Extending a Bobtail to five elements

(1/1)

:
I'm interested in extending a Bobtail Curtain to five (or possibly more) elements for use on 40 meters.  I've checked many great sites on the Web, but haven't found any articles specific to this particular type of folly!  I have several questions that I hope may be answered here.

1.  What happens to the feed point impedance when adding elements?  My intent is to direct coax feed the center element at the top, though if possible, an end fed system would be even easier with my particular supports... big trees.

2.  I have read two different articles on top coax feed.  One stated the center conducter was to be fed to the horizontal element, the other said that the vertical element should get the center feed and the braid to the horizontal.  Which is correct, or if not correct, better?

3.  I'm also considering the addition of a reflector element grouping.  To achieve the maximum forward gain without drastically affecting the feed impedance, what is the correct wavelength spacing?

Finally, I've been away from amateur radio for a long, long time and haven't exactly kept up on electronics and theory, so please keep the responses simple and as clear as possible.  My math is limited to balancing my checkbook... which seldom requires more than a few digits and a minus sign.  Please keep that in mind.

Thanks for you help.

David McKinnon:
   Have you worked with the basic Bob tail yet ? Satisfactorily ?   All my attempts in working with it didnt yield anything I was happy with so I abandoned the effort. If I were you I would walk before I ran .
 Good luck

Dale Hunt:
There is some good info on bobtail curtains including
studies of feedpoints at www.cebik.com

You can't just extend the antenna to 5 elements by
adding horizontal and vertical wires the way it might
appear.  Although I haven't tried it, I think you
would have to attach a wire to the BOTTOM of the end
vertical sections, run it back up parallel to the
existing vertical, then run it across another half
wavelength before dropping it down again.  (So the
inner vertical sections would look like quarter wave
stubs of open wire feedline shorted at the bottom.)

Regarding center feed:  it really doesn't matter which
side of the coax connects to which wire.

I've never had any problems end-feeding the wires with
100 watts, though you will have to make sure the tuning
capacitor can handle the voltage at higher power levels.  W4RNL shows that you can feed it in the top
end corners as well as in the middle (see link, above.)

Personally I'd start with a half-square and see how
well it does:  if it shows promise I'd try the standard
bobtail and work up from there.  Because of the sharper
pattern with the longer array you may have to be more
careful aiming it for your intended coverage area.

Scott W. Porter:
on my 3 element 40 meter bobtail fed at the top of the center element, it made a huge difference how it was wired... I had to connect the center conductor of the coax to the vertical elemennt and the shield to the horizontal phasing wire. It allowed me to work(and conform) my last 3 zones on 40.
gl!
Scott

Navigation

[0] Message Index