>> Anything is possible, of course, but coax would have to be
>> pretty awful for
>> enough field from the differential signal to cause
>> significant heating.
>
> "Unlikely" isn't strong enough, it would be impossible.
>
> I can buy the cheapest radio shack coax, the type that you
> can see through the shield to the center dielectric, and it
> is nearly impossible to measure the leakage through that
> shield unless I use special equipment.
Playing in a lab somtimes has little bearing in a real world situation. With
a reactive load as would exist with many commercial and home brew antennas
leakage is not only possible but very likely.
It also does not take significant power to set up conditions for ferrites to
create a non linear event. Ham magazine articles and some commercial sleeve
baluns use small Teflon or RG58 coax and a few dozen very small beads. Sharp
edges and chemical residue from rain create a perfect enviroment for a
crystal set. Some of the large beads in my own baluns have banged together
and partially fractured after the tape failed to hold over several years of
NH weather. It took awhile before I figured out was was causing funny noises
on the Beverages on those low noise but windy nights. Luckily it was the
beads on the 160 and 80/75 inverted vees at the 160 level that were banging
around to each other and even against the tower. And not out on a yagi boom
somewhere.
> Use common sense.........
Thats EXACTLY what I use, plus decades of real world experience backed up by
an equal amount of time in R&D enviroments. I wish others would do the same
instead of wasting bandwidth with demeaning comments.
I had a 1500 foot
> long open wire line spaced 2.75 inches between conductors.
> There isn't any shield on it, yet any signals leaking into
> it were barely moving the S meter on any band and that was
> over 1500 feet of length.
>
> I have coaxial lines that are really single shield, just
> aluminum foil (they have a braid over top the foil but it
> isn't very dense), and with 3000 foot runs there isn't any
> ingress to speak of. S meter pinning BC stations are S-2 or
> S-3 when the antenna is pulled and the cable terminated, and
> that is with 15dB preamps.
>
> I can wind a choke balun with twisted wire instead of coax
> and the differential excitation doesn't affect a thing.
>
> I corrected a CATV system in some apartment buildings near
> a 50kW FM and 5kW AM. The tower was 100 feet from the
> buildings. The CATV company had triple shield all through
> the building and couldn't cure the problem. I had all the
> cable ripped out and regular single shield installed, but I
> ran it parallel with power wiring and entered the buildings
> at every power entrance. Bonding the shield to the
> electrical safety ground at the TV on every drop totally
> cured the RFI, and the triple shield went in the dumpster.
>
> The real worry, unless you bundle transmitting and receiving
> cables next to each other for long distances, is common mode
> currents or high resistance in shield connections.
Also incorrect. A case in point on a grand scale follows.
The nuclear carrier USS Carl Vinson CVN-70 had a broadband 5-450 mHz local
area network installed while still in the shipyard. Designed by the company
I worked for as a R&D Broadband Development Manager the installing
contractor had a very detailed set of cable specifications.
After launching and initial testing she was berthed at Norfolk and getting
ready for sea trials and bring the air squadrons aboard.
We suddenly received telephone calls that the computer equipment didnt work
and the ships CATV studio had severe interference on all TV sets.
Local field service engineers couldnt find anything wrong and to make
matters worse the ship was scheduled to get underway in less than 24 hours.
The company CEO elected to bypass the inhouse field service management and
ordered me to get a team on the ship by 9PM that evening. Since I was ex
Navy and still had my TS Crypto clearance I jumped at the chance!
It was decided that only one other engineer was required and we arrived that
evening with a spectrum analyzer, a leakage tester and a few rolls of RG-11
quad shield plus tools, connectors, etc.
The ships skipper, Captain Richard Martin welcomed us aboard but made it
quite clear that he was very upset and his operating schedule could not be
delayed or modified. Civilian enginers are usually bunked in Officer Country
but when I showed my ID he had a big grin and asked if I had a preference.
Naturally I opted to bunk in Chiefs Quarters. Anyone who has served in the
Navy knows why!
After being assigned bunks we next were given a tour of the ships broadband
network, assigned a Navy interface petty officer and given a room with a
cable drop to work in. Since the ship is over 1000' long and MANY decks high
it was a long walk.
After hooking up the analyzer it didnt take long to see the problem! It was
as if the RG-11 was a long wire mounted high above the flight deck. Signals
were everywhere HF, FM, TV, etc. The ships main transmitters were silent but
we knew that once underway the problem would only get worse.
We quickly discovered that the installing contractor ignored the specs and
used cheap unbranded RG-11 with what looked to be around 75-80% shield
coverage. After our grabbing a few hours sleep Captain Martin was informed
that the complete cable plant would have to be replaced and why. Boy, was he
pissed! But not at us. A few late phone calls were made and we got a few
more rolls of the good RG-11 on board before getting underway. The next few
days was taken up with having ships working parties stringing cable and us
cutting and connectorizing. Testing network sections at a time and then all
of it we had ZERO ingress, even from the ships 5-10KW HF transmitters whose
feedlines ran in the same cable trays for considerable distance.
The skipper was happy and we were given some souveniers and then catapulted
off to San Juan, PR to catch a flight home.
The highlight of the trip was watching the F-14 Tomcats doing flight ops,
touch and go, full afterburner launches, etc. WOW! After 25 years I still
get goosebumps!
A few months later we received official commendations from Captain Martin,
many photos and a wall size water color of the ship which hangs in my living
room today.
>
> Let's not obsess about things that don't mean anything.
Its not me that appears obsessive. Im just relating facts and making
observations that invite anyone to do the same.
Carl
KM1H
Let
> the Audio-phools and CBers worry about those shields.
>
> 73 Tom
>
>
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