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.

First, the antenna should have a balanced feed. With coax you need a 1/2
wavelength [coax corrected for velocity factor] matching stub to keep
the line balanced. This will minimized coupling antenna current on the
outside of the coax cable.

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].

Third, adjust the feedpoint location to get 50 + j0 ohms on your
preferred frequency using an antenna analyzer [one or more members of
your radio club should have one].

I'm located in southern NH, USA and if you are reasonably close I can
assist in tuning your antenna.

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



Well, Utah is a little too far away for your offer of help. But, why
shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at
the transmitter indicate 50 + j0 ohms which is what the transmitter
wants?

Thanks, jimbo

jimbo wrote:

SNIPPED


Well, Utah is a little too far away for your offer of help. But, why
shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at the
transmitter indicate 50 + j0 ohms which is what the transmitter wants?

Thanks, jimbo

It IS what the transmitter wants! But you may be introducing
un-necessary power losses in your coax from any true VSWR 1:1.

It is possible to have a VSWR 25:1 at the antenna and still have close
to 1:1 at the transmitter! REALLY !! Your transmitter is happy. But you
are wasting power heating the coax, not radiating it at the antenna!
This is caused by impedance transformation along the coax. VSWR ~ 1:1 at
the antenna provides best power transfer from coax to antenna.

BTW, I used to live in Kaysville and Layton [ZIP=85015] in the olden
days [30+ years ago]

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.

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

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

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


Thanks for all of the advice. I have one more question. Can I tune a
J-Pole antenna in my shop and then move it to the attic and expect the
same performance? In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

It would seem to me that if the antenna is tuned and works in my shop
it should work in the attic. Otherwise, how do "store bought" antennas
work?

Thanks, jimbo

jimbo wrote:
Thanks for all of the advice. I have one more question. Can I tune a
J-Pole antenna in my shop and then move it to the attic and expect the
same performance? In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

You can tune a J-Pole antenna at one place in free space, move it
to another place in free space, and expect the same results. The
real world is not so forgiving. The conditions will not be
identical but they may (or may not) be close enough to be
acceptable to you. Welcome to the real world of ham radio.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 21 Mar 2006 06:51:16 -0700, jimbo wrote:

In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

Hi jimbo

The second, the wire you call an antenna, its method of feed, and its
environment are the radiator. With care, it may work straight out of
the box, but your numbers already offer a different story; thus the
question becomes "what constitutes care?"

It would seem to me that if the antenna is tuned and works in my shop
it should work in the attic. Otherwise, how do "store bought" antennas
work?

The market for antennas destined to be put in an attic is very small,
hence you see very few for sale.

73's
Richard Clark, KB7QHC

"jimbo" wrote in message
...
jimbo wrote:


Thanks for all of the advice. I have one more question. Can I tune a
J-Pole antenna in my shop and then move it to the attic and expect the
same performance? In other words, is an antenna an antenna or is an
antenna and it's installation an antenna?

It would seem to me that if the antenna is tuned and works in my shop
it should work in the attic. Otherwise, how do "store bought" antennas
work?

Thanks, jimbo

I have built about twenty copper-pipe j-poles and I have found that with
five feet or so between the antenna and surroundings, the tuning doesn't
change enough to be annoying.

The copper pipe j-pole can be dropped into a pipe and hoisted into the clear
for VSWR evaluations if you really want to get gnats-assey about it.

Speaking of that, I have found that j-poles usually don't hit 1:1, even
when the low point is mid-band. However, after I get the best match,
I insert a 100 pF capacitor between the coax center pin and the antenna
and "Viola!" I stole that idea from the gamma match.

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


That or it is mis-matched

--
73 for now
Buck
N4PGW