From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Test Fixture for Common Mode Chokes
>Of course. But what matters most is the common mode current at the
>feedpoint, which is where common mode current couples to the receiver.
>which For my dipoles for 80, 40, and 30M, this is 120 ft in the air,
>suspended between redwoods. How do you propose I make that measurement?
Points well taken. I was not considering the dipole between tree's
boys. For that application I would just use the K9YC cookbook CM choke,
optimized for whatever band the dipole is cut for..... then hope for the
best.
Ok, here is another scenario..... with a tower. 3-el 40m Yagi at bottom
of mast, with a 75m rotary dipole 10' above the 40m yagi.
Helical hairpin used on both feedpoints. Helical hairpin made from 3/8"
soft drawn aluminum tubing. Cookbook CM choke used at each feed point. A
1 in and 2 out remote ant switch used, mounted to the mast...or to the top
of the tower. SINGLE coax cable coming down the side of the tower. Typ
I will run the coax down the inside of the tower, depending on the
tower, but not always.
Ok, now we have the braid of each ant's coax bonded to the mast / top of
the tower...(via the remote switch box). IF any CM current gets past the CM
choke, then what happens ? Any residual current can flow down the
outside of the braid of the coax going down the tower (with coax either
outside or inside the tower). It could also flow down the tower itself.
I suspect the current would split, with a portion going down each path.
Point here is, if a clamp on RF ammeter was used downstream, like
somewhere between top and bottom of the tower, you would end up with a
false reading.
Typ, the coax shield is also bonded to the bottom of the tower. Then what,
does the coax current and tower current recombine at the base of the tower
? Assume the tower is well grounded via several grnd rods etc.
When a single feedline is used, and say several ants at the top of the
tower, (or a multiband yagi and ants for 80+40m) and a remote switch box
used, residual CM current hot spots could very well appear at different
places on the feedline, since it's being used on several freqs.
IMO, just measuring the CM current at the input to the CM choke would be as
good as it gets for all practical purposes.... ( assuming no dipoles
between trees). Plan B is to use 2 x identical CM chokes in series.
That concept works well.... for some applications.
Jim VE7RF
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|