[Premium-Rx] The 'Ideal' Logwire

[Ahmet Gundes]

Hello,

Most of the literature on Baluns is talking about how to build them. There
is no mention of their restrictions of operation and where they are
applicable and can be used to take advantage of their functionality. Including
the ARRL publications, there is no mention of one simple issue which
makes the baluns tick:

Baluns are just like transmission lines. The cunstruction of baluns is actually
a simple two wire transmission line with a certain Zo characteristic impedance
wound on a core.  The two wires are simply carrying the signal and in the
process there is a balanced to unbalanced functionality.  Just like any coax
cable and any transmission line, these two wires running around the toroid
(in most cases ) have a Zo charact. impedance and behave exactly like a
transmission line.  A transmission line is also an impedance transformer,
therefore you see impedance of the antenna change when using the Balun.
Just like the best way to use a coax cable is to make sure the load matches
the Characteristic Impedance Zo, you need to do the same for baluns.
No one eve mentions that Baluns are most effective as long as the load
is in the vicinity of their charateristic impedance.  So this is the trick to
baluns.  Most Baluns are in the 50 - 75 ohms ( 30 - 100 also ) range.

So in order to actually use baluns the best way possible, the load impedance
either better be Real ( R+ jX with X being low ) or a unpredictable
impedance transformation will occur ( since you do not know the length of
wire in the balun, you cannot determine the output impedance ). In the
case of having a Real Impedance as the load, the Balun will also make it
go through an impedance transformation, however the closer to the Zo or
Balun the less. In either case a Real impedance is easier to handle than
a complex one.

The Solution is to First Match the Antenna at the Feed Point with a
Balanced Matcher and then use the Balun to feed the Coax.  For receiving
systems this makes life hard as you need to have a remote matching unit
which is hard to make ( need to use Microcontroller or Motor ).

One way to confirm that Baluns work best with Real Impedances close to
the Characteristic Impedance of Baluns and to find out what this Zo is, is
to connect a simple Variable Resistor 0 - 100 Ohms or 0 - 470 Ohms and
connect the other end to a AEA CIA-HF and see how flat the line is
with a Real Impedance and then connect a capacitor or inductor in series
with the resistor and you will see that the output of the Balun is now totally
different picture.

Also note the Description "Broadband Balun",  this does NOT mean that the
balun will function as a balun in a broad band, it means that it will function
as a Balun in a broad band as long as the criteria of feeding it with a Real
impedance is met.

In addition most of the Short length antennas for VLF, such as 20 Feet long
wire, even if they are placed horizontally will have Vertical E Field Patterns
especially the Monopoles ( long wire ).  So the fact that you see signal gain is
perhaps due to polarization matching at that frequency with a station having
a vertically polarized signal ( common in the 5.8 - 5.9 MHz Brdcst band ).
I would go for a dipole more than a long wire.  Also Loops are great for
VLF work as well as well entire HF bands.

I spent a lot of time on the dipoles as well as loops and baluns. I can assist
if you need more details.

Regards
Ahmet



NewRadioChap at aol.com wrote:

> Does anyone know if there is an 'equivalent' circuit (or lab set-up) that will
> simulate a typical logwire (single-ended) antenna working into a co-ax? I don't just
> mean the theory - I am looking for practical work.
> I have been doing experiments and research on broadband balun designs and
> construction - particularly VLF and HF (approx 10KHz to 10MHz) to optimise my
> receive capability using a variety of longwires from 20ft to 200ft. I've found that
> most commercially made broadband balun transformers for receive do not appear to
> work very well - if at all - and have no benefit. My homebuilt ones don't fair any
> better. When frequency swept, all exhibit distinct bandwidth/resonant peculiarities.
> In fact, by incorrectly connecting certain types of balun, I get a voltage boost
> (gain) at LF, with high loses at HF! This is confirmed on the antenna sweep. There
> must a pile of work previously carried out on this that someone can point me to, I'd
> much appreciate it. If I can find a simulation, it will save me putting life at risk
> trying to do measurements up a 40ft tree, and instead do them indoors! Every dB
> counts....
>
>
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