Jimmie D wrote:
It may be YOU that is causing the vaules to be so erratic. Your pressence
can detune the antenna.

Some of us are very close to one wavelength tall. :-)
--
73, Cecil http://www.w5dxp.com

I never actually said 10 -12 feet. My coax is 20 feet long. My
movement doesn't change the values at all. These values aren't erratic
either. You can move the coax around on the boom and mast and it
doesn't make the slightest difference on the readings.

Oh well, maybe I'll have to move my prototyping to the 6 meter band.

-Scott, WU2X

Jimmie D wrote:
wrote in message
oups.com...
Its not that I am trying to get the exact feedpoint impedance values
from the antenna. I understand how 50 Ohm coax affects the values that
I've measured. I was originally trying understand how to determine if a
reactance
value was capacitive or inductive.

I do see now that when I use NEC2 to plot the values for this antenna,
the resistance and reactance appear more linear across the frequency
range I measured. I see if I apply the logic of the MFJ manual to the
plot from NEC2, it makes sense and that my data is all over the place.
For what reasons I do not know. I do not think the feedline is
radiating. Maybe it is just the meter. Or perhaps some larger deviation
from 50 ohms in the coax and connectors.

And Richard - the antenna doesn't stay outside, that is why its not
sealed. This is just rigged up for testing.

-Scott, WU2X

wrote:
Your coax is what, 10-12 feet long? That's pushing a couple of
wavelengths on 2m, but isn't quite there.

To read the impedance of the antenna directly and get a good set of
resistance and reactance curves clearly, you need probably less than a
foot of coax, or an exact multiple of an electrical half wavelength (so
exactly 2 wavelengths * velocity factor would do it). You can use your
MFJ-259 to cut the line.

It's possible to transform your impedance readings into the antenna's
impedance if you know the (random) electrical length and loss of the
coax and are handy with a Smith chart or have some other calculating
ability.

Exact multiple of a half wave is easier... it's an impedance repeater,
aside from the loss in the line. To that end, I'd use an exact
multiple of half wave of better coax... RG-8 or 9913 or something.

73,
Dan


It may be YOU that is causing the vaules to be so erratic. Your pressence
can detune the antenna.
10- 12 feet is too close.

Sorry but by the time I read the whole thread........

wrote in message
ups.com...
I never actually said 10 -12 feet. My coax is 20 feet long. My
movement doesn't change the values at all. These values aren't erratic
either. You can move the coax around on the boom and mast and it
doesn't make the slightest difference on the readings.

Oh well, maybe I'll have to move my prototyping to the 6 meter band.

-Scott, WU2X

Jimmie D wrote:
wrote in message
oups.com...
Its not that I am trying to get the exact feedpoint impedance values
from the antenna. I understand how 50 Ohm coax affects the values that
I've measured. I was originally trying understand how to determine if a
reactance
value was capacitive or inductive.

I do see now that when I use NEC2 to plot the values for this antenna,
the resistance and reactance appear more linear across the frequency
range I measured. I see if I apply the logic of the MFJ manual to the
plot from NEC2, it makes sense and that my data is all over the place.
For what reasons I do not know. I do not think the feedline is
radiating. Maybe it is just the meter. Or perhaps some larger deviation
from 50 ohms in the coax and connectors.

And Richard - the antenna doesn't stay outside, that is why its not
sealed. This is just rigged up for testing.

-Scott, WU2X

wrote:
Your coax is what, 10-12 feet long? That's pushing a couple of
wavelengths on 2m, but isn't quite there.

To read the impedance of the antenna directly and get a good set of
resistance and reactance curves clearly, you need probably less than a
foot of coax, or an exact multiple of an electrical half wavelength
(so
exactly 2 wavelengths * velocity factor would do it). You can use
your
MFJ-259 to cut the line.

It's possible to transform your impedance readings into the antenna's
impedance if you know the (random) electrical length and loss of the
coax and are handy with a Smith chart or have some other calculating
ability.

Exact multiple of a half wave is easier... it's an impedance repeater,
aside from the loss in the line. To that end, I'd use an exact
multiple of half wave of better coax... RG-8 or 9913 or something.

73,
Dan


It may be YOU that is causing the vaules to be so erratic. Your pressence
can detune the antenna.
10- 12 feet is too close.

Looks like you have a double dip in impedance typical for staggered tuned
antenna. Is one of the elements "pulling" it or maybe effect of the other
balun and coax within the antenna? (It is in the same plane as the feed and
might be resonant causing the "other" dip. Something like OWA Yagi.) Try to
remove it or check the element dimensions.

73 Yuri, K3BU


wrote in message
ups.com...
I thought the SWR curve looked strange as well. The SWR has a weird
curve across the entire band. The antenna is actually a 4 element quad.
Its fed with a W2DU style VHF 1:1 balun. The coax is a short run of
RG-8X. The RG-8X uses PL-259s on both ends and I soldered a SO-239 onto
the coax for the balun (see pictures). Any issue with using PL259s and
a SO239?

Here is a picture of the feed system:
http://www.wu2x.com:8888/usenet/P1010062.JPG

Here is a picture of the antenna under test:
http://www.wu2x.com:8888/usenet/P1010058.JPG

This is the data graphed out, so its easier to see:
http://www.wu2x.com/usenet/ReadingA-ReadingB.htm

I took measurements with it fed for vertical polarization and
horizontal polarization. The results are both shown on the graphs.

-Scott, WU2X

wrote in news:1166596897.289275.21440@
48g2000cwx.googlegroups.com:

Today I was making some measurements with my MFJ-259B. From the manual,
I read:
....

What concept am I screwing up here?

Scott, your measurments are all over the place like a wild woman's
whatchamacallits.

Is the broadband detector in the MFJ being influeced by RFenergy received
by the antenna? That will make the readings worthless. Just try a quality
reflectometer (eg a Bird 43) and see if the readings are consistent with
the '259... if they aren't ask yourself why.

My experience (like others reported in the thread) is the MFJ259B is
potentially misleading on 2m.

As for your original question, MFJ's hint is useful, but visualise where
your observations mapped on a Smith chart, and assumption of one of the
two possible signs for the reactance will often seem less sensible or
even inconsistent. Additionally, it you calculate the load impedance
(considering the line's characteristics) at the load end of the line, and
consider readings at several nearby readings, again one of the two
possible sign assumptions for reactance will seem less likely to be true.

(Pity you can't plug in a line type and length and the analyser calculate
the impedance at the load end of the line, may be do it now? The
calculator at http://www.vk1od.net/tl/tllc.php will perform the calcs for
you.)

I see advice that you use an integral electrical half waves of coax or
just a foot of coax. Neither eliminates impedance transformation by the
coax when you sweep 4MHz as you have done.

Owen


-Scott, WU2X

Here are the values I measured:


Freq R X SWR
--------------------------------------
144 44 2 1.1
144.25 46 4 1.1
144.5 55 7 1.1
144.75 60 7 1.2
145 68 0 1.3
145.25 78 0 1.4
145.5 79 0 1.4
145.75 73 11 1.5
146 63 21 1.5
146.25 53 23 1.5
146.5 46 21 1.5
146.75 40 18 1.5
147 37 10 1.5
147.25 35 8 1.4
147.5 37 6 1.3
147.75 41 10 1.3
148 48 15 1.3
148.25 59 19 1.4
148.5 75 16 1.6
148.75 90 0 1.7
149 96 6 1.9