IW5EDI Simone - Ham-Radio

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Magnetic Longwire Baluns

Posted: 07 Apr 2019 02:24 PM PDT
http://www.iw5edi.com/ham-radio/3172...ongwire-baluns



Do these things do exactly what they say they do or are they total hogwash?




Have you seen the ads? This device will match your long wire to your
receivers 50 ohm input? Firstly a long wire on HF would be on 28 MHz
longer than 120 foot and on the lower bands it just an end fed “EF�












We can also get rid of the “magnetic� and give the balun its true name of
voltage balun. My interest in the MLB was started after a friend went out
and paid ?30 for one and started to tell me how this little device will
produce a 50 ohm match at all frequencies. Any aerial will have differing
values of impeadence depending on what band it is used, a 66 foot wire on
80m is about 30 ohms and on 40m the impeadence is about 1000 ohms.




For any sort of balun to be able to match different impeadences to 50 ohms
you need to be able to vary one or more of its components. And when you can
do this, the balun will match most things to 50 ohms, but you would call
this type of balun an ATU. An ATU is a variable balun.




But theory is one thing and practice is another. So mainly for some thing
to do I decided to make some MLB’s and see what mystical properties they
have. The first balun was made “by the book� using the correct wire SWG and
ferrite ring with a type 63 core for 1 to 30 MHz. I used a friend’s MFJ
analyser for the tests and a 35 foot wire aerial.




The balun did improve the match to the receiver. Only near the resonant
frequency of the aerial was any match close to 50 ohms or a VSWR of 1 to 1,
as you would expect. On all other frequencies the match varied from 3 to 1,
to 5 to 1 VSWR. Without the balun the match was horrible!




In the real world this means I had some thing like a increase of signal
from 1 to 4 “S points� on the receiver from 1 MHz to 30 MHz.




The “VMR version� MLB was made from scrap. 3 core 2 amp lighting flex and
an old ferrite rod. The results are just the same as the posh version!




You need about one yard of 3 core, 2 amp flex and a ferrite rod with a
length of about 6 inches or longer.




Using 3 core flex it is easy to wire because it is colour coded. Tape one
end of the flex to the rod leaving 6 inches for the connections, then wind
on as many turns as possible (mine has 10 turns) and again fix the wire
with tape etc at the other end. Now you can just twist the wire connections
together or better solder them. Connect it up as shown. Blue right hand
side to Green left-hand side. Brown right to Blue left. Inner of COAX, to
Brown/Blue. Braid to single Green. Aerial to single Brown. Then just bung
it into a box.




So is a MLB worth having? They do produce a “better� match to the receiver
but you could get an ATU kit for about the same price, which will always
produce the right match. If you buy surplus VC’s you can make an ATU for
under ?10. Find a couple of broken tranny radios and you can make one for
free.




The main advantage of the MLB is you can use COAX to the feed point of the
aerial out side and get a reduction in noise pick up from TV’s etc in your
home. I would not spend money on one, but a home made MLB in a box is worth
having. Because it is a pain to keep peaking the ATU as you tune up the SW
bands and anything that helps to give a better match is better than having
nothing at all.

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