Fire In the Wire - Literally

[ka4koe]

As I contemplate installation of the Gates BC-1T, I worry about the possibility of a fire. I am currently not running anywhere near full strap with the Johnson Valiant. I've run the SB220 at 175 watts AM carrier and 1200 watts PEP on the rare occasions I get on SSB.

Currently the 600 ohm open wire feedline enters the house via soffits; the soffits are non-conductive plastic with LOTS of leedle holes that are conveniently the same diameter as the OD of #14 AWG. There are quick disconnects so I can physically remove the feedline and get it away from the house in case of incoming storms.

The feedline runs thru the attic and enters my 2nd floor shack via an interface plate on one the walls. The run in the attic is around 60 feet, if memory serves.

I took care to route the feedline away from other objects in the attic space. However, full strap is worrisome and I am considering turning the current 180' long dipole into a resonant 80/40m fan dipole type with coaxial cable. This would likely eliminate 160 as I know of no traps that will handle full strap without conversion into a powdered carbon substance. RF exposure levels are also worrisome.

The tuna would require retirement if I kept the open wire feedline as it would suffer the same fate as the sacrificial traps. K1JJ design link-coupled tuna?

Opinions / Thoughts / Advice?

Tnx.

FEELEEP

[W3RSW]

Run LMR 400 to an external wall mounted, remote tuner. Put the Tuner in a Strapping NEMA enclosure with seals, dehi resistor and other corrosion mitigating items to prevent moisture from entering the cab, particularly around the twin Lead exiting HV ceramic insulators.

Place the tuner on external wall so that it can feed somewhat directly upwards without a bunch of crooks and bends.

Design a nice control box / E-feed for band and sub band selection for chairside operation. These days it could be directly controlled by your rig via IC2 buss, etc.

Costly, but worry free unless critters in the attic like coax insulation.
If you were commercial, and planned to run a continuous RF KW, then Heliax through the attic would be better, not animal tasty either. 

Sorry, couldn't resist. Designing a high power, robust tuner for 160 down, unbalanced to balanced and not the size of a little house is a challenge. See K4KYV's lash up, etc for some ideas.

[W2VW]

Rick's suggestion is good. If it were me I'd build at least two antenna tuners in an enclosure for quick change. Giant DPST remote switch for the output and a pair of coaxial cables or remote switch for the input. I'd steer away from hammy remote coaxial switches for your application.

AM operation currently is much more spread out on the bands. A tube final won't care about the antenna system moving off resonance but accurate meter readings are nice.

A camera in the attic also works.

I had 3" spaced balanced feeder in the house for years but could see it when operating.

[w1vtp]

I've always wanted to build this one:

http://www.wz5q.net/index/shack_data/tuna.htm

Dialup alert: takes a long time to load page due to high resolution pictures

Al

PS:  Check out his home page!

http://www.wz5q.net/index/shack.htm

[N1BCG]

What power level are you concerned about? The normal "Low" setting is 250W carrier (1000 PEP@100%). I used a variac on the plate supply of a BC1-G to get the full 375W (1500PEP@100%).

You can use formulas to calculate the peak voltages for various impedances, but if you're not arcing anything over at 1200PEP then you're probably going to be okay at 1500.

[W2VW]

1500 watts peak is plenty to start a fire. Calculating peak voltages might be instructive but when there is an arc in the feedline the tuning will change and those calculations go right out the window.

WZ5Q's article is great. One thing that looks like a potential problem might be just the way the photos look. The feedline connection and general area to the tuner is very busy with other wiring near the balanced feeders. That may account for some of the unbalanced condition mentioned.

Stuff needs to be far away from those feeders for them to work as designed.

An alternative when the area is just too busy is to use less feeder spacing and step up to more normal spacing once the feeders are out in the open. A homebrew giant tuner will still be able to work into this lashup because the builder will reconfigure it until it plays

[N8ETQ]

      Like Rick said use LMR-400 in the attic.
Convert to 600 ohm outside. 1500 W @ 50 ohm is
only about 775 V P/P but at 600 ohm that voltage
goes to 2.6 KV P/P. YIKES!

      Play with this site and see your numbers.

http://www.random-science-tools.com/electronics/dBm-Watts-volts.htm

     
     Like N1BCG said at 1200 W your P/P voltage
is up around 2400 anyway so your prolly OK. 
My internal run of 600 ohm is < 2 feet and like
W2VW said, "when there is an arc in the feed line the tuning will
change and those calculations go right out the window."

    Snow, Ice and rain may create the same issues.

GL

/Dan

[WBear2GCR]

Is the problem getting the open wire out of the house/into the house??

                  _-_-

[Opcom]

A friend hung the usual plastic covered 450 ohm ladder line by nylon cords from rafters and the like, keeping it 1 FT away from everything. His matcher is at the transmitter position and the line runs through the attic to the dipole outdoors. Would this work with un-insulated line? I think so. Using bare line in the attic makes me think of dust or lint getting on the line and creating an additional hazard.

[ka4koe]

I've already routed the 600 ohm in the attic and took reasonable care in keeping it away from objects. In some areas, I used lengths of 550 paracord tied to the center insulators and then to anchor points to pull away from nearby objects; or to suspend it so it wouldn't lay on top of rafters, etc. I'd say in most cases I have at least 12" all sides. The center insulators are Ladder Snap type with #14 AWG.

I may not have a problem in the first place. I'd rather deal with potential problems than destructive aftermaths.

PAN

[Ralph W3GL]

Is the problem getting the open wire out of the house/into the house??

                  _-_-

           

     Hint,  take a couple minutes and READ THE FIRST POST !!!

[W2VW]

When using 600, 450, 300 or whatever impedance feedline on a doublet fed with an antenna coupler the voltage cannot be simply calculated by using the feedline impedance unless the feeders are flat. Flat means the load impedance is non-reactive and equals the feedline impedance.

Typical amateur doublet antenna used on several bands exhibit SWR more than one. SWR dictates maximum voltage and current.

SWR more than one is not always the end of the world. The rules must be considered however.

Selecting a BalUn ratio based on impedance of a feeder's impedance on a multiband doublet is a mistake commonly seen. Remember in an SWR situation of more than 1:1 the feeder acts like a transformer and will present an impedance different than that of the antenna feedpoint based on feeder length and feeder impedance.

Many career EEs do not grasp this at first glance for some reason based on ham radio forum posts.

[N2DTS]

I would not feel comfortable running anything through the attic.
If your house burns down insurance likely would use it as an out to deny coverage.

[Steve - K4HX]

What do you think will catch fire? If the feedline is not near anything, especially metal, I don't know what could catch fire.

It would be very easy to run the transmitter for some period of time and measure the heat rise on the feedline at some critical points. Data is always better than speculation.

[W2VW]

People will get out of your way when you on fire.

[Steve - K4HX]

Or they will stand around and cheer or make a video.

[W3RSW]

or they will lose their favorite mice in you attic who'll eat your paracord, thus lightening your path to redemption.

why am I even on here.? 40 is way hot tonight, even over the approaching strum und drang.  

Penn State , wa3fet was working ruskies at 40 over about a half hour ago.
--then again he has a full 4 ele. yagi on 40.
  and a Sterba Curtain of sorts which he switched back and forth.
 

[WQ9E]

Properly installed line shouldn't be a problem and arguably balanced line is less likely to catch on fire from deterioration than coaxial cable.  I wouldn't worry about the line since it sounds like you took proper care with installation.  Where you can have a problem is with baluns and similar devices that fail.  A contesting friend was disturbed during a contest when a passing motorist let him know that part of one of his towers was on fire.  A balun had failed and was flaming nicely against the dark sky

[W3RSW]

darn baluns. --look like terminal resistors at 20:1
hey, almost real time email. 

[WBear2GCR]

Is the problem getting the open wire out of the house/into the house??

                  _-_-

           

     Hint,  take a couple minutes and READ THE FIRST POST !!!

Hey Ralph,

Ya know I did read the post.

Says something about going through a plastic soffit... and other things

Just read it again. Still not sure exactly, precisely where his issue is. Apparently you are, so why not just clarify it?

Thanks a bunch, hugs and kisses,

                    _-_-

[Opcom]

Getting it through - excuse me for not addressing it.

I use a couple of ceramic tubes and run one conductor through each.

The old 'knob and tube' style is what I got, cheap old junk, 4" ceramic tube, works well.
They are thick-wall and don't break easily.
Glued through the wood of one wall and the sheet metal of another.
Many other kinds of quality tubular insulation are usable.

A friend used a piece of slate from an old large knife switch.
A section of 'replacement asbestos shingle' can be used. (no asbestos)

Plastic and PVC, nylon, etc. were avoided.

tubing:
http://www.ebay.com/itm/271722927610
http://www.coorstek.com/markets/chemicals/ceramic-tubes-rods.php
http://www.ortechceramics.com/ceramic_tubes.htm
http://mcdanelceramics.thomasnet.com/category/ceramic-tubes-rods
http://accuratus.com/cerrodtube.html

[WZ5Q]

Howdy Y'all,

When using Open Wire Line (OWL), You do have to be careful in your routing and support.

I had a temporary installation using 14AWG that came loose from its support and sagged down near a 6x6 wooden support pier for my River House. I kept seeing the match change during an Old Buzzard transmission so I would just reach on over and adjust the Link Coupled Tuner (LCT) to compensate. This happened a few times during the transmission and sighned by stating that I needed to go outside and check on things. Good thing I did, the 6x6 pier was on fire and climbing up towards the house, I had to get the water hose on it to put it out. The cause of this was that the OWL support failed and it dropped down close enough to a pine knot in the wood to start an arc to ground. As you can see in the pics, the voltage was very high at that point and punched thru the insulation sustaining an arc across a WIDE gap and not only melted back the wire, but ignited the wood.

As Patrick does, I also run my OWL thru ceramic tubes making sure that there are no conductive objects around (yes, even pine knots). My final installation uses 10AWG wire for the OWL using ceramic insulators to hold it far away from anything else when required.

I only use OWL for the AM High Power stuff, straight into the shack to a LCT.
Just from my experience, I would never run it in an area that was not readily visible like an attic.

Ferrite and coax are evil in a multiband antenna system using OWL. The VSWR on the coax between tuner output and the balun can get VERY high, on upwards of 20:1 depending on the antenna system parameters and the band your trying to use. This is lost as heat and the longer a run of coax your using to get to the evil balun, the more power your losing. Look at any calculator for coaxial feed line loss at a 20:1 VSWR for 100', LMR-400 calculates to almost 3db, RG-213 close to 4dB, that's over half your Power! I have used this configuration before and the coax has gotten so hot, that you could not keep your hand on the connector. It actually melted the so called Teflon on the inside of the  PL259. So if you must use coax, use the shortest possible run, using NO coax is the best.

Baluns, pugh....
Be careful on what kind of balun you use for AM operation with a multiband OWL Antenna system. Remember the advertised power ratings for the balun are for a 1:1 match at best. When you get upwards of 20:1 as seen in a multiband OWL Antenna System, then the power rating of the balun is dramatically reduced. I have blew up, yes BLEW UP baluns using a 1058' loop fed with 600 ohm ladder line. I had the balun hanging outside the window and had 6' of coax coming inside to the tuner. It sounded like a shotgun blast when that thing went off, and when the shrapnel hit the window, I hit the floor.
I installed another balun I had as a spare, and that one had the Teflon wiring melt on the inside and the ferrite cores shattered. See the attached pic. I finally upgraded to a 10Kw Balun in an aluminum case until I could build a LCT and get rid of the evilness.

73,

[ka4koe]

The Coorstek line is actually reasonable until you read about the minimum order of 150.00. Thanks for the links.

PAN

[N1BCG]

I've had damage like that occur while running a kW and I'm sure the 4kW+ PEP was the icing on the cake. Definitely can get messy.

[ka4koe]

"Run LMR 400 to an external wall mounted, remote tuner. Put the Tuner in a Strapping NEMA enclosure with seals, dehi resistor and other corrosion mitigating items to prevent moisture from entering the cab, particularly around the twin Lead exiting HV ceramic insulators.

Place the tuner on external wall so that it can feed somewhat directly upwards without a bunch of crooks and bends."

Anyone use an ice cooler? NEMA boxes tend to be expensive.