My Favorite 40m "DX" Antenna

"The 40m Delta Mono-Loop"

Of course I would rather have a full size, 3 element Yagi at 110 ft., but the reality is, I'll never be able to afford that antenna, or the QTH where I could install it.

 

With age I have learned to couple my dreams with realities.  I no longer shoot for the stars.  I prefer to shoot for targets I can easily hit.

 

Most people who want to have a good DX signal on 40m but can't afford a beam, turn to a quarter-wave vertical.  Though I often do that too, whenever I can, I put up something better; a 40m delta mono-loop.

 

This antenna is a full wavelength delta, with the point of the triangle at the top of the single support pole, and fed near one of the corners - precisely one quarter wavelength down the diagonal leg. 

 

Feeding and matching is simple;  it's fed with a quarter-wavelength of RG-11 or RG-59 (75 Ohm) coax, and then any length of 50 Ohm coax.  No balun is necessary, though an RF-choke is advantageous.

 

Advantages over a full size quarterwave vertical with a good radial network:

  • 2-dimensional (will fit in a long, narrow space)
  • About 1 dB gain, but that's not the real advantage
  • Quieter on receive
  • More broadbanded resonance than a dipole or vertical
  • Yet like a Vertical, pretty much Omni-Directional (due to its low height)

 

Advantages over a horizontal dipole (at 11m height):

  • Lower angle of radiation
  • Stronger signal for DX contacts (usually +2 S-Units)

 

Disadvantages:

  • More wire hanging in the air
  • More conspicuous to the neighbors
  • Not as good as a low dipole for local (NVIS) QSOs

 

I first became aware of this antenna from an article written by DL1BU (SK) in CQDL magazine, April issue, 1979, page 154.  I built the antenna immediately and fell in love with it.  It is low cost, easy to build and easy to bring into resonance.

 

I have used this antenna many times in contests and on expeditions, and occasionally I even built one for 80m. I was always pleased with its performance. Unfortunately I do not have any pictures, so you will just have to look at my drawings.  Why no pictures? Well if you get far enough away from it to get the whole antenna in the camera's view finder, you can't see the wire.

 

 

 

CONSTRUCTION DETAILS:

 

• The pole should be about 12m high (minimum 11m).  (39’4” to 36’).  Higher is better, but then you will have to re-adjust the total length for resonance.
• The feedpoint is located in either diagonal side near one corner of the antenna, enabling vertical polarization.  This makes the antenna an excellent DX antenna.
• The length of the diagonal is not very critical and may be adjusted to help find a better fit in the space available, but the distance from the feedpoint to the top should be one quarter wavelength.
• The exact total length will vary depending on ground conditions at your QTH.  Begin with 42.7m (137’ 10”) and then shorten the horizontal leg to bring the resonance up to the desired frequency. 
Adjust total length by adjusting the length of the horizontal wire.  (Easiest way).  
• The horizontal leg of the antenna on the bottom should be 2 to 3m high (6’6” to 9’10”) high enough for humans and animals to walk under. Changes to the height will require adjusting overall length
• The insulator shown directly on the pole at the 2m level is for mechanical reasons.  Secure the insulator to the pole, and then pass the horizontal leg through the insulator, reducing sag in the horizontal leg.
• The insulator in the horizontal leg near the left is an option for convenience.  It enables easy adjustment for resonance by removing or adding wire.  For disassembly, disconnect one side from the insulator and then roll the antenna as a single wire.  Each time I changed my QTH, I had to re-adjust the length of the jumper.  I just let the jumper wire hang down.  For permanent use, you may leave this out.
• The antenna will have an impedance between 90Ω and 100Ω.  A quarter wavelength matching stub of 75 Ohm coax will provide a good match to 50Ω.  RG-59 is good enough for about 500w.  If you want to run more power, use RG-11.
 

TUNING THE ANTENNA:

 

After installing the antenna, measure the SWR to find the resonance of the antenna and note it.  The antenna is probably too long.   Adjust length as necessary. 

 

MAKE ALL LENGTH ADJUSTMENTS IN THE HORIZONTAL LEG OF THE ANTENNA.

The antenna is not symmetrical and there is no advantage to trying to shorten or lengthen both diagonal legs.  Make the adjustments at the insulator.  If you cut the antenna too short, you can just add a loop of wire  [JUMPER]  and leave it hanging from the insulator. That is usually enough to bring it into resonance on the desired frequency.  For home use, the JUMPER is not necessary, but if you are using this portable, you will be adjusting the length a lot and this makes it easy.

 

80 VERSION:

 

Overall length:      85m

Diagonal length:    25m

Feedpoint:              3m from one of the corners

Horizontal length:   35m

Height at top:         22m

Height above Gnd:   4m

Length of Stub:     13.7m

 

But, 80m is a big wide band.

Here are some recommendations / estimates of overall length depending on resonant frequency:

  • 3520 kHz   ca. 85m
  • 3700 kHz   ca. 81m  (diagonals: -1m ; horizontal: -2m)
  • 3800 kHz   ca. 79m  (diagonals: -1.5m ; horizontal: -3m)

I have built the 3520 kHz version twice.  I've never built one for higher frequencies within the 80m band. My recommendations for shortening for higher frequencies is a gut feeling, but I don't think "where" you shorten is as critical as just getting the correct length, which has to be found by trial and error anyway.

 

NOTE:  The feedpoint is always One Quarter Wavelength down the diagonal side.

 

 

DOWNLOAD INSTRUCTIONS:

Instructions for building a 40m Mono-Delta-Loop
40m Delta-Mono-Loop.pdf
PDF-Dokument [389.3 KB]