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[KD7RDZI2]

How would perform a twisted military wire (I believe it's characteristic impedance is about 100 ohms ) feedline and then opened as a doublet? Would the feedline still be low loss or a 450 ladder line is better? Mechanically the twisted military wire seems very strong albeit very light. I have a basic MFJ tuner with an internal voltage 1:4 balun. Given the rather low impedance of the feedline would you prefer to use that internal balun or would you use a common mode filter between the tuner and the feedline or the common mode filter between the tuner and the transceiver?

[N2EY]

How would perform a twisted military wire (I believe it's characteristic impedance is about 100 ohms ) feedline and then opened as a doublet? Would the feedline still be low loss or a 450 ladder line is better? Mechanically the twisted military wire seems very strong albeit very light.

It's strong and light because it's mostly steel rather than copper.

IMHO, you're better off with real copper antenna wire and open-wire line, designed for RF.

Here's why:

1) Steel is more lossy than copper, particularly at RF.

2) The characteristics of the insulation at HF are unknown.

3) The behavior of the wire (mechanically and physically) when strung up as a dipole is unknown.

That said, such a dipole will probably "work" to a certain extent, because almost any wire antenna will "work" at HF. How much loss there is depends on a bunch of factors, and there's no easy way to calculate it.

73 de Jim, N2EY

[WA3SKN]

There are a lot of variables here.
I would not go out of my way to purchase it... however, if you already have some then why not try using it?

-Mike.

[WB6BYU]

How would perform a twisted military wire (I believe it's characteristic impedance is about 100 ohms ) feedline and then opened as a doublet? Would the feedline still be low loss or a 450 ladder line is better?

“Better” in what regard?

It certainly will “work” if that is what you have handy.
Losses likely will be higher than for 450 ohm line,
but may be a reasonable choice if the light weight and
small size are important to you.

There are three main sources of loss in a transmission
line:  the wire diameter, the insulation type, and
the type of metal used in the wire.


For copper wires using polyethylene insulation, the
effective resistance of the wires is often the limiting
factor.  Larger diameter wires have less resistance.

However, there is also the impedance of the line
to consider:  a higher impedance line tends to have
higher voltage and lower current (when averaged
over the length), and the lower current reduces
losses in the wire.


While polyethylene has low losses, PVC and many
other plastics not designed for RF use can increase
losses, sometimes significantly.  This often is a
problem when using speaker wire or zip cord.


The losses in magnetic steel increase with frequency,
as at each reversal of current some power gets used
in reversing the magnetic field of the wire.  Stainless
steel, on the other hand, isn’t as affected as much
(depending on the type).

With Copper-Clad steel, the thickness of the copper
is important:  it has to be several times the skin
depth to keep most of the current in the copper
instead of in the steel.  As a result, losses may
actually increase as the frequency is lowered
below a certain frequency.  Stranded copper-clad is
even more lossy, as the effective copper thickness
is just that of one of the strands.


So there are lots of factors.

I would expect it to have higher losses than full-
sized 450 ohm types, especially if it has steel
conductors.

Fortunately, it isn’t too difficult to check the
cable loss at different frequencies if you have
an antenna analyzer or VNA.  Stretch out a
length of cable, short the far end, and find
the resistance at those frequencies where
it drops to a low value.  Then open the far end
and take a second set of measurements (which
will be at different frequencies).

The cable loss can then be determined at each
of the frequencies the method described here.

I have a basic MFJ tuner with an internal voltage 1:4 balun. Given the rather low impedance of the feedline would you prefer to use that internal balun or would you use a common mode filter between the tuner and the feedline or the common mode filter between the tuner and the transceiver?

I would nearly always choose to use a 1 : 1 current
balun (feedline choke) between the feedline and
the tuner.  The tuner likely is a T network type, and
they tend to be more efficient matching higher
impedances (up to 1000 ohms or so) than lower
ones (under 50 ohms).  Using a 4 : 1 balun at
that point we’ll reduce the impedance the tuner
must match.


Are you planning on using this antenna for as a
single band dipole, or as a doublet on multiple
bands?   If the impedance is really close to 100
ohms (it may be higher with small wires) then
I’ve had good results using a flattened loop
with a 200 ohm feedpoint impedance in place
of the dipole, and a 1/4 wave matching of 100
ohm line as a matching section.  The loop can
be erected in the same manner as a horizontal
dipole, and the tie points at the corners adjusted
match the exact line impedance if desired.

Note, however, that small speaker cable is
often around 140-150 ohms, and the loop
impedance can’t be raised high to match
with such lines.

[K0EKL]

I don't use it as feedline, but I do use WD1A wire for dipoles, including my 160-meter inverted V with the apex at 72 feet. It think it works just fine. It's tough, strong wire, especially for it's size.

It's a combination of steel and copper conductors so I make a point of ensuring that all connections are well waterproofed because I don't want the steel strands to rust. If they do rust away the strength of the wire will be greatly compromised.

[K0CWO]

https://radionerds.com/images/3/34/MIL-DTL-49104C.pdf

Info and specs.

k0cwo

[KB1MCT]

All technical information aside, it makes great cheap dipole wire. It was made to withstand being run over by heavy vehicles and getting hit with artillery strikes.

You paid $80 for a KILOMETER of it. If it breaks, you're out less than a dollar for an 80 meter antenna. Make another one. It's strong, and takes solder pretty well. I think my roll is haunted by the souls of dead Vietnamese guerilla fighters, but that's another story all together.

On the negative, I've found it holds a memory and loves to kink, and the ends take bloody joy in stabbing you in the fingers.

[WB6BYU]

Info and specs.

Thanks!

That doesn't tell us everything we need to know, but it's
a good start...

The good news is that it has polyethylene insulation,
which as low loss at RF.

Owen Duffy has a good reference to the impact of wire material
on antenna performance, with some other useful links, of which
the one from Rudy N6LF may be particularly enlightening.

The primary issue is the magnetic properties of the
steel in the wire, which isn't specified in the military
specification.  However, since it calls for a galvanized
coating, I would assume that it isn't stainless steel,
and so likely is magnetic (some types of stainless
steel are, too.)

In this case, we have copper strands and steel
strands tightly coupled together.  Because they are
tightly coupled together, RF is going to flow in
both types of conductors.  The maximum permitted
loss is 5 dB / mile at 4 kHz, which seems pretty
good, but the curve goes up steeply, and the losses
at RF will be affected by the steel.

Looking at the second table in Owen Duffy's article,
it shows a 40m dipole made using #32 copper wire
loses 8% loss in the wire - relatively insignificant.  On
the other hand, an 80m dipole using #19 steel wire
has a loss of 84% that certainly can make a difference!

As a guide, Figure 5 in N6LF's article shows the wire
loss vs. frequency of Copper-Clad steel wire vs. the
thickness of the copper coating.  Generally, RF wire loss
will drop with decreasing frequency because of the skin
effect:  more RF flows deeper into the wire rather than
just on the surface.  But with a steel core, the center
of the wire has much higher loss.  As a result, the
curve of RF resistance flattens out at low frequencies,
and that is where the efficiency problem is most
acute.

The wire he measured with the thinnest copper
coating was about the same as copper at 7 MHz,
but stayed relatively flat as the frequency dropped
from there.  Stranded CopperWeld was worst
than solid types, because the effective thickness
of the copper was that of one individual strand.


So the presence of the steel strands likely will
increase the RF resistance of the wire, and it
seems clear that straight steel wire generally
isn't a good choice (although sometimes you
need to use whatever you have on hand).

And, as Rudy pointed out in his article, the
specific application makes a difference, too.
At higher impedances (i.e. lower currents),
the wire loss has less of an impact.

It would be interesting to actually test a piece
and measure the RF characteristics...