure strain relief between the coax and the probe body.  The power leads are spliced  to the Bell wire that comes to the ground for connection to the battery.  A fully-charged, 12V, 77Ah sealed lead acid battery will provide two months of power to the probe, which is running as 12V at 50 mA.  Using battery power ensures that power supply noise is not coupled to the system.  Thanks to Larry McFarland, KY5S, for saving a few surplus batteries that were otherwise destined for the recycling yard.  The probe is pulled up about 20 feet into the tree and the coax hangs free to ground level but there is probably no issue with using the tree to support the feed line and power lead.

As stated earlier, the coax is decoupled from the probe at ground level using a 1:1 isolation transformer.  If you build a transformer yourself, be sure to either use a non-conductive housing OR ground-isolated connectors.  I’ve seem a lot of very smart guys build an isolation transformer that wasn’t isolating anything because the case was a common ground.

 This probe works from 10 kHz to 30 MHz with only a few db of variation over the entire range.  Plenty of grabbers and even stations on the Reverse Beacon Network use them for listening over a large range of spectrum.

So how did it work?  Its been in use one night so far and what I can report is that the location it is installed is quiet.  It is recommended that the location be 30-60 feet from a house or other sources of electrical noise.  My probe is located in a pecan tree near the street and about as far away from any persistent electrical offenders that I can achieve and still be on my property.  The device did not burn up when I was transmitted and reports from others indicate that this should not be a problem.  I will probably power the unit down during electrical storms, however.

The noise floor on the probe is quite remarkable.  Its the quietest antenna that I have and signals seem to stand out of the noise several S-units while using NDB’s for ground wave evaluation.  The probe seem to do well with skywave signals also, hearing stations in Pennsylvania, Washington, Arkansas, and Arizona which represents almost all of the stations that were on the air during the first night of testing.

Another notable observation is that each of the receive only antennas at this location (beverage, loop, and probe) has its own unique sensitivity to a particular incoming wave angle.  Of course this should be obvious and is consistent with behaviors seen on other bands but I can report that the probe exhibited sensitivity to signals at times when the other antennas heard nothing.  And of course there were times when the other antennas heard signals not heard by the probe.

The value of remote battery power cannot be overstated!  Having performed a test using power from a supply and long power lead, the battery was the hands down winner.

As my goal this summer is to improve my listening ability in preparation for new, weak signals showing up on the band as well as improving the quality of QSO’s with existing operators, I can definitely report that the E-Probe will remain in the antenna arsenal at KB5NJD / WG2XIQ.