On Tue, 16 Aug 2011 11:21:20 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

Thank you for the links, appreciate your help.

Y'er welcome. However, I still don't have a clue what you're trying
to accomplish. If you want super front to back ratio for a direction
finding antenna, you need a wall, not a phased single wire reflector
as in a yagi. Something like a dish or a panel reflector will work,
but is probably not practical. As you discovered, the position of the
reflector is very critical for maintaining the best F/B ratio. It's
also very sensitive to frequency. You might be able to make it work
with a flimsy tape measure, but it probably won't be very stable in
the field.

Permit me to offer another idea. Instead of direction finding by the
peak signal, do it with a null. Two identical dipoles, a T connector,
and some coax, are all that are required. Space them about 1/4
wavelength apart. Insert a 3/4 wave coax phasing section between the
two dipoles. Signals from the front antenna will cancel when they
reach the 2nd antenna yielding a cardioid pattern. It will be just as
critical and sensitive to frequency and position as your yagi, but
methinks will work better. I've obtained 50dB deep nulls with this
method. The good news is that you really only need one adjustment on
the antenna, which is the relative distance between dipoles, which is
easily accomplished by sliding the antenna on the boom. The
adjustment range is limited, so an assortment of coax phasing lengths
will also be needed. Adjusting the antenna for best null at a
specific frequency, prior to transmitter hunting, is a necessary
complication.

I also use this method on my home base station to null out a rather
strong repeater, so I can talk to a distant repeater on the same
frequency. I've also built one using 4 identical rubber ducky
antennas for 2meters. It sorta worked but I could only get a 10dB
deep null.

Both EZNEC and 4NEC have HF examples of this type of antenna.
cardioid.ez and \models\HFvertical\cardioid.nec

I fear that in the end I will have to cut and tweak using the
antenna analyzer, and with the time perhaps find
"tape measure correction factors".

More bad news. You really do have to TEST your creations to see if
the simulations and models are correct. I tend to make far too many
simplifying assumptions in my models, which show up as errors in the
actual construction.

Want to keep the design as simple as possible,

Simple is good. Crude is not. Recognizing the difference is
difficult.

and hope it works without a balun or additional tricks.
Am not yet sure if it will work that way, or not.

It won't work well without a balun. The coax will radiate, giving
unpredictable directional indications. You can minimize the effects
with ferrite beads, a sleeve balun, or with just a coax loop.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558