Re: [HCDX]: Build a balun

[Vittorio De Tomasi <ik2czl@xxxxxxxxx>]

Mike wrote:
> 
> Hi,
> 
> I wonder if anyone out there could tell me how to build a balun for my long
> wire SWL antenna?
> >From what I have seen they look quite simple, but I can't find how many
> times I should wind each wire on the ring.
> Any help would be great.
> 
> >From Mike, QTH Gloucester UK.
> HF Radio's are: Sony 2001D and Yaesu FRG-100.
> VHF/UHF Radio is; Realistic PRO 2005.

Dear Mike, the choice of the balun depends from the configuration of
your long wire and from the coaxial cable you want to use.

If, as I believe, you are simply using a single wire antenna, its
impedance is given by the equation:

Zo = 138 Log  (4h/d)
            10
where:
h is the wire height above ground
d is the wire diameter
and of course it is supposed thath h>>d holds...

both h and d must be expressed using the same measurement units, and the
equation is of course assuming a perfect ground system, a wire that runs
straight parallel to the ground, and so on... 

Just as an example, if your wire is two meters above the ground, and the
wire diameter is 1 mm, you get Zo=538 ohm.

If you want to connect it to a 50 ohm coaxial cable (like RG-213), you
need as a balun a transformer that has a 1:10 winding ratio. You can use
say about four turns for the primary, and forty for the secondary
winding. Both winding should be done on a suitable toroidal iron-powder
core (Amidon FT series, and so on). If you have no Amidon toroidal cores
at hand, you could try to experiment with toroidal cores taken from a TV
or PC switching power supply: they are quite easy to find at flea
markets, and are usually manifactured by Siemens. I was able to use them
with very good results in the balun I built for my T2FD antenna till to
at least 30 MHz!

Hope the above is clear enough.

vy 73

Vittorio IK2CZL


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