Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

Thanks de Pierre VE2PID

ve2pid wrote:
Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

That's _Baluns: What They Do and How They Do It_, in the _ARRL Antenna
Compendium_, Vol. 1, and also at
http://eznec.com/Amateur/Articles/Baluns.pdf.

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

Not if it's a good dummy load. A good dummy load is shielded so that all
the current from the center conductor stays on the inside of the shield.
This current then returns on the inside of the coax shield.

We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

Ok, let's look at a poorly designed dummy load, say a 50 ohm resistor
connected with pig tails to the coax feedline. Remember that the current
on the outside of the inner conductor is *always* equal and opposite in
phase to the current on the inside of the outer conductor. Let's call
that current I1. So the lead connected from the coax center conductor to
the resistor has I1 flowing. If the pigtails are very short with respect
to a wavelength, the current in the whole coax - pigtail - resistor -
pigtail - coax loop is essentially the same, I1. So where the other
pigtail connects to the coax shield, it carries I1 which all has to flow
on the inside of the shield (so that the current on the inside of the
shield equals the current on the center conductor).

But a dummy load radiating 10 uW, say, would drive a nearby receiver
bonkers. If you put 100 watts into the dummy load, that means only
1/10,000,000 of the applied power has to radiate, and this can be hard
to prevent. The pigtail - resistor - pigtail loop length isn't exactly
zero, so there will be a little difference in the current at one end
than the other. And this difference current will end up on the outside
of the coax shield. In addition, there's probably some small amount of
coupling from the resistor and pigtails to ground, the feedline, and
other conductive objects. This will result in a displacement current
that causes current imbalance and consequent current flow on the outside
of the feedline. The only way to prevent this is to completely shield
the dummy load and transition to the feedline, and have no seams, holes,
or cracks which could let any current get from the inside to the outside.

I've had limited experience with leaky coax, but have found a
substantial difference between single and double shielded cable in the
amount of signal radiated. Connector assembly is also critical -- it has
to be done just right, or you'll end up with one of those sneak paths
from the inside to the outside, regardless of how good the shield is.
You usually won't have to deal with this, though, unless you need a
pretty high value of attenuation.

Roy Lewallen, W7EL

ve2pid wrote:
Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

Thanks de Pierre VE2PID

Hi Pier
There can be several reasons for a dummy load radiating a signal but
common mode current is not likely to be one of them. In a good dummy
load the load resistor is inside a shielded container. If the container
is RF tight the RF current is on the inside of the box and not on the
outside, just like a coax transmission line all the signal of interest
is on the inside surface of the shield and the surface of the center
conductor. Skin effect works on shield boxes too. There should be little
to no RF current flowing on the outside of container or the coax shield.
One place the radiation could come from is from the coax itself. The
shielding on typical coax is 95% coverage. This is a source of leakage.
Connectors are a problem too. Pl259's are not a very RF tight connector.
Type N's are better but there are some that are even better. Hard line
is 100% shielded it would not be likely to radiate but it can be spoiled
by sloppy connector installation. The most likely reason for any
radiation is poor shielding of the dummy load. Typical ham grade dummy
loads are not very well shielded. Often when using a dummy load and one
finds that a signal is being radiated it easy to blame the dummy load
when in fact it's the transmitter. Transmitters are not often all that
RF tight. The only way to tell is to probe around with a field strength
meter and find were the signal is coming from. This rambles all over the
place, hope it makes some sense to you.

73

John Passanau W3JXP

ve2pid wrote in news:66e9aa1e-ad1c-4dc8-b324-
:

Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

If the dummy load is effectively screened, and to the extent that it is
effectively screened, there is very little current coupled to the outside
surface of the outer conductor. That is very different to the feedline of
an antenna which usually has significant mutual coupling to the nominal
radiator.


We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

The through braid loss (ie from inside of outer conductor to outside of
outer conductor) is not infinite, so there is some leakage and it depends
on cable construction and frequency. There is also leakage from the dummy
load enclosure, transmitter enclosure, and other interfaces to the
transmitter (eg power cable).

Owen

Thanks de Pierre VE2PID

ve2pid wrote:
Hi to all

In a document from Roy Lewallen, we read this:

When a balanced antenna is fed with coaxial cable (Fig.1),
the outside of the shield appears as an extra, separate
conductor connected to the right side of the antenna at the
feed point. (page 157. What They Do And How They Do lt)

My question is this: Can we say the same thing for a dummy load (i.e
can we replace the words 'balanced antenna' with 'dummy load'?) so the
dummy load would 'see' three conductors?

We are trying to explain why some dummy load/Tx line systems radiate.
Perhaps, tthe current outside the coax (common mode current) exits,
while being very small, so the radiation could come from a leaking
coax... ?

Thanks de Pierre VE2PID

To cut to the chase, and past the BS ... with the best affordable dummy
load and solid coax, I can still chat a block down the street, at least
(with decent power.)

Now, how this is occurring? Is it "leakage" from the xmitter onto the
ac lines? Coupling to metal objects in the shack? Etc.?

Gawd only knows ... nothing is perfect ... when you find an answer to
this mystery, please post back here; I am positive many more than me
have wondered the same as you ... :-)

Regards,
JS

On 23 nov, 21:42, Roy Lewallen wrote:

When *a *balanced *antenna *is *fed *with coaxial *cable (Fig.1),
the *outside *of *the *shield appears *as an extra, separate
conductor *connected *to the *right side of *the *antenna *at *the
feed point. (page 157. Baluns: What They Do and How They Do It_, in the _ARRL Antenna
Compendium_, Vol. 1, and also athttp://eznec.com/Amateur/Articles/Baluns.pdf.

Fb Roy and thanks (to all) for the explanations.

But... returning to your document, I wonder why a balanced antenna
'see' that third wire, but the dummy load does not recognize that
third conductor. Is it because the common-mode current outside of the
shield has something to do with a kind of RF coupling?

ve2pid wrote:

Fb Roy and thanks (to all) for the explanations.

But... returning to your document, I wonder why a balanced antenna
'see' that third wire, but the dummy load does not recognize that
third conductor. Is it because the common-mode current outside of the
shield has something to do with a kind of RF coupling?

Current doesn't flow one way or another because the path is "seen" or
"recognized". It has to obey Kirchoff's Current Law, which it might be
useful to review. It states that the (vector) sum of currents into or
out of any node must equal zero. In this case, the node of interest is
the connection of the dummy load to the coax shield, which is
effectively the connection of three conductors: the conductor to the
dummy load, the inside of the coax shield, and the outside of the coax
shield. If the current on the inside of the shield equals the current to
the dummy load, the current on the outside shield equals zero per
Kerchoff. And this is what happens if the currents in the two dummy load
leads are equal and opposite in phase. Similarly, if the current to the
two halves of a dipole are equal and opposite in phase, the current on
the outside of the coax is zero, and conversely. The whole article is
based on that principle, and explains how it can be accomplished.

Please look at Fig. 1, and the currents going to each side of the
dipole. If the two coax conductors go to opposite sides of a resistor
rather than to two sides of a dipole, those currents have to be equal or
nearly equal, as I explained in my last posting. If they are equal,
there is no extra current available to go along the outside of the coax.

You asked about RF coupling. I explained in my last posting that
coupling to various objects could cause the currents in the two dummy
load wires to be unequal, resulting in current on the outside of the coax.

Sorry, I'm running out of ways to explain this. You might try reading
the other responses if you still don't understand.

Roy Lewallen, W7EL

ve2pid wrote:
But... returning to your document, I wonder why a balanced antenna
'see' that third wire, but the dummy load does not recognize that
third conductor. Is it because the common-mode current outside of the
shield has something to do with a kind of RF coupling?

Probably because the dummy load is a resistor not referenced
to ground. The antenna feedpoint impedance is a resistance
referenced to ground, i.e. not a resistor. The "third wire"
is also referenced to ground. You might consider it as a
kind of RF ground loop.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:
The only way to prevent this is to completely shield
the dummy load and transition to the feedline, and have no seams, holes,
or cracks which could let any current get from the inside to the outside.

A good example of such a dummy load is the Bird Termaline
Coaxial Resistor. I've got model 8020.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
news
Roy Lewallen wrote:
The only way to prevent this is to completely shield
the dummy load and transition to the feedline, and have no seams, holes,
or cracks which could let any current get from the inside to the
outside.

A good example of such a dummy load is the Bird Termaline
Coaxial Resistor. I've got model 8020.
--
All the above, N connectors and double shield coax. RG8X types seem to be
the worst for radiation, although it does have other redeeming features. I
take it you have RF in the shack even on the dummy? Don't forget about
other RF paths like the audio shields and data lines. Regular 95% shielded
coax is said to have isolation of about 60 dB (RG8, 58c) maybe as much as 90
dB for double shielded. Unless there is some bump in the line that causes
the shield to radiate. There will be resonances in the entire line that
cause points of high voltage and high current.