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MFJ-347 "HF Stick" mini-dipole mount design modification

This mount uses two (2) same-band Hamsticks or MFJ HamTenna whips to form a dipole.  The MFJ-347 bracket had a slight design issue, which I corrected.

Mount bracket touches dipole bolt (before modification):



MFJ-347 correspondence

Original question to MFJ on June 16, 2009:
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Dear Customer Service,

 I ordered and received the MFJ-2220 which included the MFJ-347 mini dipole mount.

 With no dipoles installed, when I insert a mounting pole (such as a 3/4" PVC pipe) into the bracket assembly, I noticed that the movable mounting plate touches the bolt for securing the passive dipole coupler (the non-cable connection dipole side).

 I suspect that it should not be that way - and that the bolt should be electrically isolated from the rest of the main T-shape mount.

 Can you confirm that the movable mount plate is not supposed to touch the bolt of the passive dipole coupler?  Should I put an insulating sheet on top the that bolt?

Thanks and regards,

Peter  AF6DS


MFJ's "Response to Question" on June 17, 2009:
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yes, i would suggest an insulating sheet or rubber cap or something be put there to insulate against that bolt. you see the bolt is insulated from both parts of the bracket. it should also be insulated there. i guess 1/2" piping is about all that can be used. although with some insulation material there, you should be able to use the 3/4 inch pipe without harm to your performance.

K6OIK analysis

Peter,
When the bolt touches the bracket, it puts a short between the coax connector threads and the bracket.  This effectively adds a small shunt capacitance across the antenna feedpoint.  If you were using a metal mast, there would also be the possibility of a ground loop through the bracket formed by the coax shield (presumably grounded at the station) and the mast (presumably grounded at its base).

I think it should work ok with the bolt touching, especially if you are using an antenna tuner.  However, the problem is that the bolt is nickel plated steel touching aluminum.  Over time, the point of contact can become corroded, and the short will become intermittent.  This will cause the antenna tuning to become "flakey."

From the picture, it appears that you can eliminate the short by using either a smaller or larger mast - under 1/2 inch or greater than 1.25 inches.  Of course if the mast is too big, it can touch the bolt and the same problem occurs.  MFJ might consider changing the mechanical design of the bracket so that both dipole halves are on the same side of the mast.

73

-  Steve, K6OIK


Bracket modified to clear the dipole bolt:

So won't be detuned by dipole mount or metal mast / mounting pole.



Mini-dipole mount with 1:1 RF current air choke using 3/4" PVC pipe:



Assembled dipole:



Dipole suspended from a tree using 60 lb. fishing line: