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1-9 of 9 messages
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swr's
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by COUCHPOTATOE on December 10, 2005
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i understand a little what swr's are about, 50ohm antenna + 50ohm coax = 1:1
but what is it when i turn up my dead key and the swr goes up, or when you speak into the mic the swr fluctuates? i though if you set your antenna, ran the right power into it, the swr would be what they are, but what are the fluctuations about? reactance?
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RE: swr's
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by COUCHPOTATOE on December 10, 2005
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my next antenna is trimmed for the middle of the cb band also, ch.19 from the math. it is 5/16" copper tubing 212" driven element and 223" reflector, supported by 2x9' 1.5" tubing, mounted on a 7'3" 1 1/4" square steel tubing, bolted to a spare 75 celica half-axle shaft, stuffed into appropriate fence post of 1/2wl height,split in the middle with a coupler. cheap, easy, and quick to build with good swr's compared to the one in the question above and seems to work fair... but i was curious as to what the properties of my antenna are dB-wise. i don't seem to be smart enough to run those computer analyzers, web based or downloaded. any f/b and gain appreciated greatly! thank you
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RE: swr's
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by K7KBN on December 11, 2005
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First, it's 'SWR'.
Second, I assume you're using the type of SWR meter that you have to switch to FWD or CAL, key the transmitter, adjust a knob for a full-scale reading and then switch to REF or SWR - some markings like that.
With this type of meter, you do NOT measure your SWR while talking. Ideally you should either use the CW mode or turn your mike gain completely down when making the measurement. The SWR doesn't change unless there's something seriously wrong with the antenna or feedline. This type of meter isn't actually measuring SWR - it's responding to reflected power on a meter scale that's marked in approximate SWR increments.
SWR is (or can be) a ratio of many things - forward power to reflected power, feedline impedance to antenna impedance, etc. It doesn't change when you change power levels.
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RE: swr's
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by WB6BYU on December 12, 2005
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Well, let's see what a simple model of the antenna gives us...
Start with W9CF's yagi modeller at:
http://fermi.la.asu.edu/w9cf/yagipub/index.html
Click on the "Click Here for the Yagi Modeller" button
and let the Java applet come up. By default, it will come
up with a simple 2-element yagi.
The next step is to change the data to reflect your
antenna. First, enter your design frequeny in the "Frequency"
box. (I used 27 MHz.) Hit the "CALCULATE" button and
it will draw the pattern for you. Now, you want to change
a couple of the other defaults. First, set the "Segments
in 1/2" to a larger number for better accuracy: 32 is
good in most cases. (Larger numbers take longer to run
on a slow machine.) Then go up to the list of menus
along the top of the display, pull down the "UNITS" menu
and select "inches". Then change the "Element Diameter"
field to 5/16" (presuming that is the ACTUAL element
diameter, not the nominal pipe size.) In this case it
would be 0.3125.
Now we need to adjust the element lengths. Start with
the reflector: click on the left-most element in the
drawing and set the element length to 212 inches (if
I remembered the number correctly.) Don't worry about
the element position - these are measured from the
reflector, so it should always be 0.0. Then click the
OK button to record your changes.
Then click on the red element - the driven element.
Change the length to match your antenna, then for
"Element Position" enter the spacing in inches between
the two elements. You said this was 7' or so? That
would be about 87" (which seems rather long to me.)
Click "OK" to go back to the main screen.
Now you have entered your antenna, just click "Calculate"
to see the pattern.
Hmmm... This shows the radiation in the direction of
the "reflector" is 1dB stronger than in the other direction
(the F/B is negative). The impedance Zin is 37 ohms
with a lot of reactance, which would indicate that the
driven element needs to be shortened. (But make any
other changes to the antenna first.) The gain is 4.8dBi,
or about 2 1/2 dB better than just the driven element
by itself.
Now, what if we used a more narrow spacing? Ckuck on
the driven element again in the drawing and change the
Element Spacing to 54 inches, click OK, then hit Calculate.
Now your gain has increased over 2dB, the impedance is
lower (19 ohms) but much less reactive, and the F/B
ratio is more than 4 1/2 dB.
Looking at the sketch of the pattern, the top half of
the drawing shows the pattern in the plane of the
elements (that is, the horizontal pattern if the yagi
is mounted with horizontal elements), while the bottom
half of the drawing shows the pattern when the antenna
is mounted with the elements vertical.
Once you get this working, you can experiment with
different element lengths and spacings to see what
results they give you. For example, try changing the
spacing between elements to a small number - 32".
(Click on the driven element - the red one - and change
the "Element Position" to 32".) Now you should see more
directivity in the pattern. The impedance is lower, but
can be matched by mounting a coil or "hairpin" across
the feedpoint and adjusting it for minimum SWR. The
gain is a bit lower than the 54" antenna, but not by
much. And there is much less radiation off the back of
the antenna.
Note that all these measurements are made in free space,
ignoring the ground and mounting mast. If the antenna
is vertically polarized and has a metal mast coming up
through the middle of it, it will mess up the pattern.
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RE: swr's
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by WB6BYU on December 12, 2005
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I'll presume that you have the type of SWR meter where you
set the forward power to some reference mark then switch
to read the reflected power/SWR. In this case, the SWR
is given by the ratio of the forward and reflected power.
But it is only correct for the reference output power.
If your modulation changes the forward power, the indicated
reflected power will change in the same ratio, which will
look like the SWR is changing.
Now that assumes that the SWR is low but not exactly 1:1.
If the SWR realy is 1.00 : 1, there wouldn't be any reflected
power regardless of the output power. But sometimes the
rig will generate harmonics or other spurious outputs,
especially if overdriven. If the antenna is not matched
for THOSE frequencies, the energy will be reflected and
will show up as SWR. So, if your SWR really IS changing
with power output, it either means that you have
spurious outputs from the transmitter at the higher
power levels, or perhaps that something is arcing in your
antenna or feedline when the power exceeds a certain level.
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RE: swr's
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by WB6BYU on December 12, 2005
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Oops... Going back and rereading your original post, it
seems that I misremembered the antenna element lengths
when I was using the yagi model. But there should be
enough information there for you to put in the correct
lengths and see what the pattern looks like.
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RE: swr's
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by WB6BYU on December 12, 2005
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Sorry for all the posts... I just found another mistake.
On the yagi modeller, click on the driven element and check
the box that says, "Driven Element". The original antenna
used a driven element plus DIRECTOR, which accounts for
some of the odd patterns.
If I have all the dimensions correct now, it shows a gain
of 5.6dBi (or 3.5dB over a dipole), a front/back ratio of
10dB, and an impedance of 73 + j43 ohms.
Yes, there are a number of factors that need to be entered
right to get it to work, but once you get the hang of it
it is quite easy to sit and experiment with different
dimensions. Generally, I think you would find spacings
around 4 may work better.
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RE: swr's
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by WA4PTZ on December 13, 2005
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I'll probably get chastised for my attempt to simplify
this situation but here goes.
As you talk into the mic the power output of your
transmitter changes with the intensity of your
voice and how you emphasis certain words. Since
SWR indications on your meter are relative to
the power flow through the meter you are actually
changing the current flow in the circuit each time
you speak. Now, this ain't the 8 page technical
expose but it just might make it easier to understand.
73 - Tim
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RE: swr's
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by COUCHPOTATOE on December 16, 2005
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thank you for the insight to my problem. upon inspection of antenna, found loose connection at the gamma match to d.e. and found i had to much gap between them too. seems to work very good. works better with 2in. gap than 3in. still working with the gamma match to prune out a little more reactance. i do run it as horizontal though most use verticle, but i was looking for a little noise cancelling effect that it gave me and i think i'm fairly proud of it for a total cost of about $10. thanks again!
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