Wednesday, June 29, 2011

40m 4 Square antenna


40m 4 Square antenna


This was one of the 2009 projects and the first to be functional , it has ground mounted radials and elements (with the attendant risk of our Goats eating it!).

This type of antenna is very simple in concept but involves a lot of work constructing the cables, radials, elements and measuring site layout- as everything is mutiplied by four! There are 1200m of radials using WD1/TT D10 copper and steel (anti Goat chewing) field telephone wire.

Construction history at bottom of page.

Antenna photos:

Direction NE JA, Asia (complete with radial eating Goat.) Direction NW (North America)



Direction SW (South America) Direction SE (Africa)



Simulations

they are similar to the classic Comtek plots, the differences are probably due to the

actual simulated ground used and the element sizes and wire diameter .



This antenna works well as a receive antenna on 30m, i've used it to work several dxpeditions already who I could not hear on my TX antenna due to Eu QRM.



Construction Details.

The antenna elements are 18 AWG PVC insulated wire suported on 4* 10m fibreglass fishing poles, each pole is mounted at about 0.5 m above ground using a metal standoff and an exhaust clamp at the top of the posts which are wooden 7x7cm fence posts.This easily allows 9.8 m radiating elements which resonant at 7.0 MHz individually. The post are screwed into 'Met post' sockets which allows easy installation and adjusment of the verticality. The fishing pole supports are kept in the extended position by self amalgamating tape over each section joint, then a protective layer of pvc tape. The element wire is on the outside of the pole and is taped to the pole either side of each junction to help prevent each section telescoping down.



The SO239 feedpoint is riveted to a small aluminium angle section which also has the Stainless radial connecting screws and directly bolts to the top of the 4ft copper ground rod at each antenna. The ground mounted radials are copper and steel wire ex field telephone drop cable. I have decided on 30 radials of 10.15m long per antenna according to K3LC radial optimisation paper ' Maximum-Gain Radial Ground Systems for Vertical Antennas' of the March 2004 NCJ. Element spacing is 10.56 m along the sides of the square and phasing lines are 75 ohm WF100, designed for 7.1 MHz centre when operational, of length 8.45m. Only a few test radials are fitted in the picture. The base has a plastic cover (removed for the photo) which keeps the rain and the goats off the feed point.




The controller is a Comtek COM-ACB-40-2 from DX Engineering, the phasing box is mounted on a 6ft pipe with 4 foot in the ground as an earth post and normally resides under a protective large plastic dustbin along with the 100W dummy load AEA DL-1500. All cables exit under ground including the phasing lines to the antennas, LDF4-50 to the exciter and the 4 core dc control cable (only 3 used). The dc control cable has an inline XLR connector on the Orange cable to facilitate testing and removal of the unit, as Comtek don't allow for this.

The feeder cable to the 4 square from the shack is 40m of LDF4-50A to a remote Ameritron 5 way antenna antenna switch, then a further 30m of LDF4-50A to the 4 square centre, the total path loss should be less than 1 dB.

The 4* 75 ohm 1/4 wave phasing lines are of 7mm Sat TV coax, with copper braid and tape screen and foam dielectric, 80% VF. This cable is very cheap, handles 1kW and is low loss (0.2 dB/10m at 10 MHz) and takes smaller cheaper ferrites to make the braid choke when needed (in elevated ground plane designs). All antenna connections are SO239/PL259 using soldered types and N-types on the LDF4-50.

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