[K3PZN-List] Lightning, antennas, and coax

Fri Aug 19 16:40:09 EDT 2016

Ahh we've run into the complexity of AC circuits and the differences and 
similarity of resistance, reactance, and impedance.

Resistance is essentially friction against the motion of electrons. It 
is present in all conductors to some extent. When alternating current 
goes through a resistance, a voltage drop is produced that is in-phase 
with the current. Resistance is mathematically symbolized by the letter 
“R” and is measured in the unit of ohms. Your multimeter is measuring 
just the resistance.

Reactance is essentially inertia against the motion of electrons. It is 
present anywhere electric or magnetic fields are developed in proportion 
to applied voltage or current; but most notably in capacitors and 
inductors. When alternating current goes through a pure reactance, a 
voltage drop is produced that is 90deg out of phase with the current. 
Reactance is mathematically symbolized by the letter “X” and is measured 
in the unit of ohms.

Impedance is a comprehensive expression of any and all forms of 
opposition to electron flow, including both resistance and reactance. It 
is present in all circuits, and in all components. When alternating 
current goes through an impedance, a voltage drop is produced that is 
somewhere between 0 and 90 out of phase with the current. Impedance is 
mathematically symbolized by the letter “Z” and is measured in the unit 
of ohms. This is what the 50 hm coax is referring to.

So what we need to do is measure the impedance of the coax, the tool for 
this is a little different but I have a return loss bridge that does 
this. Also there are a variety of smaller meters like the MFJ or 
Antennas Specialist ones that various people may have.

On 8/19/2016 3:50 PM, Kyle Thornton wrote:
> Thanks Andy and Phil,
> I got it to work again for the next few days maybe. I took the antenna down and nothing looked damaged how ever I'm still skeptical on the coax. I put a multimeter on it and tested its continuity which is good. How ever when I tested the resistance which should be ~50 ohms I got back on the 200 ohm scale .6 - .7 when it finally stayed at one number. I don't think this is right. It should read 50 ohms because it's 50 ohm coax correct? Otherwise it works good and tunes the bands it usually does again.
> Kyle
> 73 de KC3FMP - FM19KJ
>
>> On Aug 18, 2016, at 10:22 PM, Andy Leeds <wo3l at comcast.net> wrote:
>>
>> What this boils down to is devising a troubleshooting plan so that you eliminate components as the issue until you isolate the one (or more) that are causing the problem. Before doing anything super technical I'd start out with a physical inspection of the antenna and coax, looking for deformity, burns, etc. Connectors and where the coax touches or comes closest to the ground are the first places to look. Even without electrical damage a tree branch or flying debris may have hit the antenna and damaged it.
>>
>> Once that's clear you need to start isolating components, probably the simplest way to go here is to put a dummy load out at the end of the coax where the ladder line starts. You should have a pretty perfect match across the HF spectrum.
>>
>> For the built in tuner a 75 ohm coax jumper before the 50 ohm load creates a mismatch ~1.5:1 that the tuner should easily cope with. I've got a piece or two of 75 ohm laying around you can use for this.
>>
>> With your description my first bet is the antenna since the behavior with the external tuner has changed as well as the internal one, but that's a guess.
>>
>> Andy
>>
>>> On 8/18/2016 9:16 PM, Kyle Thornton wrote:
>>> Hi All,
>>> So with all the storms that have been rolling through the area lately I
>>> think I might have myself a slight issue. I have a terrible feeling that at
>>> minimum my HF Wire antenna strung up in an inverted V formation @ 30ft is
>>> fried if not close to it and the HF coax fried as well. Im not 100% sure but
>>> I think something is up. The HF radio has a built in tuner (480 SAT) and is
>>> usually wonderful at tuning certain bands up to 20m then is not able to.
>>> Antenna is 52ft long (Junior G5RV from MFJ) and is cut to to tune 40-10. For
>>> the jobs that the internal tuner can't do I have a jumper going into an
>>> external tuner. Technically everything is running through that jumper and
>>> into the tuner but I keep it switched to bypass the tuner when not needed.
>>> When I do need the tuner I flip it to where it needs to be adjust some
>>> things and boom I'm on 40, 80 with some work, and 160 with tons of work. It
>>> works perfectly. Up until these storms. Most of the storms I was able to get
>>> down to the shack and disconnect the coax even though everything is below
>>> roof line with plenty of other objects such as water towers for lighting to
>>> strike. From what I can tell I never took or came remotely close to a direct
>>> hit even though there was visible strikes no more than 2 miles away. Since
>>> the radio won't even tune 10 anymore much less any other band could this be
>>> sign or fried coax and antennas or could it be a failing internal tuner? How
>>> do I check all this? 2M side of everything seems to work fine and that
>>> antenna is the tallest one on the mast.
>>> Thanks!
>>> Kyle
>>>
>>> 73 de KC3FMP - FM19KJ
>>>
>>> ______________________________________________________________
>>>
>>> 73 de K3PZN Westminster, MD
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