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.