I wonder how Balun Designs keeps a low SWR without twisting their wires.  I will try a twisted wire experiment.  Looks like HyPower Antennas twists about 1 turn per inch guessing from their web photos.

While 40m now shows a good resonant dip, I am still concerned about why the balun is not 1:1 across the frequency range.

I just wound an identical version with no Teflon sleeve.  A twin-leads transmission line of #14 Thermaleze taped together with 14 turns thru each core.  The wire ends are connected parallel on the feed side and series on the antenna side as per 4:1 Guanella diagram.  It has even worse SWR at higher frequencies.

I don't know what I'm doing wrong with the winding Barry.  Your commercial product appears to have smaller wire sleeved in teflon and twisted. 

What would cause the 4:1 transformation to only occur at lower frequencies?

So I built a 4:1 Guanella balun.
Two cores, FT-240K, each with 14 turns of paired Teflon-sleeved 14AWG Thermaleze.
Bench test with MFJ259B and 200 ohm 1% resistor load:
1.0 SWR across 160m and 80m
1.1 SWR across 40m
1.4 SWR across 20m
1.6 SWR across 15m
1.8 SWR across 10m

I should be flat 1.0 everywhere.  I'm guessing the Teflon sleeve is increasing the impedance of the paired transmission line above the ideal 100 ohms.   I could remove the Teflon to get the 100 ohms, but will I give up being able to run 1500watts?  Is the Teflon overkill?

Operational test with a 69-ft 40m OCF Dipole fed 20% from end, up 70 feet and Z measurements taken at end of 70-ft feedline.

40m SWR runs 1.6 to 2.0
20m SWR runs 1.9 to 2.1
15m SWR runs 1.7 to 2.0
10m SWR runs 1.9 to 2.1

So I may be looking at fine-tuning the feedpoint location as well.

BTW, I am interested in the 20% feedpoint design so I CAN use 15m.

 

Or I could just get to the point and ask what kind of toroid should be used for a good high choking impedance?

How about the FT-240-K, two of them wound 1:1, connected in a 4:1 Guanella-style?

Well I thought some time on the web might reveal an answer but I guess not....

So then,  how does one estimate or predict the common-mode or choking impedance of a particular balun design???

And so now I understand.  Thanks Owen, Jerry, and Dave!

So now my task is to homebrew a suitable hi-CM-Z 4:1 balun.

If that fails, I guess I'll shell out for the higher-end commercial offerings that offer CM-Z data.

Thanks,
Steve KK7UV

Don't know if they are V or C baluns.  Manufacturers don't say, and glued PVC enclosures require a rebuild to find out.  I'll be attempting a homebrew 4:1 current balun for my next test as per a Guanella-type in Sevick's book.

I only used the model to locate the 200-ohm feedpoint for multiband operation.  I never automatically expect "actual" to agree with the model until I make the operational test - which is where I am at now.

I am suspicious of the baluns I've tried.  Antenna dimensions and feedpoint location seem less suspect because the higher bands would be most affected by errors in length and feedpoint location.  My last test shows good agreement between model and antenna on 20,15,10m.  It is the lack of any sort of impedance dip on 40m that has me perplexed.   

I've been attempting to build an OCF dipole.  Using EZNEC, I came up with 69 feet long, fed at the 200 ohm Z point (20% from the end) for a very useable 40-20-15-10m antenna.

I have built two of them now around two different commercial 4:1 baluns (RadioWorks and Atomic,N9HRP).   Hoisted to 70 feet high flat-top configuration, and reading impedance with an MFJ analyzer at the end of 70 feet of coax, I measure no SWR dip on 40m.  Although, there are nice dips on 20, 15, and 10m.  I get the same results hooking up my transceiver at the same location.  On the bench, both baluns measure near 50 ohms at 7 MHz with a 200 ohm resistor across the leads.   Not sure what else could be wrong now.  Also tried a different coax lead.  No other conductors anywhere nearby.  What am I missing?