wrote:
On Mon, 20 Mar 2006 17:51:28 -0500, Dave wrote:


Bob Miller wrote:

SNIPPED

I'm confused, wouldn't it be more important to have the right
impedance and swr at the end of the feedline, where the transceiver is
expecting 50 ohms?

bob
k5qwg


SNIPPED

If you have 50 ohms at the antenna, then you will have 50 ohms
regardless of the length of coax.

A 3:1 VSWR, assuming other responses to this post are correct, means
that your antenna connection could be anything from 16 ohms to 150 ohms
and all sorts of combinations of resistance and reactance within that range.

An antenna analyzer will let you determine antenna resonance, antenna
feedpoint impedance [hopefully 50 ohms], coax line loss, and allow
proper adjustment at the connection points for your coax.

Once again, find someone in your area with an antenna analyzer. I repeat
my offer if you are within 60 +/- miles of Concord NH.



It's possible to tune a Jpole with just a SWR meter. I've done ti
many times though it's more tedious than with an antenna analyser.

The simplest step is add some length to the long section, not alot
and see how the change affects it. Add more or less as needed
for a good minimum. If the minimum is still not on the mark (1:1
is ideal but anything under 1.3:1 flies well) then adjust the tap
point (very small movements) and resweep again. With patience
you (the person that asked) will get it right.

What happens with Jpoles is they often are not built identical
and small differences do show. Also like most halfwave and
larger antennas being close to "stuff" tends to affect tuning.
No harm or foul but learning to prune (aka tune) an atenna is
a skill and worth developing.

Allison
KB1GMX

Adding some length to the long leg would be relatively easy, BUT
moving the feed point would be a real challenge! The coax shield and
center conductor are soldered to the ladder line wires. When I tuned a
j-pole in my work shop, tiny movements made a change amd maintaining
the position while trying to get it soldered was nearly impossible.
(At least for my clumsy fingers.) Is there a better way to connect the
feed point?

Thanks, jimbo

I have no personal experience with your conditions.
However, the scenario is familiar. You would like to "tidy up" an
installation that is "sort of" working.

You can minimize the effort involved by going to
http://free.prohosting.com/~w0rcy/Jpole/jpole.html

That will allow you to plan the fewest trips to the attic that will
enable you achieve your goals with your resources.

OTH, if you are achieving acceptable results with what you have now I
would advise leaving it alone. If you feel 2-4db more is worth the
effort, lay out your plan!

BTW, I totally agree with Cecil. The fundamental problem appears that
the antenna is a bit short! Any change other than that is a patch!

On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote:

OK, I have a new 2 meter j-pole antenna installed in my 3rd floor
attic. I have 50 feet of LMR240 coax running to the basement. I
measure SWR at the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter
band?

Thanks for any insight, jimbo
John Ferrell W8CCW

Owen Duffy wrote:
On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote:


In your set up the VSWR tells you very little!!

That 50 feet of LMR240 is modifying the actual VSWR as seen at your
meter. It is not telling you the VSWR at the feed point.


The matched line loss of 50' LMR240 at 146MHz is 1.5dB.

Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a
reasonable estimate that the VSWR at the antenna end of the line is
3.5 (not a very good match for an antenna that should use an
adjustable matching system). Total line loss is around 2.3dB, or about
0.9dB worse than matched line loss.

Now look at the tx output power, has it decreased because of the bad
load. Add that reduction (in dB) to the 0.9% above to get the overall
degradation of transmit perfomance.

A question: Is the proximity of other structures or feedline isolation
a cause of the high VSWR, and should you resolve that before tweaking
the matching?

Owen
--

The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand
that.) Total line loss 2.3 dB. (Where did you get that?) .9 dB worse
than matched line loss. (Where did you get that number?)

There is wooden structure near since the antenna is located in an
attic. But I don't think there are any large metal structures close to
the antenna. The coax is in PVC pipe. It may go close to a metal AC
duct on it's way to the basement..

Thanks, jimbo

Owen Duffy wrote:

SNIPPED

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--

Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's
expertise/knowledge, at this point in his ham experience, to perform
those calculations.

I didn't perform those calculations until my 3rd year in engineering school.

KISS, and fix it at the source.

jimbo wrote:

SNIPPED




Adding some length to the long leg would be relatively easy, BUT moving
the feed point would be a real challenge! The coax shield and center
conductor are soldered to the ladder line wires. When I tuned a j-pole
in my work shop, tiny movements made a change amd maintaining the
position while trying to get it soldered was nearly impossible. (At
least for my clumsy fingers.) Is there a better way to connect the feed
point?

Thanks, jimbo

Jimbo, you just added another variable to the discussion. Please
describe the details of the coax to ladder line and ladder line to
antenna for us.

On Mon, 20 Mar 2006 16:51:44 -0700, jimbo wrote:

Owen Duffy wrote:
On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote:


In your set up the VSWR tells you very little!!

That 50 feet of LMR240 is modifying the actual VSWR as seen at your
meter. It is not telling you the VSWR at the feed point.


The matched line loss of 50' LMR240 at 146MHz is 1.5dB.

Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a
reasonable estimate that the VSWR at the antenna end of the line is
3.5 (not a very good match for an antenna that should use an
adjustable matching system). Total line loss is around 2.3dB, or about
0.9dB worse than matched line loss.

Now look at the tx output power, has it decreased because of the bad
load. Add that reduction (in dB) to the 0.9% above to get the overall
degradation of transmit perfomance.

A question: Is the proximity of other structures or feedline isolation
a cause of the high VSWR, and should you resolve that before tweaking
the matching?

Owen
--


One step at a time...

The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand
that.)

Ok.
Total line loss 2.3 dB. (Where did you get that?)

It can be calculated knowing the line loss and source end VSWR. The
additional 0.8dB is increased line loss due to operation with VSWR1.
Try http://www.vk1od.net/tl/tllce.php for the calcs.

.9 dB worse
than matched line loss. (Where did you get that number?)

Sorry, my rounding error, call it 0.8dB. It is by subtraction of the
matched line loss from the total line loss.


There is wooden structure near since the antenna is located in an
attic. But I don't think there are any large metal structures close to
the antenna. The coax is in PVC pipe. It may go close to a metal AC
duct on it's way to the basement..

Is the roof sarked (does it have a metal foil lining under the tiles
or shingles), presumably it isnt a metal roof.


Thanks, jimbo
--

On Mon, 20 Mar 2006 19:00:16 -0500, Dave wrote:

Owen Duffy wrote:

SNIPPED

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--

Owen, I don't disagree with you. But, IMO, it is beyond Bob Miller's
expertise/knowledge, at this point in his ham experience, to perform
those calculations.

I didn't perform those calculations until my 3rd year in engineering school.

KISS, and fix it at the source.

Dave, there is no doubt that the closer you are to the antenna, the
less error introduced by tolerance on line parameters. On the
otherhand, the instrumentation / user can be disruptive or real
inconvenient.

As for the calcs, one can be lazy and use
http://www.vk1od.net/tl/tllce.php (if one's cable type is supported).

My antenna analyser can't do the calcs internally, I don't know if
some can, but it would be a pretty handy feature.

Owen
--

Dave wrote:
jimbo wrote:

SNIPPED





Adding some length to the long leg would be relatively easy, BUT
moving the feed point would be a real challenge! The coax shield and
center conductor are soldered to the ladder line wires. When I tuned a
j-pole in my work shop, tiny movements made a change amd maintaining
the position while trying to get it soldered was nearly impossible.
(At least for my clumsy fingers.) Is there a better way to connect the
feed point?

Thanks, jimbo


Jimbo, you just added another variable to the discussion. Please
describe the details of the coax to ladder line and ladder line to
antenna for us.


Sorry for the confusion. The ladder line is the j-pole antenna. Cut to
the appropriate length, etc. The coax feed line is attached to the "J"
near the bottom, maybe 2 inches up.

Thanks, jimbo

John Ferrell wrote:
I have no personal experience with your conditions.
However, the scenario is familiar. You would like to "tidy up" an
installation that is "sort of" working.

You can minimize the effort involved by going to
http://free.prohosting.com/~w0rcy/Jpole/jpole.html

That will allow you to plan the fewest trips to the attic that will
enable you achieve your goals with your resources.

OTH, if you are achieving acceptable results with what you have now I
would advise leaving it alone. If you feel 2-4db more is worth the
effort, lay out your plan!

BTW, I totally agree with Cecil. The fundamental problem appears that
the antenna is a bit short! Any change other than that is a patch!

On Mon, 20 Mar 2006 08:12:51 -0700, jimbo wrote:


OK, I have a new 2 meter j-pole antenna installed in my 3rd floor
attic. I have 50 feet of LMR240 coax running to the basement. I
measure SWR at the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter
band?

Thanks for any insight, jimbo

John Ferrell W8CCW

Well, the thing operates OK as far as I can tell. But it doesn't take
much to hit a 2 meter repeater. I guess I am more concerned about
damage to the transmitter than I am about getting full power radiated.

Thanks, jimbo

But-out flag cleared//

"jimbo" wrote in message
. ..
Dave wrote:
In your set up the VSWR tells you very little!!

That 50 feet of LMR240 is modifying the actual VSWR as seen at your
meter. It is not telling you the VSWR at the feed point....

Second, you should measure the antenna feed point impedance at the
feedpoint, or 1/2 wavelength from the feed point [coax corrected for
velocity factor].
.....

DD

jimbo wrote:

OK, I have a new 2 meter j-pole antenna installed in my 3rd floor
attic. I have 50 feet of LMR240 coax running to the basement. I
measure SWR at the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter
band?

Thanks for any insight, jimbo


Seems to me there are several items being asked / answered here. Sort
of going backwards through the parts of the thread I can see...

1 - I think it is a safe bet that if the VSWR at the transmitter
(regardless of feed like loss BUT within normally expected values) is lowest
at the high end of the band, then the antenna system is either too short or
resonant too high in frequency - take your pick of wording.

2- As you measure VSWR further and further from the load (antenna) you will
get a better and better value because of the loss in the transmission line.
This is because as signal is lost to the loss (attenuation) of the line,
there is less to reflect at the load and less makes it back to the measuring
device - when compared to tehat sent from teh transmitter. Less power to
measure in the backward direction = better VSWR.
Side Note that the loss is twice the line loss in this case, because it
travels it twice in a round trip. Another result of this is that a simple
resistive (lossy) attenuator (or a hunk of t-line used as an attenuator) has
a return loss that is equal to twice its attenuation rating. In other
words, a 10dB pad has a 20 dB return loss and is a pretty good load, since
only 1% of the power makes it back to be measured for the VSWR measurement.

3- Then there are comments about measuring an *impedance* *at* the load, or
1/2 wavelength away. This is because the *Impedance* repeats every half
wavelength in a *LOSS-LESS* line (due to the much discussed physics of
waves). For practical lines, with minimal loss, it's pretty close, but, as
explained above, approaches 50 ohms (Zo) further and further from the load.
Therefore, accuracy in the measured *impedance* degrades further and further
from the load.

But/out flag set//

73, Steve, K,9.D;C'I

Hi Owen.