Attenuation Questions
 

On Tue, 11 Mar 2008 16:00:11 +0000, Ian Jackson
wrote:

Even at 1GHz, I don't believe that you would get as much as 2dB of loss
even if you joined two pieces of coax by stripping the ends with a knife,
twisting the conductors together, and wrapping them in scotch tape.
--
Ian

It is surprisingly easy to get significant loss at 1GHz and above, 2dB would
be very easy to achieve even when using what appears to be a 'good'
connection.

regards
Jeff

I have to admit that I've routinely worked to 870MHz, and recently even
to 1003MHz, but rarely ventured higher. However, I've never found any
excessive loss in a connector which could not be accounted for by a
straightforward 'bad connection', ie usually a total or partial open or
short circuit, whether in the making off of the cable, or in the
construction of the connector itself. Honest!

Hi All,

Anyone can do it wrong (come up with 2dB loss), but we would have a
great more deal traffic here if doing it wrong was that common.

I have measured attenuation of near everything from DC to 12GHz at the
standards bench and RG58 with BNCs for a short run (2 to 3 meters)
never presented any problems being wildly imagined here. At the
worst, the connectors (note plural) "might" show 0.2dB mismatch loss -
trivial.

On the other hand, if Jeff is so sure of this 2dB figure, it should
reveal itself in blisters to the fingers for even mild power (100W).
Since this unequivocal evidence is so obviously missing from the
records (or the testimony, as the case may be); then it is very
apparently one of those "someone heard from someone else about their
brother's friend's boss who had a customer who made this claim."

73's
Richard Clark, KB7QHC

Attenuation Questions
 

Robert11 wrote:
Hello,

First, let me say thanks to everyone for the replies and help.
New at this, and it seems these very basic questions of mine keep coming up
the more I look into it.
Great hobby, and really do appreciate the help, very much.
Super newsgroup for folks like me. Sure is a lot to learn re antennas;
reading the ARRL book on it now.

Would like to modify my question somewhat.
Was really surprised to learn about the copper on steel, vs all copper
center conductors on coax.
I thought the difference were only in types of shielding; not center
conductor construction, and that RG6, e.g., was RG6, at least concerning
attenuation and perhaps "quality". Guess not ?

I understand, and am aware of now, the skin effect depth
Will read up on it some more.

Question: any meaningful difference for the sw frequencies; say from 0.5 to
30 MHz ?
I have a sw receiving set also, and frankly, when i strung my coax for it, I
never considered the center conductor coax's construction.

Should I have, possibly ?

Thanks again,
Bob

Any quality coax will have a sufficiently thick copper coating on the
center conductor to insure that the current is, for practical purposes,
always in the copper rather than the steel down to quite a low
frequency. If the plating were unusually thin, the current would
penetrate into the steel at lower frequencies and the loss would be much
greater. However, loss in a transmission line used for receiving only
becomes very much less important at HF and below (that is, below about
30 MHz). So the bottom line is that it's not normally anything you need
to worry about.

A copper plated steel conductor is used to increase the physical
strength of the cable, particularly cables of smaller diameter and
higher impedance, where the diameter of the center conductor becomes
small. When done properly (as it usually is), it works just the same
electrically as a solid copper conductor.

The _ARRL Antenna Book_ is a very good place to start in learning about
antennas.

Roy Lewallen, W7EL

Attenuation Questions
 

Anyone can do it wrong (come up with 2dB loss), but we would have a
great more deal traffic here if doing it wrong was that common.

I have measured attenuation of near everything from DC to 12GHz at the
standards bench and RG58 with BNCs for a short run (2 to 3 meters)
never presented any problems being wildly imagined here. At the
worst, the connectors (note plural) "might" show 0.2dB mismatch loss -
trivial.

On the other hand, if Jeff is so sure of this 2dB figure, it should
reveal itself in blisters to the fingers for even mild power (100W).
Since this unequivocal evidence is so obviously missing from the
records (or the testimony, as the case may be); then it is very
apparently one of those "someone heard from someone else about their
brother's friend's boss who had a customer who made this claim."

73's
Richard Clark, KB7QHC

Well let me put you right Richard, there was no high power, so burnt
fingers, this was noticed in a test set-up on a network analyser, and
subsequently the adaptors in question were tested in isolation. 'Good'
adaptors were also substituted and their loss was measured and was very low
indeed.

The problem was first noticed by a very senior consultant in the lab in
question and then checked by several other people because they could not
believe it either. The connector interfaces were checked and were Ok. No
further time was spent investigating why the problem occurred, just 2 SMA
adaptors in the bin and the rest of the adaptors in the lab checked.

So please don't denigrate other people's comment without asking about the
facts.

My comments were not about properly made connectors, that will have very low
loss, but were a caution that at high UHF and above care has to be taken and
it is very easy to introduce loss. This is particularly the case when
launching from connectors onto pc boards with microstrip or joining on to
coax cable.

Having been involved professionally for over 10 years in the design of
microwave equipment I can assure you that there is an endless list of the
ways that loss can creep into a system at high frequencies.

73
Jeff

Attenuation Questions
 

Jeff wrote:
. . .
My comments were not about properly made connectors, that will have very low
loss, but were a caution that at high UHF and above care has to be taken and
it is very easy to introduce loss. This is particularly the case when
launching from connectors onto pc boards with microstrip or joining on to
coax cable.

Having been involved professionally for over 10 years in the design of
microwave equipment I can assure you that there is an endless list of the
ways that loss can creep into a system at high frequencies.

I sense a communication problem here.

"Loss" usually implies dissipation of some of the desired power in the
form of heat, and that's the way I use the term.

However, people who spend a lot of time in a test equipment environment
often use the term to mean loss as described above, "mismatch loss", or
some combination of the two. And I've often seen people mistake
"mismatch loss" for dissipative loss. But it's important to separate the
two, since they have different causes and cures.

Dissipative loss is caused by current flowing through a resistance,
which causes power in the amount of I^2 * R being lost as heat. When
dealing with dielectric loss, it's often easier to calculate it as V^2 /
R. The mechanism is somewhat different but the end result, dissipation,
is the same.

"Mismatch loss" is entirely different, although someone making
measurements with a network analyzer or other test system won't readily
be able to distinguish it from dissipative loss. "Mismatch loss" works
like this:

Suppose we have a signal generator with a 50 ohm fixed resistive output
impedance which delivers 1 watt to a resistive 50 ohm load. If we were
to insert, say, an inductor with a reactance of 100 ohms in series
between the source and load, only 500 mW will be delivered to the 50 ohm
load. Now, this is for a lossless inductor, so there's no dissipative
loss in the inductor. Yet the inductor is said to have an "insertion
loss" of 10 * log(1/0..5) ~ 3 dB because the amount of power delivered
to the load is less than the amount which would be delivered under
matched conditions. Consider another example -- a perfect, lossless
transformer is inserted between the generator and 50 ohm load. If the
transformer has a 2:1 turns ratio (4:1 impedance ratio), 640 mW will be
delivered to the 50 ohm load. So the transformer is said to have an
"insertion loss" of ~ 1.9 dB.

The way to reduce dissipative loss is to reduce the I^2 * R or V^2 / R
product one way or another. Power companies do this by raising the
voltage for long distance transmission, thereby reducing I. We often use
a larger diameter coaxial feedline, which reduces the conductor R -- or
use an open wire line which reduces I because of its higher
characteristic impedance.

"Mismatch loss" can be eliminated entirely by adding an impedance
matching network. In the examples above, a matching network anywhere
between the generator and load which causes the generator to see 50 ohms
resistive will reduce the "mismatch loss" to zero -- that is, it will
raise the power in the load resistor back to its maximum possible value
of 1 watt.

Now when Jeff says that loss can be introduced by coax-to-microstrip
transitions, he's speaking of "mismatch loss", not dissipative loss.
There's no mechanism which would cause the I, V, or R to become high
enough in the region of a transition to cause an appreciable amount of
dissipative loss. It is, however, extremely difficult to make a
transition which doesn't introduce a different impedance in the
transition region. (And it's very nearly always a higher, or inductive,
impedance due to the fields escaping or fringing from the regions
immediately between the conductors.) I'm very familiar with this, having
designed transitions which had to maintain the proper impedance from DC
into the tens of GHz for very sensitive time domain equipment. It can,
however, usually be compensated by introducing complementary impedances
in the immediate vicinity -- this is the equivalent to providing an
impedance match. The result is a clean transition with negligible loss
-- "mismatch" or otherwise. It's not possible to compensate for
dissipative loss in this way. If such a transition shows loss in a
network analyzer measurement, for example, it's almost surely "mismatch
loss" and not dissipative loss. It won't burn anyone's fingers if a kW
is applied, and the cure wouldn't help dissipative loss at all.

"Mismatch loss" is useful mostly to people who have to stay in a fixed
50 ohm environment with no opportunity to apply impedance matching.
That's not usually the case in amateur antenna systems. So when someone,
especially someone with a test laboratory background, says "loss", it's
important to ask whether they mean dissipative loss or "mismatch loss",
since we can make the latter disappear but not the former.

So, I'll ask: Was the 2 dB connector loss dissipative (caused by an
exceptionally high resistance series conductor and/or exceptionally
lossy dielectric), or was it "mismatch loss" (caused by a dramatic
impedance change within the connector due to a changed relationship
between conductors -- say, missing dielectric or something like a
helical conductor)?

Roy Lewallen, W7EL

Attenuation Questions
 

Snip
" So, I'll ask: Was the 2 dB connector loss dissipative (caused by an
exceptionally high resistance series conductor and/or exceptionally lossy
dielectric), or was it "mismatch loss" (caused by a dramatic impedance
change within the connector due to a changed relationship between
conductors -- say, missing dielectric or something like a helical
conductor)?

Roy Lewallen, W7EL

A good question Roy, but I cannot answer it, other than to say that the SMA
interfaces were clean and apparently dimensionally in spec.. There was not
the time or money to investigate further, the adaptors just went in the bin!
(they were good quality adaptors by the way, probably MA/com).

73
Jeff

Attenuation Questions
 

Richard Clark wrote:
On Tue, 11 Mar 2008 16:00:11 +0000, Ian Jackson
wrote:


Even at 1GHz, I don't believe that you would get as much as 2dB of loss
even if you joined two pieces of coax by stripping the ends with a knife,
twisting the conductors together, and wrapping them in scotch tape.
--
Ian


It is surprisingly easy to get significant loss at 1GHz and above, 2dB would
be very easy to achieve even when using what appears to be a 'good'
connection.

regards
Jeff


Hi All,

Anyone can do it wrong (come up with 2dB loss), but we would have a
great more deal traffic here if doing it wrong was that common.

I have measured attenuation of near everything from DC to 12GHz at the
standards bench and RG58 with BNCs for a short run (2 to 3 meters)
never presented any problems being wildly imagined here. At the
worst, the connectors (note plural) "might" show 0.2dB mismatch loss -
trivial.

On the other hand, if Jeff is so sure of this 2dB figure, it should
reveal itself in blisters to the fingers for even mild power (100W).
Since this unequivocal evidence is so obviously missing from the
records (or the testimony, as the case may be); then it is very
apparently one of those "someone heard from someone else about their
brother's friend's boss who had a customer who made this claim."

73's
Richard Clark, KB7QHC

I've encountered (defective or broken) SMA hardware that has remarkably
high *apparent* loss. Typically, internal cracks or voids. I had a
batch of Pasternack right angle M-F elbows that had losses that were all
over the place at 1 GHz sorts of frequencies. I'm pretty sure that
internally, there was something not connected, or a spring loaded
something that wasn't making contact.

In this case, the connector wasn't actually lossy, but more of an
extreme mismatch, and when hooked up with a 50 ohm signal generator on
one side and a 50 ohm power meter on the other, it looked like loss.
Actually, it was reflecting the power back to the source.

In this scenario, with 100W, your fingers wouldn't have gotten burned
(but the signal generator would probably shut down with reverse power
warnings)

Attenuation Questions
 

On Wed, 12 Mar 2008 08:36:12 -0000, "Jeff" wrote:

So please don't denigrate other people's comment without asking about the
facts.

Hi Jeff,

Well, in fact, facts were preciously few and generalities were
liberally asserted. I read facts attesting to "significant loss" and
indicated that it should be attended by heat where in my experience
(and as the head of an RF metrology lab) these connector issues far
more commonly exhibit mismatch loss (and I chose my words with care)
not "significant loss." Bird Wattmeter connectors suffer from this
very problem and I have outlined that in the past.

Bob asked about receive properties and RG6 where BNC or N applications
would be typically found for operation above 150 MHz. My response
with respect to cables (plural) of a short length and terminated with
on-par connectors (plural) indicate that loss (insertion loss both as
dissipative and mismatch) is principally invested in the cable, not
the connector. RCA or F or PL types are not taken seriously in the
real world - but they would work suitably for receive. My personal
limitation to 2 or 3 meter lengths were for jumpers application. My
personal limitation to 1GHz for BNC was for leakage issues, not loss.
BNCs/TNCs are rated to 4GHz and N higher to 12GHz. Bob can then
research cable which has far more specification available on the web.
Abstracting a one-off problem with a barrel connector into general
cautions about issues of dielectric quality brings to mind that one
data point does not denote a trend. An SMA/RG6 combination is not one
that springs automatically to mind; neither does an SMA/50 foot line
(presumably RG174 - even though I would do it in spite of demurring it
as a solution).

For 50 feet, and considering the receiver and the service, and the
link budget, none of this probably matters (not even 2dB). If this is
for EME, then yes, it matters a lot. Bob didn't indicate EME.

73's
Richard Clark, KB7QHC

Attenuation Questions
 

On Wed, 12 Mar 2008 11:55:35 -0700, Jim Lux
wrote:

I've encountered (defective or broken) SMA hardware that has remarkably
high *apparent* loss.

Hi Jim,

Modifiers abound, and such an abundance generally suggest there are
problems of understanding. Roy and I have already written to that
matter.

Returning to poor connectors, the cables that I gathered for jumpers
at the RF bench were selected, naturally. We had plenty to choose
from, but there were always poor performers for any of a number of
reasons we didn't care to investigate, simply because we tossed the
losers. Insertion loss was never a problem as long as it was
constant. The losers did not present a constant insertion loss which
could have been attributal to connector quality (or construction
quality); but some times there seemed to be the suggestion of
triboelectric effect (cable bending not associated with straining the
connector). Anyway, the cables were tossed without too much regard
for why - we had better things to do (the in shelves never seem to
empty).

73's
Richard Clark, KB7QHC