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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Aerials – Some thoughts and ideas

June 27th, 2009 by g4lds
Having recently moved to Morecambe (Good Move!!), I had my dreams of a 80 foot tower with a rotary HF beam popped as the garden is only 25 x 35 feet!!! MMM Back to plan B.  Which is wire aerials that i have used to good effect in the past.
Doublet for 80 - 10 fed by balanced feeder

Doublet for 80 - 10 fed by balanced feeder

The doublet works I found on all bands.  A dipole is really a one band aerial, cut for 1/2 λ on the band you want to work on and fed by 50 Ω coax (really it should 75 Ω as that's the correct value of a 1/2 wave dipole but rigs are 50 Ω!!) A doublet is a balanced aerial around 1/2 λ on the lowest frequency you wish to work on however in practice, it can be 85% of this value with minimum effect of efficiency.  I find that a top of 84 feet (ie 42 x 2) is good for 80m upwards.  I fed this by 300 Ω slotted ribbon cable.  It can be feed by open wire feeder between 300 - 600  Ω shop brought or homemade.It should be fed into a balanced Aerial Matching Unit (AMU or called ATU) or to a 4:1 or 6:1 Balun to coax. I have used this both Horizontal and Inverted Vee.  During a contest on 40 meters I managed to work Stateside using this at 15 feet!
Another aerial which I have used is the W3EDP. This appears to be in use from around the 1930s.  Check on the net for its history. This can be laid out in really any layout with bends, kinks and it still works!  It rocks on 5 Mhz as using 5 watts I have covered round the UK no problem.  It again should be fed to a 4:1 balun before the AMU or a balance Z-Type AMU. Very useful for portable use, laying along a hedge, thrown up a tree.
This should be feed via a balanced AMU like a Zee match or via a 4:1 balun to an unblanced AMU. If using a balun, the long length goes to the "hot" terminal and the short to the "earthy" terminal. The notes indicate that on 3.5 Mhz, the counterpoise does not need to be connected(it acts as a endfed around 19 feet longer than a 1/4 λ), 7Mhz its a halfwave with eightth wave feeder, 14Mhz its a fullwave endfed beyond the end of the counterpoise 17', 21Mhz its one and a half wavelengths with the 17' counterpoce acts as twin feeder! 28Mhz it does neeed the counterpoise as it two and half wavelenghts long (85')
Now having used these aerials at my new QTH, I found that the local noise was producing a S8 level on HF so operating was not very successful!.  My thoughts turned to going portable in the wonderful countryside and sea front around Morecambe.
Non counterpoise aerial (See table for variants)

Non counterpoise aerial (See table for variants)

I had read early about the non-counterpoise aerial on the G-QRP forum. A couple of years ago, you could buy a version of this aerial from Stateside via E-Bay. There has been lot of thought of what this aerial is, I think it looks and works like a "J" pole, or a Zepp or even an Off Centre Diople.  A lot of work has been carried out by Pete M3KXZ (see and also look on the wonderful Cebik site
Table for different coverage

Table for different coverage

After reading up the published info and mailing Pete, i thought that this might be worth a try.  I decided to build a 20-6 metre version. I cut a length of speaker cable (or called low voltage twin cable) 12.5 feet long, and a length of 24/0.2 connecting wire 12.5 feet long.  I soldered this single wire on the end of one of the twin cables, making note that this would be the "hot" or radiating element.  I then used insulating tape to cover the joint and to "weatherproof" the cut ends. I then wound 10 turns on a ferrite core to act as a 1:1 balun or RF insulator at the "feed point" of the aerial. I then used insulating tape on the soldered joints and to avoid the winding moving, I used Tye-wraps.

1:1 balun or feedpoint for my portable non-counterpoise

1:1 balun or feedpoint for my portable non-counterpoise

Having made this I connected around 10 feet of twin cable to act as ribbon feeder to an AMU which had a 4:1 balun built in. Having used my aerial analyser, checked that it could be resonated on 20 - 10 meters using the AMU, noting the settings.  I then connected the FT-817 to see how it worked and found the noise level had dropped to S1  and signals were heard!  A rush into the shack and the noise level was still S8 so the non-counterpoise was connected via a coax lead to the shack, and switching between the two aerials, the noise was there or not depending on the selected aerial!!!  I moved the non-counterpoise aerial near to the house and the noise level shot up. So a quick re-think took place and I made another version for the base station, using a  4:1 balun at the feed point so I could use a direct 50 Ω coax feeder to the shack.

4:1 balun eed point of my base version of the non-counterpoise

4:1 balun feed point of my base version of the non-counterpoise

My base version is set up with the feed point on the fence and sloping towards the house, which blends in with the rest of the domestic site. Currently  the noise level is around S1-2, I am able to work on 20m upwards.  During recent sporadic E openings on 10 meters, I worked from North to South Europe. Later I tried the portable version on the stone jetty here in Morecambe, wound helically around a 6m pole, I heard around Europe and Stateside on 20m. (would have tried longer and maybe tried working but the batteries went on the '817!

My next plan is to make a 50 foot version for operating on 40 meters upwards but firstly I need to find out the local noise one source is from the street lights and I hope to get the local authority to find the cause.  the other local

My portable version wound on a kite winder
My portable version wound on a kite winder

source I hope OfCom will help in deaing with.The next version, I may make from 300 Ω ribbon cable as it may be more weatherproof!

UPDATE (28 July 2009)

Have re-read the notes and replaced the 4:1 balun with a 1:1 choke ( 8 - 10 turns on a ferrite bead) and will try this out now. First tests make it seem a bit "more lively!". When can afford it! will replace this with 450Ω ribbon as it should decrease the losses

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a short 40m dipole

March 30th, 2009 by g4lds


The GM4JMU Shortened 7Mhz Dipole

When looking around for a 40m diople, found this and built it I can say it works even if short!  I got a good match with my MFJ analyser and on QRP, worked around Europe ok.  I also heard VK/ZL!  And only at 30 feet as an inv Vee so if stuck for space try this.
To construct each side of the antenna proceed as follows. Cut a 10.25 metre length of 24/0.076 insulated wire, and a 160 mm length of 40 mm o.d plastic tubing ( white plumbers tubing). Measure a 2.75 portion of the wire and attach the wire to the plastic former. Wind 40 turns of the wire onto the plastic former, and firmly secure the end of the winding. Make the other half of the antenna in the same way. Attach the ends of the 2.57 metre sections to a suitable centre insulator, which should also mount the choke balun, connect the 50 ohm coax, then carefully waterproof the whole assembly.
The choke balun uses RG174AU coax and a 40mm Ferrite Toriod.
Once the antenna is erected adjust it to resonance on 7.030Mhz (or wherever your fav 40m freg is!) by folding back the ends, and adjust the length to provide minimum SWR.

This could be made multiband by replacing the feed point with a 4:1 balun and feed with 50 Ω co-ax or directly with 300 Ω (or 450Ω) ribbon feeder to a balanced AMU or 4:1 then AMU. (update 8-09-10)
Article from SPRAT Issue 74 Spring 1993


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Clansman PTT and Telephone handset

February 15th, 2009 by m0rgb

Military PTT and telephone handset

I have always been impressed with the quality of UK military equipment and recently decided to try a Clansman PTT circa 1980 as a PTT for my K2.

The idea being it would be a great portable PTT for using my K2 out and about, probably water proof and very tough!

Did the usual Ebay thing and found one for £4.99, while I was there I also thought I would get a Clansman telephone style handset. If nothing else this would have useful parts in it.

So here goes....


Clansman headset PTT


Clansman telephone handset (with PTT in handle)

The first thing  noticed was that the PTT has a socket for the headset to plug into it, this uses the same connector as the telephone handset. So I decided rather than making a PTT and then wiring the handset seperately I could use the PTT as an adapter for any Clansman audio equipment....hmmm..... Read the rest of this entry »

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