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Technical Topics => Equipment => Topic started by: bandarr3000 on April 18, 2018, 1602 UTC

Title: Touching MFJ-956 increases gain.
Post by: bandarr3000 on April 18, 2018, 1602 UTC

When i touch the knob on my MFJ-956 I see a boost in the gain in the waterfall display for my SDR.  Is that normal?  It's like I grabbed the antenna.  Disconcerting.

Title: Re: Touching MFJ-956 increases gain.
Post by: Josh on April 18, 2018, 1655 UTC

At least some of the mfj antenna tuners marketed towards HAM radio variable caps aren't isolated from the adjustment knobs, resulting in the same behavior as you note. Do you have to touch the device for this effect or just get your hand near it?

Title: Re: Touching MFJ-956 increases gain.
Post by: pinto vortando on April 19, 2018, 0020 UTC

Have never noticed any hand capacitance affects with my MFJ 956.  What type antenna are you using and how is it connected to the tuner?  Do you have a convenient means for properly grounding the tuner?

Title: Re: Touching MFJ-956 increases gain.
Post by: bandarr3000 on April 20, 2018, 1735 UTC

@Josh I have to touch the device for it to happen.

@pinto I have a 90' horizontal insulated wire loop hooked up to The Squid, running LMR-400 pretty much all the way back to my SDRPlay RSP1a.  There is no way that I know of to ground the RSP.

Title: Re: Touching MFJ-956 increases gain.
Post by: pinto vortando on April 21, 2018, 1609 UTC

When hand capacitance affects tuning, that is usually a sign that something is not grounded, improperly coupled, mismatched, or mistuned.
The reference to grounding in my earlier post meant the tuner, not the RSP. 

Title: Re: Touching MFJ-956 increases gain.
Post by: pinto vortando on April 21, 2018, 1640 UTC

Another thought, take a look inside the 956 and give it a thorough inspection.  You are looking for loose connections, bad, cold solder joints, that sort of thing.  If that checks out, and you know that the connectors on all the coax are installed properly (no loose braid, bad solder joints), then you may want to experiment with the Squid and perhaps find a better match.  ;)

Title: Re: Touching MFJ-956 increases gain.
Post by: Josh on April 21, 2018, 1738 UTC

You might try some ferrites placed on the feed line near the mfj. That may cut down on the rf coming down the outside of the coax your hand interacts with.

Title: Re: Touching MFJ-956 increases gain.
Post by: moof on April 21, 2018, 1832 UTC

I've seen that lots of times on portables. Maybe you become part of the ground plane.  Like touching rabbit ears on ancient 1970 TV.  HOLD IT RIGHT THERE.

Title: Re: Touching MFJ-956 increases gain.
Post by: bandarr3000 on April 22, 2018, 0002 UTC

LOL.  I will dig inside the MFJ and go from there.  Thanks all.

Title: Re: Touching MFJ-956 increases gain.
Post by: pinto vortando on April 22, 2018, 0841 UTC

Quote from: bandarr3000 on April 22, 2018, 0002 UTC

LOL.  I will dig inside the MFJ and go from there.  Thanks all.

While you are in there, get out the schematic and make sure it is wired correctly.   ???
It is a simple circuit but double check it just to be sure it got wired correctly.   ;) 

Title: Re: Touching MFJ-956 increases gain.
Post by: ka1iic on April 27, 2018, 1828 UTC

Hi all,

I have a MFJ 945E and it is effected by hand capacity on the Antenna control... The only way to fix it is to place a isolated coupler on the antenna v-cap. I think of that every time I use the thing but I fight the urge to modify and live with it. I seldom use it anyway so what the hey hey hey :-) 

Tee network antenna tuners will do this without an isolated coupler between the Vee Cap rotator and the knob.  It's the nature of the beast.  Leaving out this coupler is just a cheaper way of making the tuner. Higher power use when using these type of tuners require the isolator... remember RF Burns hurt like all get out and sometimes don't heal well.

vw