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Author Topic: Adjusting Antenna Tuner  (Read 2357 times)
KK4HOK
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Posts: 25




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« on: May 09, 2012, 07:27:20 AM »

I have an antenna tuner that I bought used (MFJ-921) and I'm not 100% sure that it is functioning properly... not sure if the dials have been changed or what not just based on where I am turning the SWR/Power knob in comparison to what the SWR should be.

1) I noticed in the manual for the MFJ-259B you could use it to adjust the tuner.  Do you have any specific steps to do this with these two models? 
2) (Excuse the lack of knowledge on this one) but how - exactly - would you use the antenna tuner?


Thanks!
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WB6BYU
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Posts: 13013




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« Reply #1 on: May 09, 2012, 08:43:01 AM »

You connect the antenna tuner between the transmitter and the antenna.
Make sure you get the connections the right way around, or the SWR meter will
read backwards (presuming the tuner has a built-in meter, otherwise it
doesn't really matter.)

Then you generate a low power signal - a few watts should be enough - and
adjust the knobs on the tuner to get a minimum SWR on the meter.  This
requires some practice, because the settings interact, so it may take a few
times adjusting each control in turn until you get a good result.

You can practice online with W9CF's tuner simulator here:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

On the right side you set the frequency and the load impedance - try
values like 10, 50, or 300 to get a feel for it.  The field marked "X Load"
can be left as zero for now.  Then crank the "coil" knob for minimum SWR.
(It will be about at maximum of 30uH with the default values.)  Then adjust
each of the capacitors to try to lower it further.

added:  Set the righthand capacitor to full scale (250) and repeat
the process.  Do you see any difference?


Then change the frequency to 20m and repeat the process.

With a bit of practice you'll find that sometimes you want to go past
the dip with one control so the next one gets you closer, but that should
give you a feel for how to adjust it.
« Last Edit: May 09, 2012, 09:21:15 AM by WB6BYU » Logged
NA7U
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Posts: 72


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« Reply #2 on: May 09, 2012, 10:15:29 AM »

You could start off by using the 259B and a dummy load. You should have a good dummy load already, but if not, for this test, since the signal is so small, use a non-inductive 50 ohm resistor or some combo of resistors to get 50 ohms.

MFJ259B -> ATU -> [SWR meter] ->Dummy Load  is the circuit. If your ATU has an SWR meter already then you don't need another one in the circuit.

Set the analyzer to whatever band you want, probably one that matches whatever antenna you have now. Set the ATU to Bypass if it has that setting and you should read SWR 1:1 on the analyzer and the ATU/SWR meter. Doesn't have to be exact, but close. If it's not 1:1 then you probably need to adjust the SWR meter. Often there is a small screw on the meter for adjustment.

If you get that far then replace the dummy load with your antenna. Now set the analyzer to whatever frequency you want to use your antenna for and adjust the ATU knobs to get as close to 1:1 as you can (don't worry, anything under 3:1 will probably work with your rig). As you go through each band write down the ATU settings. You'll start to notice a pattern.


What might not work in the above is that the SWR meter may not be sensitive enough to register with such a small signal from the analyzer. In that case, you'll have to substitute your TX, just use low power.
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WA3SKN
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« Reply #3 on: May 09, 2012, 10:24:50 AM »

OK, so you have a MFJ921 tuner.
Do you have a MFJ-259B? (you indicated you had the manual)
Do you have a dummy load?

-Mike.
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KK4HOK
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Posts: 25




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« Reply #4 on: May 09, 2012, 11:12:24 AM »

OK, so you have a MFJ921 tuner.
Do you have a MFJ-259B? (you indicated you had the manual)
Do you have a dummy load?

-Mike.

Yes, I have the MFJ-259B (Guess I could have been a little clearer with that).  I don't have a dummy load yet -- I was in the process of bulding http://www.qsl.net/k5lxp/projects/SaltLoad/SaltLoad.html actually, as I had all the stuff laying around the house.  I'm not the *best* with soldering, but I did see plans online for one that I Could build with a small paint can . .
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WA7KPK
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Posts: 129




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« Reply #5 on: May 09, 2012, 12:23:07 PM »

One thing I wanted to add to WB6BYU's reply:

When I put up a new antenna I create a chart showing all the bands I plan to operate on. In my case that's 80-10 (with the exception of 60), with 6 added as "wishful thinking" as I'm not sure my tuner will operate on 6. I go through, tune up on each of the bands, and write down the settings for each band. That way when I QSY to a new band I can set the tuner quickly and just have to adjust the capacitors to get to optimum SWR.

The chart also has some useful information like the DX window and typical digital mode frequencies.

Actually with my tuner I've found that the only setting I need to keep track of is the inductor setting, and from there I can adjust the capacitors for lowest SWR and/or highest output power.
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WB6BYU
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« Reply #6 on: May 09, 2012, 12:50:11 PM »

Quote from: WA7KPK

When I put up a new antenna I create a chart showing all the bands I plan to operate on...



I just cut up some Post-It notes into long pointed shapes (using mostly the sticky part of
the note), write the band on each and use them to mark the right settings.  Much faster
to just set the knob to the proper marker reading a number off a table.  Often this gets
me close enough that I can operate without further tuning.
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WA3SKN
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« Reply #7 on: May 10, 2012, 04:16:29 AM »

I just looked up the MFJ921, and it is the one designed for 2m/220Mhz... so the question is what frequencies are you trying to match?

-Mike.
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KK4HOK
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Posts: 25




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« Reply #8 on: May 10, 2012, 07:09:52 AM »

I just looked up the MFJ921, and it is the one designed for 2m/220Mhz... so the question is what frequencies are you trying to match?

-Mike.

It is completely possible that I may not fully understand the purpose of an antenna tuner?  I built an antenna, and for the life of me, couldn't get the SWR any lower than 2-2.5.  I think I had read somewhere that using a tuner could help (but I'm not 100% sure at all...).  I found it at a really good price so I picked it up, and proceeded to be lost and confused with it  Huh

I operate mostly on 2m at this point.
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WB6BYU
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« Reply #9 on: May 10, 2012, 08:37:23 AM »

On one hand you're on the right track:  the purpose of an antenna tuner is to
provide an impedance match to the antenna when the SWR is otherwise too high
for the rig.

But because coax losses are much higher at VHF/UHF, antenna tuners are rarely
used in the shack on those bands.  It is more efficient to adjust the antenna to
match the coax, because then the feedline losses are lower and more power gets
radiated from the antenna.


If your SWR is 2 : 1 on 2m, then I'd suggest one of two solutions:

1) use the antenna as is, in spite of the SWR.

2) figure out what is wrong with the antenna that it can't be adjusted for a
lower SWR.

The choice depends a lot on the type of antenna you built, how it is mounted,
how the feedline is connected, etc.  Some antennas can't be adjusted for an
SWR any better than that, but you can get a better match by connecting a
short length of 75 ohm coax in series with the 50 ohm feedline.

If you give us more information about your antenna we can probably come up
with a better option than using a tuner in the shack.
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KK4HOK
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Posts: 25




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« Reply #10 on: May 10, 2012, 09:50:15 AM »

If you give us more information about your antenna we can probably come up
with a better option than using a tuner in the shack.

I built a copper pipe colinear jpole antenna (http://home.comcast.net/~buck0/Kb1dig_j.htm).  I tried quite a few things ... moving the lead points up and down quite a bit, adding some copper wire to the top to see if that would affect it, and trying a different coax.  They all seemed to yield the same results. (Mind you, I haven't tried it with the new SWR meter... it may be worth revisiting at this point possibly).
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K8AXW
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« Reply #11 on: May 10, 2012, 09:57:44 AM »

I have found the quickest way to tune an antenna tuner is to open the caps about 25% and then adjust the inductor for the loudest noise in the receiver.  Then adjust the caps for the loudest noise.  This will get you into the ball park.  There will be more than one setting on all three controls to give you a minimum SWR.  After you get one setting that satisfies your needs, mark these settings down on a card for future reference.

I have never in 55 years used an antenna tuner on 2m or higher.  Usually, antennas cut for these V/UHF bands are so broadbanded that a tuner isn't necessary.  Perhaps this is the way you should go.
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KK4HOK
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« Reply #12 on: May 10, 2012, 10:00:15 AM »

If there's no fear on operating with that high of an SWR, it may be okay.  I was debating taking off the upper half of the antenna and turning it into a regular jpole and seeing from there how it measures up.
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WB6BYU
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« Reply #13 on: May 10, 2012, 11:21:49 AM »

Quote from: KK4HOK

I built a copper pipe colinear jpole antenna (http://home.comcast.net/~buck0/Kb1dig_j.htm).  I tried quite a few things ... moving the lead points up and down quite a bit, adding some copper wire to the top to see if that would affect it, and trying a different coax.  They all seemed to yield the same results. (Mind you, I haven't tried it with the new SWR meter... it may be worth revisiting at this point possibly).



J-poles are quirky.  One reason is that the coax shield is connected directly to the antenna
at a point that is NOT at RF ground - that almost guarantees you'll get common mode currents
on the coax, and antenna performance (and SWR) become dependent on how the coax is
routed, coiled, etc.  If you check the SWR with a short coax jumper, then put a longer cable
on when the antenna is mounted on the roof or on a mast, that will change the SWR.  It can
be quite frustrating.  (Not to mention that, with some combinations, the radiation from the
coax cancels that from the antenna, and you can get nulls in the pattern where you were
wanting to have maximum radiation.)

For a standard J-pole I cut the radiator (the portion above the top of the stub) to 1/2 wavelength
and adjust the feedpoint and the length of the stub for best SWR.  Unfortunately most
copper pipe J-pole designs don't allow for adjusting the stub length at the bottom, and if you
adjust the top then you are changing the radiator length at the same time.  I haven't made one
using copper pipe, but if I did I'd probably make the stub longer with a sliding short circuit across
it to adjust the length, and use some sort of a balun on the feedline as well. 

But try the antenna as it is and see if it works well enough for you.  A 2 : 1 SWR isn't likely to
hurt the radio.  Or remove the upper section and phasing stub and see how that works, and if
you can get a better match.


One point that concerns me is your comment that moving the tap points and other changes
didn't seem to affect the SWR:

What type of feedline are you using, and how long is it?

Making such adjustments SHOULD change the SWR, even if it doesn't lower it to 1 : 1.  But
a long length of relatively lossy coax can make the SWR low even with no antenna connected.
For example, 50' of RG-58 coax would give a maximum SWR on 2m of 2.7 : 1.  So just because
the SWR doesn't look too bad doesn't mean that your antenna is anywhere close to being
tuned properly.  An SWR of 5 : 1 at the antenna would look like 1.9 : 1 at the rig end due to
the losses in the coax.

So if you are measuring the SWR through a significant length of coax (particularly RG-58) then
it may be difficult to see the effect of changes in the antenna matching.

And that comes back to why we rarely use antenna tuners in the shack on VHF:  it can
match the 2 : 1 SWR to the rig, but you still may be losing 3/4 of your power in the feedline.
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KK4HOK
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Posts: 25




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« Reply #14 on: May 10, 2012, 12:27:36 PM »

Wow, didn't know that at all about the coax.  It's about a 25' run from the antenna to the rig, and that's the cable I was using to test it.  Seeing as how I don't have to have the radio at my side with the new SWR meter, I can use a shorter coax alongside the antenna and hopefully get some more detailed results in testing the antenna (I guess I found my project for tonight!)
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