Ground Probe Antenna

Ground probe antenna which some call them Earth Probe Antenna.  It consists of a long wire terminated with ground rods at each end.  Looks similar to a Beverage antenna which is a very long wire terminated at the end.  The wire being only a few feet off of the ground.  These are low noise antennas directional off the end.  Best used for DX on 160 meters as their directivity and pickup of local noise is much less than an Inverted-L for example. 

The Ground probe antenna is used for reception of ELF and VLF signals. This post will describe the one I put out on my property in the suburbs.  My back yard is around 175 feet wide and I was able to run a wire out 158 feet (41 meters).  For now the wire is just laying on the ground.  I drove 8 foot ground rods at each end.  I connect to the antenna approximately in the middle.  I am not sure how to feed this antenna so I did so in the middle as that was a convenient location.  Some on suggested the impedance magnitude was around 1000 ohms.  With further measurements I think it is more like 500 ohms. 

I have in my junk box a ferrite core of unknown properties.  I measured the AL value at 6490 mH / 1000 turns.  The material appears to be "P" or "F" ferrite used in switching power supplies running under 1.5 MHz. I am feeding the antenna with RG-6 cable.  I used 50 turns on the antenna and 14 turns connected to the RG-6.  I am going to change that when I get a chance assuming a 500 ohm Zmag rather than 1000 ohms.

I drove the ground rods below grade with a 12" long section of 4" PVC at the top with a cap.  The cap is flush with the ground so I can run the mower over the top.  I can still get to the ground rod connection by removing the cap.  I did the same for the coax connection in the center.  The transformer sits inside of the 4" PVC. 

 

First tests

It was 20:00 EST on February 3^rd when I connected the HP3586C up to the antenna.   Wide band noise was -8 DBm with the broadcast stations.  Cutting out the big three (WHIO, WING, WONE) with my trap filter the broadband noise was -21 DBm.  Noise with 3100 hz bandwidth on 630 meters sits about -86 DBm, except…… well I will get into that shortly.

Daytime levels for WHIO are -13 DBm and -10DBm for WING.  They are +10 DBm on the 160m inverted-L.

First to test was the broadcast band.  The antenna did very well from 550 to 1500 KHz.  Picking up many stations at night.  Better reception on the typical ones such as KDKA, WBZ,  WHAS, etc.  I did not attempt to dig down for the weaker stations.  I tried FT8 on 160 meters and that was a failure.  Only decoded three stations.  

Next test was WSPR on 630 meters.  A while back I ran this test on the K9AY loop before I added the 160 meter high pass filter to cut out everything below 1.7 MHz.  That did very well.  So how would this antenna compare?  Well…. I ran into a brick wall.  Or I should say, a noise wall.  Before I said the background level was -86 DBm on the 86C.  That was after midnight.  At 8pm the level was more like -66 DBm.  It started around 400 KHz and ended below 550 KHz.  I tried to determine a signature of the noise but could not find any frequency peaks or repeating pattern. It has a low frequency buzz similar to 120 Hz.  I could not hear anything through that junk.  To my surprise it abruptly ended around 23:15.  Someone shut something off.  It is not present during the day.  I am leaving the receiver on to try to determine the hours of operation of the QRM.  When it is off I hear 630 meter WSPR very well.  Image of map attached.  On this map notice a K9 stations in Hawaii.  At first I thought that was bogus but looking at QRZ he does live there.  There was another “W4” stations in the southern pacific.  If that guy is really there he has probably drown by now.  His QTH is listed in Tennessee.  As far as I know Tennessee has not annexed that part of the Pacific.  I assume he has his grid square setting messed up.  There are three stations on this map that did not appear on K9AY test.  A station in western Canada, the K9 in Hawaii, and a station in California. 

Next test was reception of Non-Directional Beacons (NDB) for aircraft.  List of the stations attached.  A couple of stations to note.  DDP is a 1KW beacon in San Juan, PR.  I am not surprised at that one.  The one I question is BBD in Brady, Texas.  According to the database that is MH class station. Those have a maximum power of 50 watts.  The database shows it as 25 watts which is typical of the class “compass locator”.  It was very weak but it was there and the frequency agrees with the database.  As you can see from my list all the other 25 watt “compass locators” were more local.  Last night I picked up three new ones.  One being QT which is a 1KW beacon on 332 KHz out of Thunder Bay, Ontario.  So far I have logged 29 NDBs.  I also received numerous of the Differential GPS (DGPS) stations.  I found a list of them all but did not make any attempt to log or identify those.

Of course the strong NAA out of Cutler, Mane, on 24 KHz was present.  I also could detect two other near 24 KHz. 

I have not heard anything on the Amateur 137 KHz allocation. 

Last night I could hear lightning static crashes.  Looking on a live lightning map the nearest thunderstorms were in the Atlantic near Bermuda.  The map indicated it was running a 4 second delay.  When I heard strong crashes four seconds later I could see a dot light up on the map.   So I got lightning DX into the middle of the Atlantic.  

As for that noise in the 630 meter band I plan on winding a ferrite loop stick antenna and see if I can track it down.  I fear it is neighbor's TV set.  At least they shut it off when they go to bed.   Maybe I will have more information in the future to post in this blog.  

Ground Rod below grade in 12" PVC pipe so I can get to the connections

Cap covers rod and transformer connections

Transformer for feed point to RG-6 coax

630 Meter WSPR station I heard the night of February 4, 2019