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