View Single Post
  #8   Report Post  
Old October 3rd 05, 03:40 AM
Richard Clark
 
Posts: n/a
Default

On Mon, 03 Oct 2005 02:02:13 GMT, David wrote:

If this is correct then the electrical length of the sleeve must be
modified by the presence of the RG174 coax outer sheath. The sleeve is
sort of performing a dual function.


Hi David,

I think I pointed that out already. The effects of this jacket are
variable across many suggestions offered here over the years.

As it is difficult to cut the tube once soldered in place, I will need
to make heaps of these to get the length right. At least if I had a
reasonable starting point it would minimize the number of iterations.


Ah yes, the pain of it all. I mentioned that too. You may as well
grit your teeth and set to it, there is no shortcut to this all.

The other issue is that I read about cutting lengths for elements and
not that even an element in free space is trimmed down by about 3% to
account for some "end effect" ? Would this also apply to my sleeve ? If
so, then the length may be difficult to get right because if I make
allowance for the end effect then the choking effect of the sleeve will
not work as well.


You might want to work this backwards. That is start off with a
successful choke section of the sleeve. Let that dictate what follows
because its isolation will mediate what I describe. If you have any
experience with what is called an Offset Center Fed Dipole, you would
find that it offers a closer match to 50 Ohms. You are using the
dipole as a variable match by finding the 50 Ohm portion along its
full length. This means that the portion above the sleeve's
attachment to the drive point may not be the classic length (and
certainly not accounted by the 3% of the "end effect" - not even
close).

That is neither here nor there because at that point you will have
simultaneously achieved resonance, a match, and isolation; and yet by
no fixed formula pulled out of a hat, nor accredited by an institution
of higher learning.

And, by the way, it isn't going to pull together in the first pass. I
hope you have a stack of tubing, so put on a happy face and get down
to it.

This is why I am now wanting to understand how these work and have a
good starting point for making them. The commercial units used molded
construction that was difficult for me to emulate. I have started with
RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4"
nylon tubing as a radome. It goes together well and looks good but does
not perform very well.


How do you know? It might be doing the best job you could ask of any
design. Your statement requires FAR MORE qualification than a rather
subjective toss-off.

In other words:
1. To what parameters?
2. Compared to what?
3. By what measure?

73's
Richard Clark, KB7QHC