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

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

"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

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

"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

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.

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

On Nov 2, 7:30*pm, "Jim, K7JEB" wrote:
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

We have used these at work. The 50 ohm value is very "nominal".


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

Jimmie

On Sun, 02 Nov 2008 23:23:26 GMT, "JB" wrote:

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.

Yep. However, they recommended using 75 ohm coax cable. The loss of
equal lengths of similar size 75 ohm coax is less than 50 ohm. For
example:
RG-58c/u 0.20dB/meter at 150 Mhz (cheap 50 ohms coax)
LMR-240 0.09dB/meter at 150 Mhz (much better 50 ohm coax)
RG-6/u 0.07dB/meter at 150 Mhz (75 ohm CATV coax)

However, if you wanna run 50 ohm coax, the mismatch loss at the 75 ohm
antenna is about:
reflection_coef = (75-50)/(50+75)= 0.20
voltage = 1 - (0.2^2) = 0.96
20 * log(0.96) = 0.35 dB mismatch loss.
No big deal.

Hmmmm...
0.35 dB / 0.02dB/meter = 17.5 meters
At 17.5 meters, the losses of the 50 and 75 coax systems are
identical. Beyond 17.5 meters of coax, the 75 ohm coax delivers more
power.

I've been using RG-6/u for 2.4GHz wireless for quite a while. The
main incentive is that I can get the 75 ohm coax quite cheaply. For a
while, Hyperlink (http://www.hyperlinktech.com) had a rooftop 2.4Ghz
amplifier that was fed with 75 ohm coax. Alvarion/Breezecom also used
75 ohm coax in some of their BreezeAccess LB radios.

Someone eventually asks why 50 or 75 ohms. See:
http://www.microwaves101.com/encyclopedia/why50ohms.cfm

Motrac? Those are 30-40 years ancient. I used them for boat anchors.
Back then, I preferred GE radios:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/index.html



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

Jeff Liebermann wrote in
:

....
However, if you wanna run 50 ohm coax, the mismatch loss at the 75 ohm
antenna is about:
reflection_coef = (75-50)/(50+75)= 0.20
voltage = 1 - (0.2^2) = 0.96
20 * log(0.96) = 0.35 dB mismatch loss.

The analysis you give assumes that the notional 'reflected power' is lost
from the system as heat. The old 'reflected power is dissipated as
increased heat in the PA' line.

In the real world, the power that a transmitter delivers to a non ideal
load is not so simply predicted, and it is entirely possible that it
delivers more power to the mismatched load.

Owen