Sunday, June 8, 2014

Simple Ham Radio Antennas: The "Down To Earth Antenna." Post #279

Are you looking for an inconspicuous amateur radio antenna that is largely invisible, yet delivers acceptable DX and local results?  An antenna idea described by Robert McGregor (VK3XZ) and published in the May, 1993 edition of "Amateur Radio" (Australian magazine) may suggest a few alternatives for hams facing restrictive HOAs and CC&Rs.

A few weeks ago, Dean Manley (KH6B), gave me a copy of this fascinating article for my antenna reference library.  I've built several disguised antennas, ranging from K3MT's "grasswire" antenna to low-lying dipoles running just above my home garden.  But none of this antennas worked as well as "The Down To Earth Antenna" from Robert McGregor (VK3XZ). Here are some pertinent paragraphs from Robert's article.

DOWN TO EARTH ANTENNA

Australia and the African Desert have a common need for radio communication and a mainly sandy terrain. VK5TL's letter, AR Jan. '92 caused me to dig deep into a pile of ancient documents and in "Instructions to Australian Signallers" there was a piece of information for our guidance.  I quote:

1.  A ground aerial is a length of insulated wire attached to the aerial terminal of a wireless set and laid out along the ground in the direction of the distant station.  Although it lies horizontal, it radiates ground waves; it is thus an exception to the general rule that vertical aerials are used for groundwave work.  A ground aerial gives approximately the same range as a 6 ft. rod (vertical whip?  Dean's, KH6B, note).

2.  A ground aerial is useful in positions where a rod would be dangerously conspicuous. It may be buried to a depth of about an inch, to prevent people tripping over it, without losing much of it efficiency.  Ground aerials are less likely to screening than rods, and may provide the only means of getting communication where the sets are in woods or amongst buildings.  A station using a ground aerial can work satisfactory to a station using a rod.

3.  A ground aerial radiates well in the direction of its length, but very badly at right angles to its length.  Ground aerials are unsuitable for large groups on account of their directive properties.

A counterpoise may sometimes be used with a ground aerial.  It consists of another length of insulated wire connected to the earth terminal of the wireless set and laid out along the ground in the opposite direction to the aerial.  When a counterpoise is used, it should point towards the distant station, and the aerial in the opposite, because the radiation will probably be greater in the direction of the counterpoise than in that of the aerial.  The directivity of a ground aerial may be helpful in attaining a degree of secrecy, since little radiation is sent out at right angles to the aerial.

4.  The best length for a ground aerial is:  Distance in feet=(375/Frequency in MHz) or (114.3/f/MHz) in meters.  These lengths are not critical.  If the aerial is short, it will work almost as well; if it is too long, however, the efficiency will be reduced.  Therefore a ground aerial should always be made shorter than the best length rather than longer, which means it should be cut for the highest frequency that may be used.

5.  Insulation--it is important that the aerial and the counterpoise (when used) should be properly insulated.  The most common fault is failure to seal the end of the wire distant from the set.

I point out that in all sets referred to, the output circuit was designed to feed something less than a quarter wavelength.  An outboard series fixed condenser was normally available where the available antenna was too long.  Frequencies were between 4 and 7 MHz.

Tests over this frequency range showed an average difference of 16dB between a 6 ft rod and one of 18 ft. Beyond this height, a small capacity hat, four radial rod, 2 feet long, gave the best results.

Amateur Radio, May, 1993.

McGregor's idea seemed workable, since my new home sits on a full acre of grass and forest land  in the Puna District.  I decided to build  the ground "aerial" with counterpoise as my new experimental antenna.  In some ways, McGregor's antenna resembles the "grasswire" antenna of K3MT, but without the use of a 9:1 balun.  Using the formula 375/f (MHz)=L (ft) and a design frequency of 7.088 MHZ (the frequency of the Hawaii Afternoon Net), I came up with a length of 52.90 feet/16.12 meters for the ground "aerial" and its matching counterpoise.  I rounded off the dimensions to 53 ft/16.15 meters for each wire.

I attached each wire to a Budwig HI-Que Center Coax Connector, with the ground "aerial" being soldered to the + terminal of the connector and the counterpoise wire being soldered to the - terminal of the connector.  I used some #20 AWG hookup wire I had in the shack for the antenna and counterpoise wires.

I then attached a 50 ft/15.24 meters length of RG-8X with UHF connectors to the Budwig Center Connector.  The Center Connector with its attached wires was placed in a small Rubbermaid plastic container to protect the connector from the weather.  The antenna assembly was placed in the garden, with the counterpoise element pointing NW and the ground "aerial" pointing SE.

The coax was led to the patch panel in the shack window, where a short piece of RG-8X from the inside of the patch panel was connected to my Drake MN-4 antenna transmatch (i.e. tuner).  Short lengths of RG-8X connected the Ten Tec Argosy II, the low pass filter, and the Heathkit Dummy Load to the Drake MN-4.  A counterpoise "bundle" consisting of 1/4 wavelength sections of #20 AWG hookup wire for 40, 20, 15, and 10 meters was attached to the ground lug of the Drake MN-4 antenna transmatch.

INITIAL RESULTS

With the Drake MN-4 in line, I was able to get a 1.1:1 SWR across the entire 40 meter band.  Since I made the antenna after the daily net time had passed, I only was able  to test the antenna with stations not working the net.  Signals within the state of Hawaii were excellent, with CW reports ranging from 579 to 599 and SSB reports varying between 57 and 59.  I was running the Argosy II at around 05 watts QRP.  When I reoriented the antenna so that the ground "aerial" pointed to Australia (SW) and the counterpoise was aimed towards the U.S. mainland (NE), I got some good reports on both CW and SSB.  CW ranged from 549 to 579 and SSB varied between 53 to 55.  Nothing to write home about, but the antenna does work, even with low power.  And best of all, my neighbors can't see it.  Out of sight, out of mind.  Give this antenna a try.  It could stop antenna problems before they begin.

REFERENCES:

Personal conversation with Dean Manley (KH6B), 30 May 2014.

McGregor, Robert (VK3XZ).  "Down to Earth Antenna."  "Amateur Radio", May, 1993.


Enhanced by Zemanta