On Sun, 02 Nov 2008 14:00:40 GMT, "Jerry"
wrote:

Hi Owen

Another way of avoiding the math is to use both a Smith Chart and an
overlay of a Z Theta Chart. The problem of choosing line lengths and their
Zo them becomes intuitive. But any "perfect match" does depend heavily on
knowing impedance rather than VSWR, as you know.
The load impedance ploted on the Smith Chart can be assummed to translate
to any impedance on the circle of constant VSWR for any load impedance.
The impedance moves along the line of constant "Theta" on the Z Theta Chart
for a change of Chart Z.
With the overlay of the two charts, it is fairly easy to see what lengths
and Zo will produce the best match.

Jerry KD6JDJ

Hi Jerry,

Your solution is rather exotic for this group, but I have encountered
it in my Metrology days as part of the HP legacy. The method you
described is missing from this article, but it gives the group a
picture of the chart, none-the-less:
http://www.hpl.hp.com/hpjournal/pdfs...Fs/1950-04.pdf

73's
Richard Clark, KB7QHC

Hi Jerry,

The Z-Theta Chart has actually been around a long time, well known as the
'Carter' Chart, developed by Philip Carter, an early RCA transmission-line and
antenna inventor, around the same time as Philip Smith developed the Smith
Chart. In one of HP's Application Notes it is incorrectly called the 'Charter
Chart'. But you are correct, overlaying those two charts is often very helpful.

Walt, W2DU

On Sun, 02 Nov 2008 03:22:09 GMT, Owen Duffy wrote:

If you did want to incorporate an impedance matching system that doesn't
compromise the portability you have described, you could try a twelfth wave
transformer with 29.3° of 50 ohm coax from the feedpoint, then 29.3° of 75
ohm coax then any length of 50 ohm coax to the transmitter.

For example, for 146MHz, that could be 137mm of Belden 9258 (RG8/X) then
139mm of Belden 1189A (RG6/U) then any length of 50 ohm coax to the tx.

Owen

Very nice. Here's a bit more on how it works:
http://ourworld.compuserve.com/homepages/demerson/twelfth.htm
http://ourworld.compuserve.com/homepages/demerson/12thfdbk.htm

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

Dave, Jeff, Roy, ...

Thank you for the excellent responses to my questions. I am
considering any and all of them.

One note: I erred on my previously stated power levels.... it was
1.25 Watts forward and about .05 watts reflected. Same ratio, but less
energy.

Ed

"Ed" wrote in message
. 192.196...

I'm trying to build a decent performing 2M coaxial skirted antenna and
have a question about its design for maximum efficiency.

I based the "hub" on an SO-239 connector. I soldered my RG8X cable
center conductor to the solder pin center conductor and brought the
shield braid out in two places.

I cut a 19" piece of half inch copper pipe, cut 4 half inch slots on
one end, fanned the slotted end out slightly to fit nicely against the
SO-239, and slid this pipe over my coax and up to the SO-239. I brought
the two braid lengths previously prepared out through two of the rather
fat slots, soldered the pipe to the SO-239 and the braids were soldered
to the pipe where they came protruded out the slots.

I soldered an 18 1/2" brass welding rod to a PL-259 center conductor
and screwed that the the SO-239 for my radiator. The copper pipe assy
and coax slide nicely down into a length of 3/4" PVC . This makes a very
nice break-down package for transportation and portable work.

Back to my question: I am measuring about 125 watts forward and 4-5
watts reflected at my desired frequency ( 146.000 ). While this may be
acceptable to some, I would like to get the match down to 1:1 SWR.

Are there any design deficiencies in my proto-type? Can anyone
suggest something I might try to improve the match? Is there a "rule of
thumb" regarding the construction of such antennas?... length of coaxial
skirt vs. antenna element?

I appreciate any feedback on this. Thanks.

I suppose I could provide a picture if anyone requires it....

Ed K7AAT
Hi Ed

If you are in search of precission, I'd suggest that you consider using 70
ohm coax to feed the 70 ohm antenna. There are some very good 70 ohm coax
available at low cost. Their loss will be a fraction of a dB less than
your RG-8X. Their OD may be smaller (that helps a little to decouple the
stub from the coax). TheVSWR on the 70 ohm line will be lower than on the
50 ohm line. You will then be able to construct a tuning network at the
junction from the transmitter to the 70 ohm line. There will be
essentially no meaureable VSWR anywhere in the system.

Jerry KD6JDJ

Ed wrote:

...
Back to my question: I am measuring about 125 watts forward and 4-5
watts reflected at my desired frequency ( 146.000 ). While this may be
acceptable to some, I would like to get the match down to 1:1 SWR.
...

Ed K7AAT

You could always construct one of the "fancy" and esoteric 50:75 ohm
ununs to accomplish, however, you would probably not gain much if
anything, as there will be some loss--even if you construct the unun
from optimum material.

However, if ONLY the mismatch bothers you (and not the insignificant 4-5
watts) and not the actual loss of watts--it is an option. But any
"gain" you would get from this option would be virtually
"insignificant", or so in my humble opinion.

Regards,
JS

On Nov 1, 7:12*pm, Ed wrote:
* I'm trying to build a decent performing 2M coaxial skirted antenna and
have a question about its design for maximum efficiency.

* I based the "hub" on an SO-239 connector. *I soldered my RG8X cable
center conductor to the solder pin center conductor and brought the
shield braid out in two places.

* *I cut a 19" piece of half inch copper pipe, *cut 4 half inch slots on
one end, *fanned the slotted end out slightly to fit nicely against the
SO-239, *and slid this pipe over my coax and up to the SO-239. *I brought
the two braid lengths previously prepared out through two of the rather
fat slots, * soldered the pipe to the SO-239 and the braids were soldered
to the pipe where they came protruded out the slots.

* *I soldered an 18 1/2" brass welding rod to a PL-259 center conductor
and screwed that the the SO-239 for my radiator. * The copper pipe assy
and coax slide nicely down into a length of 3/4" PVC . *This makes a very
nice break-down package for transportation and portable work.

* * Back to my question: *I am measuring about 125 watts forward and 4-5
watts reflected at my desired frequency *( 146.000 ). *While this may be
acceptable to some, *I would like to get the match down to 1:1 SWR.

* * Are there any design deficiencies in my proto-type? * Can anyone
suggest something I might try to improve the match? * Is there a "rule of
thumb" regarding the construction of such antennas?... length of coaxial
skirt vs. antenna element?

* * I appreciate any feedback on this. *Thanks.

* *I suppose I could provide a picture if anyone requires it....

* *Ed * K7AAT

Hi Ed, I think your SWR is about as good as you are going to get it.
What you have created is a center feed dipole with a feedpoint
impedance of about 70 ohms or so. One way to get a better match to 50
ohms is to flair out your coaxial skirt. Skirt at 90 degrees to the
radiator the impedance will be 36 ohms, 70 with the skirt at 180
degree. You will find 50 ohms somewhere in between, about 45 degrees I
think. This may be done at the expense of your radiation pattern. I
built something like what you are building a few years ago using sheet
metal rolled in a trumpet shape for the skirt. I think my SWR was
about 1.3:1. My metal mast and skirt connected together at the
feedpoint. Whether or not the mast was insulated from the skirt of not
didnt make any difference. If itis not clear what I am talking about
think AEA isopole. While theirs was a 5/8ths we are talking about a
1/2 wl antenna.


Jimmie