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Author Topic: 6 meter antenna tuner  (Read 5723 times)
W8RID
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Posts: 45




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« on: January 13, 2012, 09:51:38 AM »

Been searching the net to find plans for a home brew 6 meter antenna tuner.
So far nothing that jumps out at me.

Anyone have any plans or ideas where to find something like this??

Bob
W8RID

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WB6BYU
Member

Posts: 13243




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« Reply #1 on: January 13, 2012, 12:58:12 PM »

What range of impedances do you want to match?

The old ARRL books used to have link-coupled circuits for matching balanced line.

One of the early 6m solid state rigs published in QST included a small tuner for
matching to a long wire antenna.

The small tuners sold for CB use may match as high as 6m, depending on the
load impedance.  You could reduce the coil size and probably make one work.


But there are some important principles that you need to understand.

(1) The exact values of the variable capacitors and the coil aren't critical as long as
they are adjustable over a sufficient range.  For example, you may find that you
usually need 10 to 30pf for a capacitor to match the range of impedances you expect
to encounter.  You could use a variable with a maximum of 30pf, or 50pf, or 70pf, as
long as the minimum in each case was less than 10pf.  All would give you the ability
to tune the required range.

(2) The range of values required will depend on the range of impedances you want to
be able to match.  This can also affect the choice of circuit.

(3) The parasitic reactances in the internal leads become significant as you go higher
in frequency.  The inductance of the wire from the coil to the switch and back may be
more than you need to get a match.  This makes tuner layout much more critical.

(4) Tuners aren't used as often on VHF because coax losses are more signficant, especially
when the SWR is high.  That means it makes more sense to adjust your antenna for a
low SWR (which minimizes feedline losses) rather than using a tuner at the shack end.


That said, here are some links that may help you:

You can determine the range of values you need for a T network tuner by feeding W9CF's
tuner simulator a range of impedances and seeing what values it uses to match:

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


W4RNL's discussion on designing tuners for the higher frequencies (specifically for 10m,
but applies even more for 6m.)

http://www.cebik.com/content/a10/ant10.html


PA0FRI has a pi-network tuner designed for 20 - 10m.  A smaller coil and capacitors
would work on 60m - possibly as low as 100pf for the capacitors.

http://pa0fri.home.xs4all.nl/ATU/PiATU/pifiltereng.htm


The PI network is a reasonable choice because the shafts of the capacitors are
grounded, and getting reasonable capacitor values is not as difficult as it is at
80m or 160m.  One such design I saw used a half-wave coax balun on the
output, feeding two SO-239 coax connectors.  Balanced line was connected to
the center pins of the two connectors using banana plugs.  A single coax
feedline was connected to one jack and tuned in the normal manner. (Though
a good 1 : 1 current balun may be a better choice.)
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KB1GMX
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Posts: 772




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« Reply #2 on: January 14, 2012, 04:43:18 PM »

The first question I'd as is, Why?

It's far easier for 6M to run a matched antenna and due to size easier to build than
at lower frequencies.

With that said I've done it to match older and somewhat cranky radios to RG6 or 11 cable.

What you don't want to do is try and match a really poor SWR to the radio as at 6M
the cable losses are up there enough to start seriously sucking up power (on TX)
and signal (RX) to be wasteful for all but emergency cases.


Allison
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AD4U
Member

Posts: 2164




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« Reply #3 on: January 23, 2012, 05:55:15 AM »

Like Allison said, I have to ask WHY?

If your intention is to use any coax fed antenna on 6 meters that is "cut" for lower freqencies (1.8 - 30 MHz), you will be able to achieve an acceptable SWR with a "tuner" in the shack, but the losses in your coax due to the mis-matched antenna SYSTEM will probably be so high that your on-the-air performance will be poor.  Feed line losses for mis-matched coax fed antennas increase dramatically as the frequency increases.  Also using a low band antenna on 6 meters can result in some really screwy radiation patterns.

With coax fed antennas on ALL bands (but much more so as the frequency increases) it is much better to cut the antenna to resonance so it can be used without a "tuner".

All that being said, if you still want to build a 6 meter "tuner" do a google search.  I did and I found a number of references to articles on the subject.

Dick  AD4U
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ZS5QM
Member

Posts: 2




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« Reply #4 on: February 15, 2012, 12:15:58 PM »

In the past I also would have said, WHY!
Well I've been active on 6 meters since the 80's and all my antenna's were always well matched etc....
YET, through time I have always noticed that although my SWR is around 1.2:1 the output power on my external watt meter always read around 70 watts on a 100 watt rig.
I just always thought this was the max capability of the rig and never thought further of it.
Also I was always against antenna tuners and saw them as a lazy way to build a decent antenna etc.
Sometime back I acquired a TS-570S with auto antenna tuner etc.
Then I let rip with this auto tuner on 6 meters and BINGO! suddenly my watt meter read 100 watts.
So I decided to look into this and found that although your beam might have a good SWR, in my case tuned with a gamma match, does not mean its actually resonating on the frequency you desire, in my case 50.150 as that's where I designed the antenna for.
We tend to adjust the antenna with some means like a gamma match etc for the best SWR but that does not mean you have resonance, so at the end of the day you are only fooling yourself and your rig to thinking it's tuned to a resonant circuit.
The output transistors on modern rigs have good protection and fold back on the power when resonance is not reached at a set frequency.
After much experimenting I found my antenna was actually resonating at around 53.5 mhz and a that's where true resonance was reached.
After adding approx 30mm to each element of the antenna, did I finally get this beast tamed at around 50.250 where a true resonance was now applicable and max power transfer available without the antenna tuner.
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KB1GMX
Member

Posts: 772




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« Reply #5 on: February 19, 2012, 03:53:19 PM »

If your getting 70W on a 100W rig there may be other forces at work like the meter calibration, not compensating for the SWR to correct the read power or you are in lucky node along the line and the SWR is actually worse.

It also could be that the radios SWR protection circuit is maladjusted and over compensating.

Keep in mind we are talking 1.5 DB power difference and that's well within most power meters error.

The old ARRL handbook (and Mobile manual) had 6 and even 2m antenna tuners as antennas
of the era were often fed with twinline(homemade or 300ohm) or marginally matched.

Getting the antenna tuned for the desired frequency likely not only made the radio
happier but also helped with coax losses.


Allison
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WX7G
Member

Posts: 6039




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« Reply #6 on: February 19, 2012, 04:18:15 PM »

The MFJ-903 is a 6 meter tuner and lists for $70.

Here are plans for a tuner that AA3SJ designed.

http://www.qsl.net/aa3sj/Pages/50MHz-Tuner.html

« Last Edit: February 19, 2012, 04:20:32 PM by WX7G » Logged
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