A very Popular Misconception
 

It is a very popular misconception that METAL in the vicinity of a
ladder-line or open-wire line causes bad loss in the line.

It doesn't! Simply because metals have a very low resistance. Line
conductors are made of it.

There IS an effect however, the magnitude of which depends on whether the
foreign metal is in small pieces, as with staples or clamps, or continously
distributed as when an insulated line is lying across a galvanised
corrugated iron sheet, touching only at intervals.

Each of the two wires has a stray low-loss capacitance to the foreign metal.
The pair of stray capacitances are in series which approximately halves the
effect. The effect is a decrease in Zo.

Now line attenuation in dB per 100' is directly proportional to R / Zo where
R is line conductor resistance. So the small extra capacitance does in fact
result in a negligible increase in line loss. But even less loss is induced
in the foreign metal especially if it is bulky or has a large surface area
and is not resonant.

The small increase in capacitance is equivalent to an un-noticeable shift in
tuner settings.

Where loss DOES occur is when the close-by foreign material is lossy such as
the surface of damp garden soil. Loss falls off again when the material is
an insulator such as dry sand although the increase in capacitance still
occurs.

Maximum loss occurs when the foreign material has a resistivity which
fortuitously "matches" into the line impedance.

But the existing rule still applies - avoid suspending twin-line nearer to
ANY material less than 3 or 4 times conductor spacing over appreciable long
distances. Even though copper sheet may be OK.
----
Reg, G4FGQ

Hi Reg

Thanks for the post!

I just wanted to add something that has never made sense to me.

I had a section of ladder line that I affixed to the roof joists under
the eaves using thumb tacks. Three of these thumb tacks kept coming
out on me, the rest held just fine. It was always the same three.
The last time I used three push-pins with plastic heads and found the
heads missing from the push-pins shortly thereafter.
It wasn't until I discovered the puddles of plastic on the ground that
I realized that they had melted. Yet the ladder line itself seemed
unharmed.
So I picked up a small box of nylon screws at the hobby shop and
replaced all of the thumb tack and the three push-pins with these
nylon screws.
After doing same, I had to make adjustments to my tuner on a few
bands.

After that I never had a problem again!

I never noticed a change in transmission or reception before or after.
But feel the RF is what caused these thumbtacks to keep coming out,
probably burning the hole in the rafter larger and larger each time
and that could be what melted the heads of the push-pins off.

The three thumbtacks that kept coming out were equal distance from
each other, about 64 inches apart. There was a thumbtack every 16
inches along the part of the ladder line that ran under the eaves.

TTUL
Gary

"Gary V. Deutschmann, Sr." wrote
Hi Reg

Thanks for the post!

I just wanted to add something that has never made sense to me.

I had a section of ladder line that I affixed to the roof joists under
the eaves using thumb tacks. Three of these thumb tacks kept coming
out on me, the rest held just fine. It was always the same three.
The last time I used three push-pins with plastic heads and found the
heads missing from the push-pins shortly thereafter.
It wasn't until I discovered the puddles of plastic on the ground that
I realized that they had melted. Yet the ladder line itself seemed
unharmed.
So I picked up a small box of nylon screws at the hobby shop and
replaced all of the thumb tack and the three push-pins with these
nylon screws.
After doing same, I had to make adjustments to my tuner on a few
bands.

After that I never had a problem again!

I never noticed a change in transmission or reception before or after.
But feel the RF is what caused these thumbtacks to keep coming out,
probably burning the hole in the rafter larger and larger each time
and that could be what melted the heads of the push-pins off.

The three thumbtacks that kept coming out were equal distance from
each other, about 64 inches apart. There was a thumbtack every 16
inches along the part of the ladder line that ran under the eaves.

TTUL
Gary

========================================

Hi Gary,

Very peculiar. But what it wasn't due to was I-squared R losses in tiny
metal objects such as thumb tacks. You were not working in the GHz range.

Always the same set of tacks could have had something to do with voltage
standing waves on your most-used favourite band.

The structural material was timber which has a highish dielectric constant
and a poor power factor.

A thin insulating material on the metal pins would not make much difference.

Tacks and pins are very small and pointed. The voltage stress would be very
high in a very small volume of timber. Arcing could possibly, but need not
occur. But only a small amount of energy is needed to take the tacks and
timber up to a high enough temperature to dry out the timber, contract the
timber, and allow the tacks to fall out under weight of the line and
gravity.

And as mentioned in my first message, the poor power factor or resistivity
of the timber at HF could have resulted in an impedance match between the
timber and line Zo.

You didn't burn the house down because of your gravity-operated safety
precaution. ;o)

I would guess you were using more than 100 watts.
----
Reg, G4FGQ

Thanks Reg

Never saw any damage to the ladder line, but figured it must be the
points along the line these particular thumbtacks were placed.

No power other than what the TS-830Sugar puts out.
But I was using a Tuner if that made any difference on the line.

TTUL
Gary


"Reg Edwards" verbositized:


"Gary V. Deutschmann, Sr." wrote
Hi Reg

Thanks for the post!

I just wanted to add something that has never made sense to me.

I had a section of ladder line that I affixed to the roof joists under
the eaves using thumb tacks. Three of these thumb tacks kept coming
out on me, the rest held just fine. It was always the same three.
The last time I used three push-pins with plastic heads and found the
heads missing from the push-pins shortly thereafter.
It wasn't until I discovered the puddles of plastic on the ground that
I realized that they had melted. Yet the ladder line itself seemed
unharmed.
So I picked up a small box of nylon screws at the hobby shop and
replaced all of the thumb tack and the three push-pins with these
nylon screws.
After doing same, I had to make adjustments to my tuner on a few
bands.

After that I never had a problem again!

I never noticed a change in transmission or reception before or after.
But feel the RF is what caused these thumbtacks to keep coming out,
probably burning the hole in the rafter larger and larger each time
and that could be what melted the heads of the push-pins off.

The three thumbtacks that kept coming out were equal distance from
each other, about 64 inches apart. There was a thumbtack every 16
inches along the part of the ladder line that ran under the eaves.

TTUL
Gary

========================================

Hi Gary,

Very peculiar. But what it wasn't due to was I-squared R losses in tiny
metal objects such as thumb tacks. You were not working in the GHz range.

Always the same set of tacks could have had something to do with voltage
standing waves on your most-used favourite band.

The structural material was timber which has a highish dielectric constant
and a poor power factor.

A thin insulating material on the metal pins would not make much difference.

Tacks and pins are very small and pointed. The voltage stress would be very
high in a very small volume of timber. Arcing could possibly, but need not
occur. But only a small amount of energy is needed to take the tacks and
timber up to a high enough temperature to dry out the timber, contract the
timber, and allow the tacks to fall out under weight of the line and
gravity.

And as mentioned in my first message, the poor power factor or resistivity
of the timber at HF could have resulted in an impedance match between the
timber and line Zo.

You didn't burn the house down because of your gravity-operated safety
precaution. ;o)

I would guess you were using more than 100 watts.
----
Reg, G4FGQ