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(Possibly) a stupid question on a RF failsafe....

Discussion in 'General Ham Radio Discussion' started by iiiquaziii, Aug 6, 2017.

  1. iiiquaziii

    iiiquaziii
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    So....I was talking to my buddy who recommended I upgrade one of my coax antenna feeds because it is really cheap cable, and old, and he mentioned the possibility of it shorting out. I think the probability of this happening while transmitting at 100w on 10m is probably low, but cable does degrade/age in the weather. Its probably good advice to upgrade the cable, but it got me thinking/wondering....



    Is there any kind of device out there, than can provide an RF "circuit breaker like" fail safe, perhaps where it routes RF output to a dummy load? Something that if your antenna feed suddenly fails due to a short from age/weather/water, it can fail safely? Does such a gadget exist? If it does, what is it called?

    It almost seems like something like this SHOULD exist, but I don't know if it's even possible. Any thoughts?

    Thanks.
     

  2. iiiquaziii

    iiiquaziii
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    I am just theorizing, but it seems like you could hook an swr/power meter, up to some kind of RF capable relay, and have it switch/transfer over to a dummy load darn near instantaneously.

    If it wouldn't work, go ahead and discuss why, but it seems like someone would have invented something like this?

    I am also not sure about the likelyhood of coax shorting out at a mere 100w, but I guess if someone had some really old suncracked cable, and water got in it, it could/would. At higher powers where you risk melting cheap feed lines, it may be impossible to just instantly re-route without melting components in the "fail safe" device as well.
     
    #2 iiiquaziii, Aug 6, 2017
    Last edited: Aug 6, 2017
  3. bob85

    bob85
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    They have invented it, the lp-100/a has a ppt relay to shut the transmitter down if vswr goes above a threshold,
    you could use the relay to do things other than unkey the radio.
     
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  4. Paul-W5lz

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    There are a lot of 'myths' you hear about coaxial feed lines and how they can 'destroy' your radios etc. In something like 50 years of playing with radios I have never see coax -melt-, burn up. I've certainly seen it damaged, but there's quite a difference between the two. Coax gets blamed for a lot of things that just aren't it's fault. In general, if it ain't broke why fix it? Because it -could- break?? Nope, you could fall and break your neck too, gonna crawl so you can't fall as far? (See where that's going?)
    Take that cable down, examine it for breaks in the insulation. Measure it for shorts. Put it on a dummy load through an SWR meter. If it passes those test there's nothing wrong with it. It my look old and nasty, but who cares?
     
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  5. Captain Kilowatt

    Captain Kilowatt
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    I have coax that was in use for 20 years and never an issue including a section that was used around a rotator and lots of flexing. I have had coax in commercial service for longer than that and never an issue unless it was hit by lightning but that is a different story altogether. There certainly is a solution to what you propose but the reasoning for wanting it is pretty far out there.
     
  6. nomadradio

    nomadradio
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    In my experience, old coax just becomes lossy. Turns into a partial dummy load, losing some percentage of your power per foot. Of course new coax does this, but old coax just loses more.

    One side effect is that the SWR you read on the antenna 80 feet away from the meter will read really, really low. This is not necessarily because the antenna has a good impedance match. In the same way that old coax can lose 80 percent of your power on the way up to the antenna, it will also lose 80 percent of the reflected power coming back. By the time it reaches your SWR meter, the reflected power reading is only 20 percent of what a meter placed ON the antenna at the top of the tower would read.

    Over and over I would hear a base-station operator wanting someone to "fix" his base antenna the day after replacing the coax. "My SWR went up to 1.5! It was a lot lower before". I would congratulate him on improving his signal and suggest the antenna was probably just fine. The 1.5 SWR was there all the time. But if the coax wouldn't let him read it at the bottom of the cable, it was also killing most of his power on the way up to the antenna.

    High SWR that can hammer your equipment tends to start with either poor waterproofing or from poorly-soldered coax connectors. Getting a good connection to the coax shield braid can be tough. Some folks simply "fold over" the braid and call it good enough. They forget that copper will tarnish once exposed to the air. And if water gets into the plug, this just speeds up oxidation. Eventually, there is no more metal-to-metal path for your RF current.

    And if you don't waterproof the connectors outside, they will soak up rainwater and feed it into the coax. The space between strands of the shield braid will suck up water like the threads of a towel. It's called "capillary attraction". If you see a puddle on your desk under the wattmeter after a storm, your coax is full of water. Tends to boost your SWR in a big way. Takes forever to dry out, as well.

    Loose parts on the antenna are a hazard, especially gamma-match clamps and telescoping-element clamps. This can boost your SWR when the wind blows, but give you a low SWR reading when weather is calm.

    Of all the things that can go wrong with your antenna system, the coax is probably at the bottom of the list of most-common hazards. Just checking SWR with a barefoot radio before firing up would have saved trouble and expense for more than one of my customers over the years. An automatic system is not a bad idea, but prevention is better.

    73
     
  7. Paul-W5lz

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    Ready for some 'blasphemy'??
    That SWR meter is very easy to fool. It's positioning in the feed line will determine what the resulting SWR reads, with absolutely no change in the antenna system (antenna and feed line). It's terrible way to try to gauge how your antenna is doing. If the antenna's input impedance is 50 ohms, and the feed line is 50 ohms, and you measure SWR a 1/4 from the antenna, the SWR will read high. No changes at all except move that meter to a 1/2 wave from the antenna and it'll read very nicely.
    Is there a 'better' way of doing all that? Sure, but it's more costly and it ain't exactly simple...
     
  8. iiiquaziii

    iiiquaziii
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    So, basically, what I'm getting from this is, don't worry about the coax. Beyond that, visually inspect/test the coax, and beyond that, the theoretical device I mentioned is unnecessary/irrelevant...even if it may potentially exist. So yeah, the coax LOOKS fine. Nothing visually wrong, and the ends were installed fairly well, I think.

    I get the point about measuring SWR at the start point vs the end point in the antenna feed. If you measure it at the radio and have a crappy loss ridden cable, you are measuring the SWR after the loss as well....which will read lower. It probably makes it more radio safe, but not necessarily SWR accurate. I am not worried about my SWR though, its been tested multiple times at multiple locations/cable lengths....and the auto tuner will still tune it safely, loss ridden or not. It's not gonna break anything.

    As far as the coax cable itself causing harm....you guys pretty much summed up my thoughts on the odds of the cable shorting and causing damage too. I deem it unlikely unless I was putting out a kilowatt or something. I'm just stock 100w max.

    I am using some uber cheap @ss, low grade, old as frack cable that was sitting outdoors under a guys porch for years before I hooked it up to my 10m vertical antenna though. My friend (who is admittedly smarter than me with this stuff) gave the cable to me, when I was poor, and it was intended to be temporary, but as this point has become semi-permanent. He's worried about it more than me I think....but my friend is a worrier/perfectionist, and I am not. I'm more of a good enough/gambler guy anyways. From the responses, it seems like a pretty safe gamble. I do need to solder a new end on the cheap coax at the radio end though. I'm having an issue at the connector where if I touch it the signal gets louder or quieter, and it messes with my auto tuner when I wiggle it around....but that's just the connector end for sure. I know this isn't good....in fact its really bad. I'll remedy it shortly by cutting the end off and installing a new connector.

    The cheap azz cable I'm talking about on the A99 vertical is Belden RG-58/U....real thin stuff with minimal shielding....basically crap. Its going to my Antron a99 for 10-15meters....its probably 50-60 feet of cable. My other 20/40meter di-pole antenna is fed with Tramflex RG-8/X. Seems like a better quality cable, but I'm also wondering how much it really even matters? We are talking distances of like 60 feet on the a99 feed w/RG-58 cable, and maybe 100ft on the RG-8/X di-pole feed.
     
    #8 iiiquaziii, Aug 7, 2017
    Last edited: Aug 7, 2017
  9. Robb

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    Never heard of Belden cable being accused of being crummy; that is a first.

    Just because it is RG-58 and thin doesn't make it bad. Sure, it won't hold as much power as RG-8 will - true. But I've seen some shoddy Tandy/Radio Shack RG-8 coax that was made with such cheap materials it was laughable. Poor materials on the outer jacket, no foil shield on the braid, thin braid, etc. Never buy that stuff again, or will I even use it even if given to me. Quality RG-58 is nothing to sneeze at - IMO.

    I still have a piece of Belden RG-58 that is thirty years old and it is still good.
     
  10. bob85

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    That Is not blasphemy it is "W5lz karaoki physics 101",

    You are telling us that a 50ohm antenna or dummyload only shows a low vswr when you use 1/2wave multiples of coax,

    YOU CANNOT TRANSFORM A 50OHM LOAD WITH 50OHM COAX,

    when the antenna's input impedance is 50 ohms and the coax is 50 ohms the vswr will be the same anywhere along the coax,

    even when the antenna is resonant but not 50ohms vswr will be almost the same everywhere along the coax reading slightly lower the further away from the antenna you place the meter due to coax loss,

    if moving your vswr meter 1/4wave along the coax changes vswr reading significantly you have VERY lossy coax or common mode on the coax outer braid causing load impedance to change with coax length,

    IT IS IMPOSSIBLE TO FOOL A VSWR METER, ONLY THE OPERATOR CAN BE FOOLED.;)
     
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  11. Paul-W5lz

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    :) Sorry, you are mistaken, The reading of an SWR meter can certainly change because of it's placement in the feed line. Try it, see for yourself. ... I am not telling you that you get a 'true' 50 ohm impedance match by only using a 1/2 wave feed line. I basically described common characteristics of various electrical length of feed line. ... Oh boy! Here's some more of that "W5lz karaoki physics 101". There used to be an article titled "SWR Makes You Stupid" (I think), read that thing you might be surprised. And by the way, that changing impedance by using a different impedance matching section of coax is called 'conjugate matching'. It's done a bunch... sometimes called a 'phasing' line.
     
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  12. KD2GOE

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    The coax he is talking about was legit sitting in mud under a friends deck for years with the metal spool nearly rotted away. It was originally just to git him going with his CB Radio... now that's he is running the Icom soooo yeah i would play it on the safe side....

    Edit: I restored this post as it made no sense to delete it. It also adds pertinent information not presented in the original post.
     
    #12 KD2GOE, Aug 7, 2017
    Last edited by a moderator: Aug 7, 2017
  13. Captain Kilowatt

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    Agreed. Belden cable is highly respected and has never been accused of being crappy.
     
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  14. Captain Kilowatt

    Captain Kilowatt
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    Paul I think you missed a word. It should say "If the antenna's input impedance is NOT 50 ohms, and the feed line IS 50 ohms........" then you will not get an accurate reading of antenna impedance.
     
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  15. Tallman

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    Anytime I have a choice for the brand of my cable I always choose Belden products. New cable withstood abuse of laying on a hot tin roof in direct sunshine for years and when I moved I took it down and used it for another ten years.
     

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