duplexers, antennas, repeaters
 

All it takes is ONE user who believes himself to be above
any technical standards in his quest to maximize profit.

These are the same clowns that strip a site of any and all
hardware that isn't nailed down or currently connected to
something.

Or throw together a "repeater" out of junk bought at the
swap meet and nailed to a piece of plywood.

Jeff-1.0
wa6fwi

--
"Everything from Crackers to Coffins"

duplexers, antennas, repeaters
 

On 10 Sep 2011 12:57:07 GMT, dave wrote:

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

The initial reaction of most techs is that the PIM is sufficiently low
level that it would not have an effect on receiver performance. Wrong.
In cell sites, where squeezing every dBm of sensitivity out of the
receiver is necessary to deal with perpetually marginal cell phone
handset signals, that install cryogenically cooled front ends and
tower mounted preamps to do this, can definitely see the effect. Look
at a cell site install and try to find anything other than Heliax.

"SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample)
http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html
"C. All cabling from the building to tower including on the
tower to the antenna, shall consist of a minimum of 1/4 inch
jacketed corrugated copper "Heliax" type cable. Semi-rigid
"LMR-400", "LMR-600", etc. cable and non-rigid cable, such as
RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as
transmission cable exiting the building."

In most cases, this has been extended to include internal coax cabling
that carry transmit RF.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

duplexers, antennas, repeaters
 

On Sat, 10 Sep 2011 08:12:23 -0500, Jeffrey Angus
wrote:

All it takes is ONE user who believes himself to be above
any technical standards in his quest to maximize profit.

Yep. However, sometime they actually have a clue. One of my friends
recently orchestrated a site cleanup and purge, emphasizing coax
cables and isolators. After the complaining, yelling, and screaming
stopped, so did the intermod. On the other foot, the county decided
to do the same things on a crowded tower that we were sharing. All
the LRM-400 came down, and was replaced by Heliax. Much of the
intermod went away, but the mixes generated in the receiver front ends
remained.

These are the same clowns that strip a site of any and all
hardware that isn't nailed down or currently connected to
something.

They're probably the same clowns that steal my scope probes that I
leave plugged into the scopes at various sites.

I had a weird problem related to unused equipment. There was an
unused "smog alert" receiver at one site, connected to an external
ground plane antenna half way up the tower. It was turned off as the
system was obsolete. Someone noticed that if they unplugged the
antenna connector, some of the intermod would magically disappear. The
outside antenna was picking up RF from the tower, delivering into the
building, and the badly shielded receiver front end was re-radiating
it all over the rack. The building manager immediately instituted a
reign of terror, demanding that all unused equipment and antennas be
removed, resulting in most of the junk exiting the building and tower.
There was a slight but noticeable decrease in intermod. Oh well.

Or throw together a "repeater" out of junk bought at the
swap meet and nailed to a piece of plywood.

Ahem... You must have been looking at my photos. Please don't do
that. Here's our unfinished plywood APRS weather station, built on a
plywood (with ash veneer) bookshelf:
http://802.11junk.com/jeffl/WR6AOK-WX-Station/
The 2m bottle is not in the picture. I use screws, not nails. There
was a good (political) reason to use plywood. Also, the rack in my
living room has plywood shelves, as are the radios in my Subaru.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

duplexers, antennas, repeaters
 

Jeff Liebermann wrote:

On 10 Sep 2011 12:57:07 GMT, dave wrote:

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

The initial reaction of most techs is that the PIM is sufficiently low
level that it would not have an effect on receiver performance. Wrong.
In cell sites, where squeezing every dBm of sensitivity out of the
receiver is necessary to deal with perpetually marginal cell phone
handset signals, that install cryogenically cooled front ends and
tower mounted preamps to do this, can definitely see the effect. Look
at a cell site install and try to find anything other than Heliax.

"SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample)
http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html
"C. All cabling from the building to tower including on the
tower to the antenna, shall consist of a minimum of 1/4 inch
jacketed corrugated copper "Heliax" type cable. Semi-rigid
"LMR-400", "LMR-600", etc. cable and non-rigid cable, such as
RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as
transmission cable exiting the building."

In most cases, this has been extended to include internal coax cabling
that carry transmit RF.


Cell sites are a different animal . We were talking about 2-way,
point-to-point, VHF/UHF broadcast type sites. YouTube is pretty intense.

duplexers, antennas, repeaters
 

In article ,
Jeff Liebermann wrote:

I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South
Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John
Hancock building, the router room at Channel 4, etc.) and I have never
seen a blanket ban on LMR because it leaks.

It's not leakage. The problem is the plated steel wire used over the
foil wrap on the shield. The steel is non-linear and subject to PIM
(Passive Intermod) problems. The aluminum foil to steel junction can
easily become a diode if the mylar coating is penetrated. I've seen
it with LMR-400 on a lab test similar to the YouTube video that you
apparently didn't watch. The problem was bad enough that Times had to
conjure a special mutation of LMR-400 with low PIM:
http://timesmicrowave.com/products/lmr/downloads/126-129.pdf
I think (not sure) that the only difference is that the braid over the
foil is now aluminum.

That same diode-like effect also seems to be capable of causing the
cable to generate a nontrivial amount of broadband noise, when
energized by a sufficiently strong transmitter signal. In simplex
applications this seems not to matter, but in repeater applications it
tends to cause enough of an increase in the noise floor at the
receiver to appreciably de-sensitize the receiver.

The system I work on, was originally build with LMR-type feedlines
within the cabinet, and didn't "hear" particularly well. When the
chief hardware guru threw out all of those (well-constructed)
pigtails, and replaced them with 1/4" heliax... the problem went away
and has not returned.

Heliax is good. Double-braid shielded cable (with silver-plated
copper braid, not aluminum) seems to be almost as good.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

duplexers, antennas, repeaters
 

"Jeff Liebermann" wrote in message
...


I had a weird problem related to unused equipment. There was an
unused "smog alert" receiver at one site, connected to an external
ground plane antenna half way up the tower. It was turned off as the
system was obsolete. Someone noticed that if they unplugged the
antenna connector, some of the intermod would magically disappear.

Such is the case on USN ships of my acquaintance. If the ship buys a
commercial transceiver and throws the antenna any old place, the front
end becomes a mixer. There's a reason why (most) military gear is
pricey. It's been engineered not to do that.

Aside:
It's not always an active device that causes problems. I had one ship
that was getting massive interference on UHF comm circuits between
about 325 MHz to 399 MHz. from a radar operating around 430 MHz.
Normally not a problem. The cause was a tangled hunk of wire I
found in the field of the radar. It had been used to secure scaffolding
during the ship's previous inport period.

Every time the radar lit up that bailing wire, the resulting arcing
and sparkling generated broadband RF pulses at the radar's rep rate.

I was climbing around on the mast, looking for just something of the
sort. When I saw that wire, I actually spoke out loud to it.

I said, "Well, hello there!" True story.

It's one of several reasons we preached "Topside Housekeeping"
to our Sailors during ship visits. Leave nothing on the mast that
doesn't have to be there.

"Sal"

duplexers, antennas, repeaters
 

Dave Platt wrote:

In article ,
Jeff Liebermann wrote:


That same diode-like effect also seems to be capable of causing the
cable to generate a nontrivial amount of broadband noise, when
energized by a sufficiently strong transmitter signal. In simplex
applications this seems not to matter, but in repeater applications it
tends to cause enough of an increase in the noise floor at the
receiver to appreciably de-sensitize the receiver.

The system I work on, was originally build with LMR-type feedlines
within the cabinet, and didn't "hear" particularly well. When the
chief hardware guru threw out all of those (well-constructed)
pigtails, and replaced them with 1/4" heliax... the problem went away
and has not returned.

Heliax is good. Double-braid shielded cable (with silver-plated
copper braid, not aluminum) seems to be almost as good.


Interesting. I always hear people bragging about LMR. Are we using the
term "Heliax" generically? Is semi-flex no good too, (It's all aluminum
and brass metallically isn't it?)

Did your guru make a profit on the replacement cables? How do you know a
$15 can of Cramolin wouldn't have helped just as much?

Tinned copper braid is OK, no?

http://www.corrosionist.com/Corros1.gif

duplexers, antennas, repeaters
 

Dave Platt wrote:
That same diode-like effect also seems to be capable of causing the
cable to generate a nontrivial amount of broadband noise, when
energized by a sufficiently strong transmitter signal.

Yeah, I've seen some of that. I spent several days finding the
culprit on a UHF repeater where the over the air rx sensitivity varied
substantially and erratically. Watching the IF noise level, showed it
going up and down with the sensitivity changes. In frustration, I
grabbed a broomstick and beat on the accessible coax cables. I
eventually found a length of 9913 coax that was apparently involved. I
replaced it, and the noise problem disappeared. Inspecting the coax
carefully, the outer jacket was slightly corroded and white dust was
visible.

In simplex
applications this seems not to matter, but in repeater applications it
tends to cause enough of an increase in the noise floor at the
receiver to appreciably de-sensitize the receiver.

Yep, but the mechanism isn't obvious. All transmitters belch some
level of synthesizer or oscillator noise. The notch type duplexer
does a great job of getting rid of the noise in the receiver bandpass
produced by the transmitter. However, when there's a diode present,
the very low level tx synthesizer spurs, or other signals picked up at
the antenna, mix with the tx synthesizer noise, and land on the
receiver frequency. It's intermod, but instead of dealing with a
collection of individual frequencies, it deals with broadband noise.
The same mechanism is a problem in broadband mux, broadcast, and
cellular systems.

The system I work on, was originally build with LMR-type feedlines
within the cabinet, and didn't "hear" particularly well. When the
chief hardware guru threw out all of those (well-constructed)
pigtails, and replaced them with 1/4" heliax... the problem went away
and has not returned.

Requiring Heliax is a good but expensive solution. Requiring Heliax
on initial installation makes is somewhat less expensive.

Heliax is good. Double-braid shielded cable (with silver-plated
copper braid, not aluminum) seems to be almost as good.

Sorta. I have problems securely attaching connectors to RG-213/u.
Unlike the rigid and semi-rigid cables, crimp type connectors are
problematic. In addition, much of the RG-213/u floating around is NOT
silver plated, but bare copper. That will corrode, and form diodes.

I also don't like the attenuation of RG-213/u. 5.1dB/100ft at 450MHz,
while LMR-400 is 2.7dB/100ft at 450MHz.

On 11 Sep 2011 13:32:15 GMT, dave wrote:
Interesting. I always hear people bragging about LMR. Are we using the
term "Heliax" generically? Is semi-flex no good too, (It's all aluminum
and brass metallically isn't it?)

A picture is worth 1000 words:
Heliax:
http://www.google.com/search?tbm=isch&q=heliax
LMR type coax:
http://www.google.com/search?tbm=isch&q=LMR+coax

I've never heard of semi-flex. Perhaps you mean semi-rigid coax,
which includes aluminum outer jacket coax as used in the CATV
industry? The coax is fine, but where it transitions to a brass or
silver plated connector, there's a problem. In general, it's a bad
idea for reducing PIM (although I use CATV coax because I'm cheap).

Did your guru make a profit on the replacement cables?

Guru's do not stay in business very long unless they're profitable.
Even the glorified poverty style of guru has to eat.

How do you know a
$15 can of Cramolin wouldn't have helped just as much?

Cramolin is now DeOxit and has been reformulated.
http://store.caig.com/s.nl/sc.2/.f
It's a total disaster on RF connectors because it contains oleic acid,
which is great for removing oxides from electrical contacts, but
equally good at rotting off the plating from connectors over long time
periods. The reformulated DeOxit allegedly contains a different
anti-oxidant, which allegedly has the same effect.

Tinned copper braid is OK, no?

Dunno. I never use tinned braid except for some semi-rigid microwave
coax, which is quite stiff. Most often I see tin plated braid. Since
tin is not magnetic, there's no PIM problem.

http://www.corrosionist.com/Corros1.gif

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

duplexers, antennas, repeaters
 

Jeff Liebermann wrote:

Cramolin is now DeOxit and has been reformulated.
http://store.caig.com/s.nl/sc.2/.f
It's a total disaster on RF connectors because it contains oleic acid,
which is great for removing oxides from electrical contacts, but
equally good at rotting off the plating from connectors over long time
periods. The reformulated DeOxit allegedly contains a different
anti-oxidant, which allegedly has the same effect.


Cramolin is still alive and well. It was and still is made in Germany,
DeOxit is made in the US, Caig used to be the US distributor of Cramolin
products, but went their own way, with a different formula.

I'm not sure which one is the one that you call "a total disaster", but
AFIK neither is to be used for anything except cleaning. Caig sells
solutions (pardon the pun) for use on connectors.

I have them because I can only buy DeOxit in small tubes off of eBay and
still get it shipped here, and that was the only way I could get fader lube,
but I have never used them.

Don't go looking it up and show me auctions of just fader lube, after I
ordered the sets, which did not show I up, I commented to the vendor
that I wanted just the fader lube and now they list it.
They also replaced the missing packages.

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

duplexers, antennas, repeaters
 

In article ,
dave wrote:

Heliax is good. Double-braid shielded cable (with silver-plated
copper braid, not aluminum) seems to be almost as good.


Interesting. I always hear people bragging about LMR. Are we using the
term "Heliax" generically?

Yes... I meant "cable with a solid or near-solid outer conductor".

Is semi-flex no good too, (It's all aluminum
and brass metallically isn't it?)

I think that the key is avoidance of (1) contact of dissimilar metals
and (2) ferromagnetic materials. And, you want a cable where you can
get a really good electrical contact between the outer braid, and the
connector shell. I believe that solder beats crimp in this
application, since you end up with a connection which will reliably
remain gas-tight and won't oxidize. A good semi-flex would probably
be fine, I'd guess.

LMR-400 seems to be dodgy (for repeater use) over several of these
issues.

I do like it for simplex applications.

Did your guru make a profit on the replacement cables? How do you know a
$15 can of Cramolin wouldn't have helped just as much?

Nope... he's one of the volunteers in the repeater group. He donates
his time, we use donated materials (e.g. the heliax cable,
ex-cell-site) when possible, and any supplies the repeater needs are
bought from independent commercial suppliers (e.g. the connectors, in
this case). He was annoyed at having to go to the trouble of
replacing the existing pigtails, but *very* pleased at the result...
it turned the system from a "basket of snakes" into one in which the
three repeaters in the cabinet can all operate simultaneously and
independently without any cross-band interference that we can detect
in any way.

We're also sharing the hospital-roof site with at least one cellphone
system, one pager transmitter (for several years - gone now), and
several public-safety LMR repeaters... and so doing things carefully
is quite important!

Both sets of cables (the original LMR and the replacement hardline)
had high-quality commercial-grade connectors... no cheap nickel-plated
imports. Hence, I do think it's accurate to ascribe the difference in
performance to the cable itself.

Cramolin is nice stuff for dealing with connector-to-connector contact
issues, but it doesn't do a thing for problems internal to the cable.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!