Owen Duffy wrote in news:Xns9B4AAC084EA4nonenowhere@
61.9.191.5:

....
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 .

If you read the article, you might try evaluating the systemusing TLLC with
say 10m of feedline configured with:
- the twelfth wave transformer near the antenna and 10m of RG8/X, and
- the twelfth wave transformer near the tx and 10m of RG6.

Intesting, the cheapest option (mostly RG6) is the one with the least loss!

Owen

"Roy Lewallen" wrote:

But you'll feel better seeing zero "reflected" power on your meter, ...
_____________

The meter indication will be essentially zero only if the directivity of the
device measuring reflected energy is extremely good (as in 60+ dB).

The leakage of forward energy into the reflected sample that is present in
typical commercial products will lead to (mis)adjusting the load Z to
whatever produces zero on the reflected meter. However some amount of
reflected energy will be needed to cancel the leakage of the reflected
coupler, in order to obtain that "zero" reading.

And then the reverse energy is not really zero, and the true load SWR is not
really 1:1.

RF

"Owen Duffy" wrote in message
...
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


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

"Richard Clark" wrote in message
...
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 Richard

As you know, I have been far away from the antenna design community for
such a long time that I feel like a beginner today. So, I wouldnt have
considered anything i know to be "exotic".
For those who havent used the Z Theta Chart, it is identical to the Smith
Chart, but expressed in polar coordinates ( Impedance magnitude with its
angle).
With both a Smith Chart and a Z Theta Chart of equal radius and overlayed,
a pin hole thru both will identify any impedance with real Resistance. The
value identified on the Smith Chart is given as R+/- jX.. The value
identified on the Z Theta Chart is given as Z Angle Theta.

The use of both charts together is a great tool for impedance matching
with transmission line sections. It is quick and simple yet it is quite
accurate. An impedance, plotted on the Z Theta Chart moves along the lines
of constant angle when the chart Impedance is changed. That lets the
designer plot any impedance on the Z Theta Chart with, for instance 50 ohms
as its center, then immediately see where the impedance will move if the
Chart impedance is changed to the value of the new transmission line
impedance chosen for the transformer, for instance 70 ohms.

Jerry

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

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

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

Gee it's too bad he didn't have a bunch of CATV hardline and a Motrac. All
of this would be real simple.

I recall that there was a commercial AS "fire engine" antenna that never
bothered with the matching at all because adding all the extra hardware for
matching, wouldn't have justified the potential losses that might be
introduced. Of course the main advantage of the antenna was that it could
be elevated without need for reflecting plane or radials and thus wouldn't
poke eyes out or get tangled. Otherwise a regular mobile mount or base
radial kit would be advantageous.

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

Kreco Antennas in Cresco, PA makes a line of
coaxial dipole basestation antennas that exhibit
a 50-ohm feedpoint impedance. Here's the
website for their high-band basic model:

http://www.krecoantennas.com/hbcaxial.htm

They pull off this trick by, *I THINK*, shortening
the top element slightly and lengthening the skirt in
*just the right way* to achieve a match at a spot
frequency.

An interesting variant on the basic antenna is their
"shunt-fed" coaxial dipole that places the entire
antenna at DC ground for lightning protection. Here's
the webpage for it:

http://www.krecoantennas.com/shuntfed.htm

I've used their antennas in the past with excellent
results, but they are a bit pricey.

Jim, K7JEB