On Sun, 3 Apr 2011 09:43:27 -0700 (PDT), Tom Horne
wrote:

What I hope to achieve is to maintain
the gain of the stacked, out of phase, dual half wave antenna on two
meters AND trap the lower have wave to a single half wave on seventy
centimeters.

Hi Tom,

Right off the bat (yes I can see the article now) you offer a
conflict. Out of phase blurs gain (lowers it). Your statement
suggests you are losing from the start.

However, it could simply be wording. Let's make this simple:
1. State the gain at the frequencies of interest before the change;
2. State the gain at those frequencies you expect after the change.

Here are the steps I was thinking of:
Fabricate a trap from 1/2" copper pipe coming out of a tee that has
been cut in half through a plane perpendicular to the foot of the
tee.

Lost already - sorry.

Remove the top half wave and the phasing loop from the existing
collinear dual half wave antenna
Attach the stub to the lower half wave so that the open bottom end of
the stub is one half wave on UHF from the top of the matching stub.
1Measure the SWR on 446 MHz. of the newly trapped two meter radiator.
2Adjust stub location for minimum SWR at 446 MHz.
3Measure SWR at 146 MHz.
4Shorten that radiator to lowest SWR on two meters or until the two
SWR
readings for VHF & UHF are equal.

Probably not. You are going to be repeating steps 1-4 to even
approach this desired end condition.

5Reattach the upper two meter half wave and phasing stub.
6Measure antenna for SWR on both VHF & UHF again
7Adjust length of of upper two meter half wave if necessary.

This will disturb the work done in steps 1-4. You cannot rely on that
configuration being isolated from the work in steps 5-7 which will
also demand repeated attention. However, proceed with 5-7 repeats
until you obtain the desired end condition.

NOW, revisit steps 1-4 with everything attached. I may be wrong, but
I suspect you will need to trim things here again. And guess what?
This means you need to visit and repeat steps 5-7 again.

And guess what? You will be visiting steps 1-4 AND 5-7 again, and
again.

Take heart, these revisits "should" result in ever smaller
adjustments. That means you will approach your desired end condition
asymptotically.

The antenna analyzer that I will use will be an AEA Technology SWR
Meter: 140-525. It happens to be what I have available to me.

Does my process seem to be along the right line or would you suggest a
different order of operations?

The order is quite rational and fully expected. You are entirely on
the right track. Your experience will reveal how much they are
interactive and how sensitive all this is. If you anticipate the
amount of repeated operations and plan accordingly, it will go far
more quickly.

Epilogue

I would note that your writer does not offer any testing of sufficient
caliber to support the claims of the EZNEC model. He merely provides
signal reports using a sophisticated instrument (a spectrum analyzer
that is not being used for spectrum analysis). Those reports also
tell me that there is at best 1.2dB gain over a 1/4 wave design - very
underwhelming. The only SWR data is reported as "The SWR was low."

73's
Richard Clark, KB7QHC