[TowerTalk] Question for common mode choke balun experts
Jim Thomson
jim.thom at telus.net
Mon Aug 12 11:41:25 EDT 2013
Date: Sat, 10 Aug 2013 20:46:04 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Question for common mode choke balun experts.
On 8/10/2013 7:34 AM, Steve Hunt wrote:
> You need to remember that there is a large resistive component in the
> choke impedance, so you'll not see "High-Q" effects.
Right. Typical circuit Q on the order of 0.4.
>
> I just checked four turns of RG213 on four stacked FT240-31 cores;
> there was a broad peak in impedance between 13MHz and 27MHz, at just
> over 3000Ohms.
>
> Then I added an 8.2pF capacitor in parallel; the peak dropped to 5MHz,
> but it was still pretty broad and the peak impedance had actually
> dropped to about 2000 Ohms.
No surprise. Note that since we're looking for as much choking Z as
practical, within size and cost limits, so if we want to move the
resonance down, we are far better off with more turns and/or more cores.
I don't understand your comment that you can't add more turns or cores
-- with reasonable care, I can nearly always fit five turns through the
1.4-in i.d. cores with a 83-1SP attached. Are you using a larger connector?
BTW -- as noted in the "Cookbook," it's also possible to have two or
more chokes in series, either to cover a wider frequency range, or to
raise the Z without going too low in frequency.
73, Jim K9YC
## I have a pair of Array solutions baluns, both in small nema boxes. I had Jay
at AS modify both of them, by installing 7-16 DIN connectors on both the input
and output sides. Inside is 4 turns of 393 coax passed through the inside of the
4 x stacked type 31 cores. 393 has 2 x silver plated braids, and each braid is
carefully separated at both ends, and terminated in 2 places on the 7-16 DIN, and
silver soldered. Small ty-raps keep the windings etc, carefully aligned, and the cores
and coax in mid air between the 2 x connectors. The boxes now become line
isolators.
## I don’t want to have to take it all apart to get 1 more core in there. They are only
going to be used on 80-40-20m. Type 43 bead baluns will be used at the ant feedpoints
for the 80m rotary dipole and also the 40 +20m yagis, like about 4-5 foot worth of beads on 80
and 40m, and 3 ft on 20m. The 393 coax from each ant is then fed to a mast mounted
remote switch box, where all 3 braids end up grounded to mast. One of the AS line isolators
will be located at the INPUT of the remote switch box, also on the mast. The nema box is
either plastic or fiberglass composite material. So in effect, I end up with a choke balun
on either side of the remote mast grounded, and mounted switch box. At bottom of tower
is a hoffi brand lightning protector. So coax is bonded to bottom of tower. Base of tower
has 3 x 8 foot copper clad ground rods jack hammered into the dirt, just beyond the concrete
base. 2 ga copper wire is cadwelded to the rods, just below ground level. The other ends of 3
2ga wires have compression lugs..and bolted to tower leg plates.
## The lightning protection casing is bonded to tower. It also serves as an interface between the
LMR-1200DB going to the radio..and the 393 going up the side of the HDX-689 crank up tower.
It also bonds the braids of both 1200+ 393 coax to base of tower. More ground rods 40 ft away,
just outside basement window. 2 ga bare wire run between base of tower and rods outside
basement window. Large SPG plate just inside basement, that is connected via 2 ga to the pair of
rods outside 3 foot away. Henry radio LPF mounted to SPG, so braid of LMR-1200db is bonded
to SPG. Just inboard of the LPF, is the 2nd line isolator....then off to the amp, etc.
## Between the bead balun at feedpoint, and choke balun at mast and also in the basement, plus braids
being grounded at mast...between the 2 choke baluns, then again at base of tower,then again at SPG,
I think I should be ok. If 3 x chokes and 3 x grounding points does not do the trick, I give up.
later.... Jim VE7RF
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