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Ferrite cores for balun

I am a relative newcomer so forgive me if I am repeating a topic.
I have some FT-82-75 cores, would these be suitable for making a 9:1 balun for 40-20? Mainly I ask if they will handle the 5 - 10 watts okay.
I have a bunch of others that are un-marked but I don’t know it they will work. How critical is the mix type?

Hi Thomas,

Material 75 is good for up to 10 MHz according to AMIDON data. For your 40m Balun the 75 would be ok, but for 20m I’d use material 43 or 77. The 75 should be ok for QRP.

73
Peter, OE5AUL

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Depending on what antenna your running surely have effect what Balun or Unun your going to run.

I use two end fed portable antennas 1/4w 40m vert and inverted L 40m 1/2w both fed by 9:1 Unun with 1/4w counterpoises cut into four lengths/ Baluns better for center fed Di poles 4:1.

Never forget me first contact off it via 10w straight into Australia from SE UK from G/DC-003

karl

Hi Thomas,

Those cores will be fine. Use three cores for a Guanella type of circuit. The high coupling inherent in transmission line baluns means the cores have more effect on the common mode current than the matching.

To check you need an analyser and a 450 ohm resistive rf load or make up two sets and connect them back to back into a 50 ohm dummy load and run your tests. With two baluns you can use your tx and SWR meter to evaluate them.

Once you have a balun using the "good " cores you can build others with the unknown cores- (they should be the same even if unmarked) - and test them. Wind 10 turns on each and measure the inductances. They should be equal to a few percent if of the same type.

The balun windings should have enough inductance so that they have an impedance of no less than 1,000 ohms at the lowest frequency.

73
Ron
VK3AFW

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Thank you all for the feedback. I have a commercial 4:1 balun but been told 9:1 would be better for an EFHW which I will try first. 73 Hope to get active in S. AZ soon.

Hi Thomas,

End fed half waves are not my favourite antennas, but you can get QSO’s with them and many people like them.

The 9:1 ratio recommended is probably the turns ratio, not the impedance ratio. The end impedance is thousands of ohms so an 81:1 ratio, 50 to 4,050, gets close to matching. What you then have is an rf transformer, not a balun. Losses will be higher than other feed arrangements for a dipole but you won’t be making any smoke at 5 W. At 10 W you may get some rf on the mike cable leading to feedback or an rf nip. Many people seem to have no trouble with their end fed arrangements but the arrangement relies on the outside of the coax and your rig and mike lead to be a counterpoise… Some include a capacitor across the transformer to bet a better match on one band. Your cores should be OK.

I recommend feeding the antenna 10% from one end, about 6 1/2 feet for a 40 m half wave. Then your 9:1 balun would be more appropriate. You might be able to get an acceptable VSWR without a tuner on more than one band.

Have fun.

73
Ron
VK3AFW

Interesting. I have a 20 metres long wire fed by a 9:1 unun which is working well but I have wound several turns of coax round a ferrite core not far from the unun and another one near the tuner, and get no RF problems and more to the point, reduced local noise pickup. In effect there is no counterpoise. As I say, it appears to be working well, but I will experiment by adding a wire counterpoise and see what happens.

Brian

Hi Brian,

You put those ferrites on for a reason. You effectively have defined your counterpoise and isolated the rig. And it works for you so that’s all that’s counts.

If you feed current into the end of a wire an equal amount of current of the opposite polarity must flow in the opposite direction. (Law of Conservation of Charge), or one of Maxwell’s laws that says current into a closed sphere must equal current out of that sphere)

Now that current might flow into the outer of your coax via direct connection or via a couple of pf of capacitance to that braid. It has to flow somewhere.

The counterpoise or a radial on the rig does not have to be resonant, just big enough to keep the high voltage off your mike or key.

73
Ron
VK3AFW