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

"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

remember, too low an swr can kill you!

To get exactly 50 ohms you may need a fatter copper pipe, or some other
change to the geometry. if ti works well as it is i would leave it alone.

On 02 Nov 2008 00:12:29 GMT, Ed
wrote:

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

125 watts forward with 5 watts back is a VSWR = 1.5:1

The characteristic impedance of a coaxial antenna is about 75 ohms.
The best you can do with your present arrangement is therefore about
1.5:1 which is what you're getting.

To do any better, you'll need some way to match the 75 ohm antenna to
your 50 ohm system. That's usually an odd multiple of 1/4 wave coax
section, with an impedance of 61 ohms. Something like:
http://www.repeater-builder.com/rbtip/matchingstubs.html

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

A JPG is worth 1000 guesses.

Also, you might want to do your testing with something less than 125
watts, as accidents and miscalculations might become expensive.

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

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.
. . .

Actually, this is acceptable to just about everyone. And those for whom
it's not don't usually have a rational reason for it.

Here are some ways to do it:

1. Lower the impedance at resonance by making the top of the antenna
into an inverted cone shape, by using fanned out multiple conductors.
You'll have to adjust the length of the conductors and possibly the
sleeve to maintain the same resonant frequency.
2. Build and use a simple matching network, preferably placed as close
to the antenna as possible. Then you can adjust it for a perfect match.
3. Use some other form of impedance transformation such as a stub
matching network or transmission line transformer.

If you do things right, it'll work the same when you finish as it did
when you start. But you'll feel better seeing zero "reflected" power on
your meter, and the placebo effect is not to be sneezed at.

Roy Lewallen, W7EL

Ed,

You might have expected the feedpoint impedance to be around 70 ohms. It
will depend on the feedline configuration, because you haven't taken much
is the way of measures to decouple the feedline.

Your measured fwd and ref indicates VSWR~=1.5 which is consistent with 70
ohms, but you haven't measured 70 ohms.

Assuming though that such an antenna should be close to 70+j0 at
resonance...

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

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

Owen Duffy wrote in
:


Ed,

You might have expected the feedpoint impedance to be around 70 ohms.
It will depend on the feedline configuration, because you haven't
taken much is the way of measures to decouple the feedline.

Your measured fwd and ref indicates VSWR~=1.5 which is consistent with
70 ohms, but you haven't measured 70 ohms.

Assuming though that such an antenna should be close to 70+j0 at
resonance...

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, Owen. Saved me a lot of difficult math.... since I have
those materials on hand I may see what I can throw together tomorrow.

Ed

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

Ed wrote in
36.82:

Owen Duffy wrote in
:


Ed,

You might have expected the feedpoint impedance to be around 70 ohms.
It will depend on the feedline configuration, because you haven't
taken much is the way of measures to decouple the feedline.

Your measured fwd and ref indicates VSWR~=1.5 which is consistent
with 70 ohms, but you haven't measured 70 ohms.

Assuming though that such an antenna should be close to 70+j0 at
resonance...

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, Owen. Saved me a lot of difficult math.... since I
have
those materials on hand I may see what I can throw together tomorrow.

I didn't do the math, I punched the numbers into TLLC
(http://www.vk1od.net/tl/tllc.php).

Of course, the reason I was so specific is that translation from the
29.3° depends on the velocity factor... so use the velocity factor for
the cables you have at hand. (For example, if you use RG59, it has a
very different velocity factor to te 1189A, and you need to adjust
accordingly.)

There is a little on the twelfth wave transformer, including a graph of
the lengths for different transformation ratios at
http://www.vk1od.net/RG6/index.htm .

Have fun.

Owen