On Wed, 30 Mar 2011 10:23:51 -0700 (PDT), Tom Horne
wrote:

Richard
I wanted to explore whether it is practical to have my collinear dual
half wave J-pole serve as a dual band antenna. If it were practical I
would want the same gain on seventy centimeters that I have been
getting out of the dual stacked half wave on two meters.

Which would be quite horrible.

The
available testing that I was able to find says that it is 6 DB over a
quarter wave vertical. What I would happily settle for would be for
it to have the same gain on seventy centimeters as the dual band
simple J-pole I am using now.

Hi Tom,

It is extremely hard to accept claims for J-Poles when nothing is said
about the care in choking the feed point, and further choking the
section of line a quarter wave away from the feed point.

Typically this discussion is arrived at with some surprise on the part
of the J-Pole user who posts here (I know you have participated here
before, and are thus not a newbie). Some (which means too many)
respond that choking is unnecessary. They are satisfied with its
performance (never daring to examine that it could be vastly
improved).

We also have writers here who condemn the J-Pole vehemently in equal
measure. They, too, have not examined the necessity of choking and
they suffer from the knowledge that things could be vastly improved.

This is the pitiable lament of feeding halfwave elements in any form.

Let's examine what you call "available testing." I presume this means
in software, and not in the lab (never mind alternatives such as out
in a field). I could be wrong and you may correct this on your
response.

However, moving on with whatever presumption, the reason why choking
is important is that with High Z antennas, they tend to drive the
transmission line into radiation. This extends the length of the
radiator, and too often this raises the lobe of maximum radiation up
into the sky (you are very near that with the half wave where 5/8ths
is considered the limit of physical height before this trips over).

These are all issues related to a vertical, and its elevation goes
into the mix too to further confuse comparisons.

Now, basically you are asking the same antenna to operate at roughly
triple the frequency. This also means either element of the native
radiator will stand like something under 3/2 wavelengths tall - truly
a cloud burner (not good).

You speak of stubs to fix this. You would have to start with two 3/2
halfwave radiators, one over the other. Fixing the phase for both 2M
and 440 would be a miracle in achievement. I presume you would also
trap the individual 3/2 wave length sections into two 5/8ths (but the
ratios don't quite work out that way); or three half waves; or six
quarter waves - and then do it again for the section above. Whew!

This is a monumental task - but you have simpler goals as the
following would suggest:

just putting an open blocking stub for UHF at thirty five
centimeters ~ up the lower two meter half wave and therefore below the
two meter phasing stub between the two meter half wave segments. That
has the virtue of being simple and still giving me a dual band antenna
that has better gain on UHF then the unmodified two meter antenna
would.

The blocking stub I presume you to mean a trap for 440. It is going
to upset the matching stub between the two halfwave 2M elements. Here
you will have to juggle between tuning them both on each band.

It is less than monumental, but still quite a job, and one that
demands that you cut and try and fully erecting your last attempt to
see how it works (doing this on the ground is going to lead to grief -
especially if you ignore proper choking).

73's
Richard Clark, KB7QHC