In message , John Smith
writes
On 8/7/2010 9:56 AM, spamhog wrote:

I am rebuilding my 15+ yr old longwire+MLB antenna due to broken coax
etc etc.

I checked the DC resistance between the 3 terminals of the MLB:
IN - high impedance longwire tiepost
OUT - low impedance coax center conductor
SHIELD - coax shield.
Weird results:
IN-OUT = 2 ohm
IN-SHEILD = OUT-SHIELD = ~50 ohm (can't really notice a difference)

Inverting + and - does not change result, hence not an obvious
semiconducting corrosion cell.

AFAIK
- the circuit is just a very broadband impedance transformer
- there should be no resistor
...however I don't remember what the normal resistance might have
been.

QUESTIONS
1- what would be normal resistance values?
would near zero be reasonable?
2- guesses on what might have happened?
is 50 ohm compatible with a poor contact
3- did anybody crack it open?
suggestions for repair?

Thanks for any wisdom on this matter!

N1JPR

What many call a balun is not, it is an RF auto-transformer; The common
schematic on the net uses a 10 turn primary with a 29-30 turn
secondary; This is an auto-transformer, but it works fine in matching
an an antenna in place of a 9:1 balun, for general SWL.

A 9:1 auto-transformer:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12673

A true 9:1 balun (schematic down on the page):
http://users.belgacom.net/hamradio/s...0Balun_MLB.htm

One importance difference you will notice, the auto-transformer has two
windings, the cold ends of the two windings running to gnd/coax-braid.
The balun has three windings, or a trifilar winding, these 3 windings
are connected in series with the 50 ohm connection across one winding,
the high impedance connection is made across all 3 windings.
Transmission Line Transformer is also used to describe some types of
baluns. Baluns are frequently wound using coax for the windings ... etc.

In the above, I have made no difference between balun and unun, there
is a difference, and most often you find ununs employed in SWL, with
monopoles--unbalanced antennas. Baluns are utilized with
dipoles--balanced antennas.

Regardless of whether you use the 9:1 impedance transformer as a, 'unun'
or a balun, the obvious way of winding it is to wind it trifilar by
filling the ring with N turns, then interwinding with a further N turns
(so the wires never cross), then finally interwinding a further N turns.
That isn't made clear in the DXzone article.

The ON6MU balun seems to be wound with purpose made trifilar wire,
rather than ordinary off-the-shelf single wire, interwound. [Or maybe
it's just wound very, very carefully!] Some Cable TV equipment used to
use ready-made wire (both bifilar and trifilar) for the windings of
their miniature wideband torroidal 'transmission line' RF transformers.
There were typically 5 turns around the ring, and they were normally
connected as 2:1 turns ratio (4:1 impedance) or 3:2 turns ratio (2.25
impedance).

Whether the transformer ends up as a balun or an unun simply depends on
how you connect the windings, and where you make the external
connections.
--
Ian