My version of a 66′ top Off Centre Fed Diople

February 10th, 2012 by g4lds

Firstly the design is found on the web so I cant claim it!!

The design is based on the 1929 aerial by Loren Windom W8GZ. (This design used a single wire resonant feeder). The theory on this aerial, is depending on where you feed to aerial can change the impeadence. (Fed in the centre of a diople is approx 70 ohms in free space and fed at the end is around 2000 ohms). Feeding at approx 14% is around 200-300 ohms. So using a 4:1 or 6:1 current this point can be fed by Coax. This makes the possibility of multi-banding real as at the resonant frequency and at all even haromincs the impedance at the feed point is around 300 ohms, and can be changed to a lower value by the balun. The use of an ATU will allow the aerial to be used on the bands but as a high SWR, there will be increased feedline loss.

Most version's of the Off Centre Fed Diople (OFCD),are for 80m upwards or even the Carolina Windom. My space being limited so went for a 40m - 6m version, 66' top and legs of 22' and 44'. 66' is approx 1/2 wave on 40m when feed at 1/3 way in produced a feed point of around 300 ohms. This is a full  wave on 20m & 2 x full wave on 10m, so around 300 ohms.  Other bands need an ATU.

(There is another design with 69' & feed at 14% from end giving 14' + 55' legs. This may give better multiband match but I havnt tried it as would struggle with 55' leg!!)

Figure 1

Figure 1

Figure 1 show the basic layout I use. A 4:1 current balun is used at the feedpoint rather than a 6:1 balun due to the weight. A 1:1 choke balun follows this at the feedpoint to trap any pick up on the coax feeder & a 1:1 "ugly choke" is also used at the transmitter end to try to block any common mode currents being picked up.

I have built 4:1 current baluns using both a single and two cores, both worked ok under test using 100W of RF.

The details of the 4:1 single core are as follows, figure 2 gives the layout on a FT140-61 core.

The construction of the 4:1 balun is quite simple. I used two windings of approx 200 ohms on a single FT140-61 core. The windings were of enamelled copper wire of around 1mm diameter (0.8 – 1.2mm should be ok),

  • Ensure the wire is straight and “kink free” by putting free end of wire in vice or around door handle and gently pull to remove any kinks but without stretching it!
  • Cut off around 2 meters of wire and fold it in half to make a short length of 100-ohm transmission line.
  • Using finger & thumb, carefully wind around 10-12 turns onto the core.
  • Keep the wires flat don’t let them twist or overlap
  • Wind over about half the core

I find that finding the half waypoint of the wire using a cable tie fix it to the core then wind half the turns one way anchor it using another cable tie then repeat on the other end of the winding. Thus making a 1:1 balun.

Repeat for the other winding.  Then “buzz” through the windings to get the separate turns.

Now comes the wiring up, on one side the two windings are parallel connected to give a impedance of 50hms, the other side are wired up in series to give an impedance of 200ohms.  This is where most problems occur. To check it, use an aerial analyser, connect a 200 ohm resistor at the high Z side and, check it should be around 1:1 from 3 – 60 Mhz. if its more than say 2:1 check the balun windings wiring!

Bare balun

Figure 3 shows the bare balun.

The next step after testing and wiring it ready is to weatherproof the balun and make it sturdy to handle strain. I used an ABS box, with a SO239 coax connector for the 50 ohm feed & two M6 bolts for the aerial connectors. I used part of an ex chopping/cutting board trimmed to fit, as the support for taking the strain of the aerial wire & anchoring. When connected to the wire, I waterproofed the balun and SO239 coax connector with PVC then amalgamating tape.

Balun under constrution

Figure four give the layout.

I cut two lengths of wire 22' & 44' (plus a couple inches to allow the end insulators). Soldered ring terminals on the  wires to allow connection to the balun. I Drilled a couple of holes through the backplate (ex chopping board!)  to thread the wire through to take the strain and I then put modelling clay around two wire connectors! The balun was fitted to the backplate via a couple of screws.

Below is the aerial in place, right under the eaves of and next to the house and above the telephone lines. Not ideal!!

The aerial in situ.

The set up is now a 1:1 balun at the aerial feed point consisting of around 10-12 turns of RG58 coax around 12’’ diameter then at the ATU/Tx, I made a 1:1 ugly current balun of a 40mm pipe with 30 turns of RG-58.

I found that this aerial gave good matching on 14/28/50 Mhz & acceptable on 40/10/18/21/24 with an ATU, as can be seen by the frequency sweep charts at the end of this article, including SWR results on each band.

2 Core 4:1 current balun

I also made a backup balun using 2 x  FT114-43 cores wound with 14 turns of 22Swg enamelled wire as per above notes. Each core was mounted above each with a piece of paper between them using a couple of cable ties. (I used different colour cable ties to show the start/finish of the winding) The wiring was as per figure 2 above.

The two cores ready for testing & wiring. Each start and end on the winding is indicated by different colour cable tie. There are around 14 turns on each FT114-43 core. The two cores were mounted above each and another cable tie used to lock them as one

The  4:1 current 2 core balun tested & wired ready for mounting.

Both the single core & two core  gave a flat response of 1:1 from 2 – 60 Mhz when loaded with a 200 ohm load on my MFj-259

The graph at the end of this article shows the freq sweep from 2 to 54 Mhz using my MFJ-259. I also noted on each amateur band from 40m to 6m, using both the MFJ-259 then using low power, my RF meter. Note: RF = my SWR reading at the TX,MFJ = SWR reading on my MFJ-259

Further infomation & references this article is based on.

66’ OCFD:

Balun kit: Manual - final.pdf

OCFD design:

1:1 ugly balun

The above graph shows the frequency sweep of my aerial set up at the Transmitter.

7 Mhz

10 Mhz

14 Mhz

18 Mhz

21 Mhz

24 Mhz

28 Mhz

50 Mhz

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