The AM Forum

For 80/75/40 in line dipole that uses jumpers across insulators to change bands.  Fed with RG8X.  Number of beads, ferrite mix and vendor source, or someone here on the forum that might have some to sell. Currently working at the ART-13 power level, but likely going higher at some point with a home brew 2x813 x PP 805s Class C CW/AM and AB1 linear (on the retirement bucket list).  First responder gets lunch/dinner  on me at the Polish Kitchen at spring Nearfest (apple crisp included)!

The way I do it here (and I did the same thing at Rattlesnake Island) is I run the feedline through a bunch of type 43 cores.

I also had a multiband antenna fed with RG8X at the Island.  I used approximately 15  FB-43-1020 cores - ran the feedline right up the middle.  Worked great!

Here in Townsend I use a similar arrangement, but I use cores with a wider hold in the center because my feedline here is 3/7 inch cable TV hardline.  But, the same principle.

Simple, but works well.

Here is a part number:   2643102002   Fair-Rite Ferrite Cable Cores.

Available from Mouser at good prices.  Using more is better than using fewer.

Google k9yc value choke cookbook.


Really needs to be a sticky.

--Shane
KD6VXI

Boy that was fast feedback!  Thanks Steve!  

Jim Brown, K9YC, thinks bead baluns aren't good enough. Of the bead baluns, the only one that comes even close is the W2DU balun, 50 mix 73 beads on RG-303. It provides about 1000 ohms of choking impedance. Jim and others think you need more like 5000 ohms. He advocates coax looped  through a stack of mix 31 toroids. He has a simple chart of how many turns, how many toroids. That's what you'll get from Jim's article, that and the fact that he's done a lot of analysis and careful measurements.

I've spoken to Jim on the air several times but I didn't let on that I have W2DU baluns all over the place. I would appreciate it if you don't tell him either (just kidding).

Chapter 6, page 24 starts the discussion of baluns.
Page 36 has the cookbook for using the mix 31 stack choke for various frequencies.

http://audiosystemsgroup.com/RFI-Ham.pdf

RIP Walt Maxwell, W2DU

The BIG THING is turns² function..... the Z goes up by the number of turns through the core. 2 turns yields 4 times the Z than 1 through the core.

Walts work (balun) still is high value.

KLC

Indeed, but more turns aren't always better with coax chokes. Just look at 6 turns, 6 cores and 5 turns 6 cores in this graph of measured impedance. At 5 Mhz and above five turns beats six turns. I don't claim any expertise in this area. I'm just reading Jim's paper (again for the nth time) and looking at the charts.

Very interesting thread, so much more to learn.

In the real world, nothing is perfect.  In the engineering world, it seems everything is a compromise.

A couple points come to mind.....  Multiple turns through a single bead can cause saturation earlier; this is not always obvious when testing with a network analyzer.  Also, multiple turns involve capacitance between turns, not present with a single path through a string of beads. 

It is also important to avoid series resonance at any of the operating frequencies.  Mechanically, it is often more practical to package and mount coax through a string of beads at the antenna, especially when supported via wire antennas.  As I recall, these were some of the factors considered in the original W2DU bead baluns.  Testing was performed with the HP Vector Voltmeter and Vector impedance meter, as well as at legal limit power testing on the relevant amateur bands.

Of course, there is always room for improvement.  Many alternative methods may be employed with success.

I just use a bunch of those FB-43-1020 beads (type 43 material).

16 of these will give you about 1000 ohms on 75 meters, give or take.  I used something like this at the Rattlesnake island shack.  I use 20 on my main antenna now.

It seems to be sufficient.  Not too much current is going to flow at 1000 ohms XsubL, and of course the center of a dipole is a fairly low impedance as compared to 1000 ohms.

This is an interesting to me since I'm about to re-do my dipole for 75 meters.  It's a long story, but my drain waveforms aren't happy loading into this antenna. 

So, running RG-8 through a bunch of cores helps to eliminate current flow through the shield?  Is that correct?  And it makes the shield a higher impedance as compared to the feed point?  That's pretty cool.

Jon

I went digging through boxes last night and came up with more than enough to install 20 cores on the coax.  Although I went to my local Fry's and they were out of stock on PL-259's.  You know, I mean... uh.  They're even in the candy aisle with all the semiconductor stuff.  I eBay'd them and the coax.

Jon

Here's the thing with ferrite chokes (as discussed by Walt and I back when I had broadcast power aspirations)....

If you don't have enough impedance in the ferrite choke, that is when they start to heat up, crack, etc. That is why Jim Brown goes for such high levels of choking impedance on his multi turn chokes.

Until speaking with Walt about it, I was under the impression that as long as a choke had the same Z as say a filament choke, I'd be fine.  Nope, due to the amount of induced currents on the line.

Its also been found to reduce noise levels on rx, when chokes are used at the point of connection to the wattmeter / point of ingress to the station.  I did knock down a switching ballast noise level on an elevated ground plane from. S8 to S2 with chokes at the feed point of the antenna (coil of coax tuned per k1ttt guide) and ferrite at the wattmeter.

G3txq (might be tqx, his name is Steve) has quite a good pdf on ferrite chokes as well.

--Shane
KD6VXI

This is very valuable!