[Antennas] Some thoughts on baluns

Wed, 8 Jan 2003 12:56:27 -0700

Sandy - how very timely!

One of the local no-coders who shows lots of promise
of becoming a top-notch HF operator has purchased
himself a "short" G5RV for his station.  His intention
is to haunt the novice bands and work CW to gain
proficiency and eventually enough familiarity to upgrade
his ticket.

I think this is great, and have volunteered to help him
where I can.

Sandy's message regarding baluns is very timely as I
was asked last night by the new op if he should consider 
a balun at the junction of the bottom of the twinlead 
"feeder" portion of his G5RV, and the RG8 that he needs 
to run from that point into where his HF station is set up.

What do you think?

Thanks to Sandy and the rest of you for your thoughts
on this.

73 de John
..

----- Original Message ----- 
From: "Sandy and Kees Talen" <[email protected]>
To: <[email protected]>
Sent: Wednesday, January 08, 2003 9:19 AM
Subject: [Antennas] Some thoughts on baluns

> I've been trying a few things with ferrite choke current baluns,
> similar to the bead balun first covered by W2DU.
> 
> All balanced loads being fed with an unbalanced line need a
> balun of some sort to prevent currents on the OUTSIDE of 
> the shield (the current on the outside is in phase with the 
> center conductor). If ALL the current flows in the center 
> conductor and the inside of the shield, you will present a 
> "balanced line" (equal and opposite current flow) at the 
> end of the coax, ready to connect to your "balanced load". 
> 
> You may have just connected the coax to the antenna and 
> have made contacts all over the place, but I can pretty well 
> guarantee that the radiation pattern is not what you think it 
> is and you may be communicating with your neighbor's 
> intercom. Current on the outside of the shield WILL radiate.
> 
> Electrons always follow "path of least resistance" so if you
> add impedance to the outside, you will force that current to
> the inside, hence the ferrite choke current balun. Saturation
> is not a problem because you are creating a "blocking path"
> forcing the inside of the shield to carry all the current.
> 
> The average coax choke balun on a balanced beam is 
> typically 6 turns of coax 6" in dia (or around that). 
> Testing shows this to make a very effective choke balun 
> for 20/15/10m. Coils for 80m and 160m are too large and 
> pose other problems, so we use large, high u (2000 and 
> up), ferrite "beads" for the choke baluns. The materials 
> which appear to work best are #75 followed by #73 
> and #77. The #43 material won't work well at the lower 
> frequencies.Those materials are found in many of the 
> large clamp-on split and non split "beads" used by the 
> computer industry on monitor, keyboard, and power cables. 
> Use the ones with a 1/2" hole so you can wind 4 turns of 
> RG/58U (4 turns roughly increases the effectiveness by 
> the square ....or x16). These are pretty easy to find and 
> Mouser has some if you can't. 
> 
> How do you know what material you have ? Take your 
> MFJ Antenna Analyzer and run a single turn through the 
> bead. The performance will be worst at the lower 
> frequencies, so on 80m measure the shorted loop without 
> the bead, should be <<5ohms. With #75 material it will
> measure around 70-80ohms, with #73 or #77 material 
> it will measure around 40-50 ohms. #43 material will
> measure around 10 ohms or lower.
> 
> What does this mean ? A 4 turn 4" dia coil of RG/58U
> with two #75 or three #73 or #77 large "beads" makes
> a very effective choke balun for 80m - 10m and a pretty
> effective 160m - 10m choke balun. Both sides of the
> balanced line looked about equal on the scope. You can 
> maybe back off one of the beads and still be OK if you 
> have tested the it. A 6 turn 6" dia coax coil by itself 
> makes a very effective current choke balun for 20 - 10m. 
> Relative to the 4" coils, don't make the bending radius 
> too small or the center conductor will migrate, especially 
> with softer foam coaxes and it creates necessary 
> stresses on the "snap-on" bead mechanism. The two 
> halves must completely touch.
> 
> To take this one step further it's probably a good idea 
> to put one of these choke baluns in the coax as it exits the 
> transmitter, on coax(s) (used or not) as they exit the 
> tuner, maybe one or two along the coax transmission 
> line ....may not do anything, can't hurt anything, probably
> more important with cheaper (minimum shield) coax. All 
> you are adding, from the transmission line view is a few 
> extra feet of coax. This is in addition to the choke balun
> at the balanced antenna load.
> 
> In fact, a few turn coil in the power cable, as it exits the 
> transmitter, might help a 20m - 10m radiation problem 
> if that was the source.
> 
> Fire away.
> 
> 73  Kees K5BCQ 
> - - - 