40 Meter OCF DIPOLE

[ Version 3.1 ]

 

VERSION 3:

  • This is a radical deviation from the design used in Ver. 1 and Ver. 2.  It has a very lopsided feedpoint split of about 18/82.
  • This version uses 2 baluns, though you only see one balun in the picture.  The second balun is on the back.
    • The Balun you see in the picutre is a 4:1 "New Improved Balun" by N7ZWY.  It is claimed to be better than a 4:1 Guanella.
    • The Balun you don't see is a 1:1 Guanella with 10 turns of RG-174 wrapped on a -43 Toroid.
  • Version 3.1 is identical to Ver. 3.0, but has had its legs pruned proportionately to the chosen split, to bring the antenna into [apparent] resonance on the desired bands.
  • I managed to get excellent SWR out of this version, but it had too much trouble with Common Mode Current.
  • The antenna with a specific length of coax could be tuned to give a good SWR on all bands, but if you add more coax, say 25 ft., it would cause the apparent resonance (point of minimum SWR) to move as much as a couple hundred kHz.
    • Note:  I said "apparent" resonance.  I know that the point of minimum SWR is not necessarily the actual resonant point of the antenna. 

 

 

 

Version 3.1:

 

The SWR is less than 2:1 on all 4 classical hf bands and 6m.

 

In addition the SWR is good enough on 17 and 12m for use with a matchbox.

 

Using it on 30m with a matchbox is NOT a good idea.

 

 

THIS IS IT.  Ver. 3.1

 

That's all there is to it.

 

Total weight: 360 gr.

That's about 25 gr. more than Version 2 weighs, but still less than one pound, including 11m (36 ft.) of coax.

 

5 bands with good SWR using just 66 feet of wire.  That's about 13' of wire per band. Perhaps that's some kind of new record!

 

The SWR curves shown above of of this version, Ver. 3.1.

 

Update Saturday, April 13, 2013

 

This is the newer Ver. 3.2 of the antenna.

  • I flipped the center insulator upside down.
  • I added more turns to the CMC choke
  • And moved the choke below the NIB Balun.  

UNFORTUNATELY this did not improve the problem with CMC.

If anything it has gotten worse.

 

Back to the drawing board.

 

So far the only solution that has worked as I want and tamed the CMC was when using a 2-core Guanella, AND the 1:1 CMC choke (3 cores altogether).  This of course is too heavy.

 

Stay tuned.

CONCLUSION

Common Mode Current (CMC) is a real beast when trying to feed an antenna this far off center.

 

The "New Improved Balun", at least in this applications, shows ZERO improvement over a normal 4:1 Guanella.

 

A 4:1 Guanella on a single Toroid also will not have enough CMC attenuation.  Using this split is going to require more attention to the feed method, which I will address in Version 4.

 

VERSION 3 WAS A FLOP!