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Author Topic: MFJ-945E tuner  (Read 2185 times)
KA3VEZ
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Posts: 92




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« on: December 07, 2011, 06:30:18 PM »

Here is a dumb question.  Thinking about getting this tuner. Now I know what you are going to say being MFJ.  In any case my question is at the lengths they recommend in using for 160, 80, and 40m.  The lengths they recommend. Will they really work and if so are those lengths the total length or what   both sides of a dipole should be? For ex

160 meter dipole: 35-60, 170-195 or 210-235 feet [avoid 130, 260 ft]
80 meter dipole: 34-40, 90-102 or 160-172 feet [avoid 66, 135, 190 ft]
40 meter dipole: 42-52, 73-83, 112-123 or 145-155 feet [avoid 32, 64, 96, 128 ft]

Now this is from a pdf file for the MFJ-945E.

Just wonder and would like any input. Thanks in advance.


~Kirk, KA3VEZ
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K2DC
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Posts: 1354


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« Reply #1 on: December 07, 2011, 11:32:25 PM »

Kirk,

   Don't mean to be rude, but go back and read the manual more carefully.  It clearly states:

To avoid problems matching or feeding any dipole antenna with high impedance lines, keep the lines around these lengths [ The worst possible line lengths are shown in brackets ]:

What's listed to avoid are odd quarter wavelength feedlines when using 300, 450 or 600 ohm balanced line to feed a doublet or dipole.

73,

Don, K2DC
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AC4RD
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Posts: 1236




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« Reply #2 on: December 08, 2011, 04:09:48 AM »

Kirk, I used a 941E for ... don't even know, close to 20 years, I guess.  (Still have it, too.)  I think it's very similar to your 945E.   The first reply was right--about avoiding certain lengths of feeder line.

As far as your antenna, it's easy as pie if you're feeding it with balanced line:  Make it a half wavelength for the lowest frequency you want to work.   So my dipole is 34' on each leg, fed with twinlead, and it works great on 40 meters (two quarter-waves there) and on every higher band, with the tuner.

A doublet fed with balanced line, as you appear to be considering, is easy to build, inexpensive, fun to modify and experiment on, and will give you good performance besides!   So you're on the right track!  73 and have fun!   --ken ac4rd
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KG6BRG
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Posts: 119




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« Reply #3 on: December 08, 2011, 06:56:22 AM »

I'm no MFJ fan. They sell some things that don't live up to the claims that are made, but, I have owned the 949E for 11 years and it tunes everything I throw at it including random wires which can be problematic for some tuners.  Coax fed balanced antennas are a snap. Cheers Scott
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KA5IPF
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« Reply #4 on: December 08, 2011, 07:34:39 AM »

The 945E has a coax input only. Works OK for me with an 80/40 trap dipole. Anywhere on either band.

Clif
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KA3VEZ
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Posts: 92




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« Reply #5 on: December 08, 2011, 10:44:29 AM »

Opps. Don't I feel stupid now.  I should added that I am going to use coax.  But I was also going to use a (I hope I am saying this right) 4:1 balun and use it as a long wire with 65 feet of wire on the "hot" side and the other going to a ground rod.  I was told by someone that it SHOULD work but just want more opions.

~Kirk, KA3VEZ
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WB6BYU
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Posts: 13152




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« Reply #6 on: December 08, 2011, 01:11:07 PM »

It is quite interesting that the manual talks about the limits on feeding a dipole with lengths of
open wire line when the tuner isn't designed for it.  Probably the standard text that they put
in all their tuner manuals.

Actually I don't find high impedances that difficult to match - low impedances are much more
of a problem.  But I suspect that the 945E won't handle the rated power at high impedances
due to high voltages arcing over the capacitors, which is why they include the warnings.

It's the same basic circuit used in a number of MFJ tuners (and other brands.)  Nothing special
about it.  Efficiency can be poor with low impedance loads on 80m and especially 160m due
to the limited capacitor sizes, and stray reactances start to become a problem in the upper HF
range.  But it is a reasonable all-band tuner, and there are lots of the MFJ 945/949 units in use.


Will it work for your proposed antenna?  Hard to say.

First, you have chosen a quarter wave wire on 80m.  Such an antenna fed directly with coax will
present a low impedance at some point in the 80m band (probably near the low end.)  I used the
same thing with my old HW-12 without any tuner.  The ground losses will raise the impedance
close enough to 50 ohms to get a good match.  Adjust the wire length to change the resonant
frequency.  Improving your ground connection by adding radials, etc. will improve your efficiency
by reducing ground losses, and make the SWR worse.  But you should still be able to tune the
antenna across the 80m band with your tuner.

Adding a balun at the antenna, however, changes things.  First, you really don't want to use a
BALUN in this situation - you want an UN-UN, because both the antenna and the feedline are
unbalanced.  Using a BALUN will cause a lot of common mode current on your feedline because
the outside is no longer at ground potential as it would be if it were connected directly to the
ground rod.  (That doesn't guarantee you don't have common mode currents, but it is better
than forcing them to happen.)

The feedpoint impedance on 80m will be around 50 ohms:  the transformer will step it down to
12.5 ohms, giving a 4 : 1 SWR on the coax (if we assume the transformer works as planned,
which isn't always the case at the low end of the operating range.)  The tuner should still
match the antenna.

On 40m the antenna will be close to 1/2 wavelength long and will have a high impedance at the
feedpoint, perhaps 2500 ohms or so.  If you don't use a transformer, this represents a SWR
of 50 : 1.  The transformer (theoretically) would step this down by a factor of 4, so you only
have 12.5 : 1 SWR.  But many transformers aren't designed to work well with high impedances.
The length of the common 43' vertical was chosen to avoid such resonances to keep the
feedline losses down.

On 160m the antenna is 1/8 wave and will be a low impedance with some reactance.  Let's
say 8 - j400 or so.  Nominally the transformer would step this down to an even lower
impedance of 2 - j100.  That would be an SWR over 100 : 1, and, depending on the length
of the coax, your tuner may not match it very efficiently.  (Melted coils are not that uncommon
in such tuners that have been used on 160m.)  But this is another case where the transformer
may not act the same as a theoretical model - likely the SWR will still be high, however.


Increasing the length of the wire will reduce the impedance on 40m and raise it on 160m.
On 80m you would no longer be resonant, but performance should still be reasonable.
Something in the 75 - 110' range is probably better, though the coax losses may still
be high on some bands and you may find that you can't get a match in some cases
without changing the length of the feedline.

If you want good multiband operation of an end-fed wire, it works best with the tuner right at
the feedpoint to reduce the coax losses.
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KA3VEZ
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Posts: 92




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« Reply #7 on: December 10, 2011, 07:40:15 PM »

Ok all.  Thanks for the info.  So I am guessing my random wire antenna won't work.  Back to the drawing board. So I guess having a dipole antenna that is 63'(31.5' on each leg) long fed with coax will not work either.  Right?

~Kirk, KA3VEZ
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WB6BYU
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Posts: 13152




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« Reply #8 on: December 10, 2011, 08:41:03 PM »

Your random wire idea might work better with a different wire length, depending on the bands that are
most important to you, or if you can put the tuner at the wire feedpoint rather than at the other end
of a length of coax.  And the tuner probably can match the coax, depending on the length - the
limitation is the coax losses at high SWR.  If you use a relatively short length of low loss coax then it
will be more efficient than a longer length, though that will put a greater stress on the tuner.

If that is the only antenna you can put up, it will be better than no antenna at all.  And the performance
should be reasonable on 80m.  But I don't know of any simple wire antenna, either vertical or dipole, that
will be efficient on all bands when fed through a long length of coax because of the high SWR on some
bands.  Now, perhaps you have a 10dB loss, which means that 90% of your power is dissipated in the
coax and only 10% is radiated by the antenna.  With 100 watts out from the rig only 10 watts escapes
from the antenna.  But I've made lots of contacts running just 5 watts, so, while not particularly efficient,
at least you should make some contacts with it.

To make the antenna efficient on 160 / 80 / 40m you either need to get the SWR lower (less than,
say, 5 : 1 or 10 : 1 at the antenna) or use a lower loss feedline.  There are several approaches available
for each - remote tuner, traps, open wire line, multiple wires in parallel, etc.  The option that works best
for you will depend on a lot of circumstances and conditions that we don't know.
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AD6KA
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Posts: 2237




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« Reply #9 on: December 10, 2011, 11:19:42 PM »

So I guess having a dipole antenna that is 63'(31.5' on each leg) long fed with coax will not work either.  Right?
~Kirk, KA3VEZ

Kirk, almost anything will "work".
One guy loaded up a 250 watt light bulb with a
balun and made dozens of CW contacts.
It was in an artical in a magazine years ago.
Whether it will work well or to your expectations
is another matter......

Good luck with your antenna project.
Hope you find a compromise which will
fit your needs.
73, Ken AD6KA
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