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Author Topic: Can't get SWR to flatten on 15 meters....  (Read 1902 times)
K3NRX
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« on: February 02, 2014, 12:59:26 PM »

Tuner is an MFJ 962C 1.5kw bought at a hamfest in June 2012...Tuner tunes all other bands flat in all areas...will not flatten on 15 meters on any/all inductance settings....the antenna is a G5RV....on my MFJ 941 B 100 w tuner, I can get a flat match on all bands....This is the case using both my rigs.....the G5RV is the ONLY antenna I have.....what is the problem here?....I don't get it...and I want to install an amp eventually with this 962C, which I can't use with the 941 B.....anyone have any ideas what the deal is with 15 meters, this antenna and tuner (962C).....Help!

V
KA3NRX

« Last Edit: February 02, 2014, 01:02:58 PM by KA3NRX » Logged
WB6BYU
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Posts: 13353




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« Reply #1 on: February 02, 2014, 02:36:36 PM »

A G5RV has a very high SWR on 15m.  My first guess is that the impedance is just out
of the matching range of the tuner.

How low can you get the SWR on 15m?

What are the capacitor settings at that point?

It may be that adding an extra 6' to 12' of coax between the antenna and the tuner
will change the impedance to where the tuner will match it.

It is also possible that there is some sort of internal problem - for example, if the
roller inductor is dirty in the part of the coil where one would normally tune for 15m
(possibly from using it at high power into a G5RV) or the "Self-Resonance Killer" circuit
might not be switching properly.  But I'd try the extra length of coax first and see if
that makes a difference.
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K3NRX
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« Reply #2 on: February 02, 2014, 03:56:29 PM »

A G5RV has a very high SWR on 15m.  My first guess is that the impedance is just out
of the matching range of the tuner.

How low can you get the SWR on 15m?

What are the capacitor settings at that point?

It may be that adding an extra 6' to 12' of coax between the antenna and the tuner
will change the impedance to where the tuner will match it.

It is also possible that there is some sort of internal problem - for example, if the
roller inductor is dirty in the part of the coil where one would normally tune for 15m
(possibly from using it at high power into a G5RV) or the "Self-Resonance Killer" circuit
might not be switching properly.  But I'd try the extra length of coax first and see if
that makes a difference.

Lowest SWR reading is 1.8 to 1 set on Letter E....& it goes higher the higher I go up in the phone spectrum, with the power output decreasing.......No other settings budge the swr in any way....I don't have this issue on the smaller tuner....flat match all the way......I will take heed of your advise and see if I can clean the points and extend the coax....

V
KA3NRX

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WB6BYU
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« Reply #3 on: February 02, 2014, 04:52:46 PM »

It may have to do with a larger amount of parasitic capacitance or inductance in the
larger tuner due to larger components and/or longer leads.  That increases the
minimum inductance and capacitance available, and if you are very close to the
lower limit in the smaller tuner you might be just over the line in the larger one.

Letter E would be a lot of inductance on my MFJ-949 tuner, which would suggest
that at least one of your capacitors is at minimum capacitance and can't go small
enough to get a good match.  That's often associated with very sharp tuning and
higher tuner losses.


Meanwhile, watch the power ratings with that tuner if you get an amp:  it is rated
for 1.5kW input power, or about 800W output power.  And the power
rating of a tuner is rather difficult to establish anyway, because it depends on the
impedance you are trying to match.  I've seen the coil supports melt on a 300W
tuner at just 100W when it was matching a radical load.
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KA4POL
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Posts: 2041




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« Reply #4 on: February 02, 2014, 09:39:24 PM »

See: http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=28004
There you can see the SWR for the different bands.
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N5IFH
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« Reply #5 on: February 12, 2014, 06:25:06 AM »

Best suggestion!  Try to find a Ten-Tec 238 A, B, or C antenna tuner.  Problems will be solved!  73 & Good Luck.
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