RE: VHF Baluns

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From: JEVERHART@cayman.VF.MMC.COM
Date: Tue Mar 14 1995 - 10:35:37 EST


Peter, you queried:

> Just wondering if anyone has tried using those cheap cable tv baluns on
> HF?
> Regards,
> Peter N2KPY

and Jim, N0CT replied:

> I have used the little binocular balun ferrites found in the backs of VCR's
> etc. (not really sure what you mean by calble TV baluns). I'm currently
> using one as a power splitter in a txcvr I'm building. Although many will
> swear upon heaps of old QST's, Ham Radio Magazines, etc. that one can never
> be *absolutely* sure about unmarked ferrites, I think one can usually spot
> type 43 (which is what I'm 99 & 44/100% sure these suckers are made of).
> Type 43 just LOOKS a lot different than the other ferrite materials. Kinda
> sparkly. Kinda special.

> I would suggest winding some windings and try them out. If they don't work,
> sell them as VHF bartenderators or some such hokum.

> 73, Jim N0OCT

I want to beat the drum some more for a favorite technique of mine. You can
measure the baluns and ASSURE YOURSELF THAT THEY WILL WORK!

To do this, you simply measure the impedance of the windings in question
either directly or indirectly. I won't go into the internal mechanics of the
baluns, as that has been done both here and in several excellent publications
by Jerry Sevick, Wes Hayward, Doug Demaw and lots of others. A pretty good
idea of a balun's worth for QRP amateur radio applications can be gotten by
just measuring it as an inductor. The following ASCII sketch shows the balun
as an impedance transformer:

                         __________
                 o------| |------o
           LO Z | | HI Z
                 o------|__________|------o
                   
A crude rule of thumb is that the impedance of a winding must be at least 4
times the impedance it will see at the lowest operating frequency (I like ten
times).In the above picture, a tv balun is to be checked out for use at hf. AT
VHF the "LO Z" side would see 75 ohms and the "HI Z" side would match 300 ohms.
The ratio of resistances is set by turns ratios inside the device. The
operating impedance level is really set by the characteristic impedance of the
 windings but can often be ignored. So potentially the balun can be used to
transform 50 ohms to 200 ohms. So you can get some idea of the usability by
measuring the impedance of the windings as inductors.

For various reasons, the LO Z side is probably easiest to measure. Using an
inductance measuring device you can check the inductance at the operating
frequency, then calculate the impedance using the familiar formula:

Z = XL = 2 * PI * F * L

A more direct way is to measure the impedance directly at the operating
frequency using an impedance bridge or antoher device such as the Autek RF-1
ANALYST, or an MFJ SWR analyzer with "rf resistance" capability.

Just be sure that the impedance is at least 4 times the resistance level for
the winding at the LOWEST operating frequency. Only one winding must be
checked. If it is ok, the transformer's turns ratio will probably ensure that
the other winding is adequate.

If you are adventurous (and I hope you are), you can use the balun core to
wind your own. Just check the impedance of one of the windings as described
above to bes sure that the balun will work.

Purists will no doubt say that this method is not exact. They're right, but it
should get you in the ballpark for most low power usage over a few ham bands.

Try it an enjoy - Love my RF-1!

72/73,

Joe E. N2CX


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