Re: [HCDX]: Build a balun

[Nick_Hall-Patch@xxxxxxxxxxxxxxx (Nick Hall-Patch)]

>
>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).

Vittorio and the group...

A problem with the above is that the IMPEDANCE ratio is 1:10   

The winding ratio is the square root of that, so is closer to 1:3.2

In fact, it's not easy to derive the impedance of a random length inverted-L
antenna, but a good guess at the tropical and mediumwave frequencies would
be in the 1000 ohm range, so the impedance ratio of the transformer could be
1:20 .

Overall, the winding ratio for a random wire matching transformer could be
in the 1:3 to 1:5 ratio or even higher; you will notice an improvement in
received signal strength with any of those unless your receiver has a high
impedance antenna input.  More importantly, you can often get improved
signal to local noise ratio by putting the matching transformer at the point
where the antenna lead-in is closest to ground and using a separate ground
for the antenna (and the antenna winding of the matching transformer) from
the chassis (and coax) ground of the receiver.

For clearer details, contact me directly...schematic diagrams are tough to
do with just text.

In other news, slight signs of the Norwegian  on 1314 recently...just a
carrier, and noted best around 0400 17 Oct UTC here in British Columbia.

best wishes,

Nick




      
                   
                   
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Nick Hall-Patch
Victoria, B.C.
Canada

e-mail:  Nick_Hall-Patch@xxxxxxxxxxxxxxx
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