k2 KPA100/KPA100 schottky diodes

[jferg977]

Is there any reason to choose one specification over another for the  
1N5711's which are found in the KPA100 and KAT100?  The ones currently  being shipped
appear to be GTE's vs whatever the kits came with in 2006.
 
I had thought that the frequent frying of these diodes on my k2 was due to  
lightning strikes but it looks more like the product of static discharges via
my  23 foot vertical - you can hear the snaps as the potential arcs across the
coax  connection at my Balun.  I had thought that this only happened in storms
 but it turns out to start as the storm is coming.  Clearly the answer is to  
disconnect the radio from the antenna unless I'm using it.  It's also been  
suggested that I put an inductor shunt between the lead in and ground, but  I'm
at a loss to size it.
 
It also occurred to me to bring a wire up from the grounding system to  about
the height of the antenna and put one of those wirebrush static wicks on  it.
 
Is what I'm considering crazy?  If you think so, it will be ok to tell  me.
 
 
73   AI4TO       john  ferguson    M/V Arcadian



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[Jim Wiley-2]

John -


A 1N5711 is pretty much the same from any manufacturer, but instead of
addressing the repair, let's address the actual problem, which is
static  buildup on the antenna itself.


You might try a RF choke, perhaps somewhere between 1 and 5
milli-henries (mH), from the antenna base (coax center conductor) to
ground.  The exact value is not important, as long as the inductance is
not too low.   The RF choke is will bleed off static charges, but won't
help all that much for fast rise items like nearby lightning strokes.  
Lightning arresters are better for that purpose.  Of course nothing will
protect against a direct hit, so  disconnecting cables  when a storm is
expected is always  a good idea.  


If you don't have an RF choke handy,  you can make one easily enough by
putting 20 to 30 turns of 20 to 24 gauge enamel insulated wire on a
FERRITE core, such as an FT-80 or a  FT-200  (or anything else that is
handy.  The same type wire as is used to wind the toroids in your
Elecraft radio is what you want.   If you don't have some wire handy,  
any electric motor repair shop can help you out, and if you ask nicely,
they might just give it to you for free.   You will need about 3 to 6
feet, depending on how many turns you put  on the core. Smaller gauge
wire is easier to wind, and power rating is not an issue here.   A
larger core is better, in that it will be easier to wind and have more
room for the wire).


Lacking a handy toroid core, then a resistor rated between 2000  and
10,000 Ohms at  1 or 2 watts might do, but the resistor could cause
problems if you are feeding the antenna on multiple bands, as I suspect
you are.  On some frequencies, the feed-point  impedance of the antenna
could be several hundred to even a couple of thousand ohms, and the
resistor would then be absorbing a large portion of your outgoing signal
- which is why the RF choke is a better solution.  However, this is one
instance where a  wire-wound resistor is OK for use around RF, so if you
happen to have a 5 to 50 watt  wire-wound power resistor handy, it  will
work just fine.


The overall idea is to drain off the static charge before it has a
chance to build to potentials that can arc across something or damage
your set.    Back in the vacuum tube days, we didn't have to worry so
much about such things,  but that was then, this is now.


- Jim,  KL7CC



[hidden email] wrote:

> Is there any reason to choose one specification over another for the  
> 1N5711's which are found in the KPA100 and KAT100?  The ones currently  being shipped
> appear to be GTE's vs whatever the kits came with in 2006.
>  
> I had thought that the frequent frying of these diodes on my k2 was due to  
> lightning strikes but it looks more like the product of static discharges via
> my  23 foot vertical - you can hear the snaps as the potential arcs across the
> coax  connection at my Balun.  I had thought that this only happened in storms
>  but it turns out to start as the storm is coming.  Clearly the answer is to  
> disconnect the radio from the antenna unless I'm using it.  It's also been  
> suggested that I put an inductor shunt between the lead in and ground, but  I'm
> at a loss to size it.
>  
> It also occurred to me to bring a wire up from the grounding system to  about
> the height of the antenna and put one of those wirebrush static wicks on  it.
>  
> Is what I'm considering crazy?  If you think so, it will be ok to tell  me.
>  
>  
> 73   AI4TO       john  ferguson    M/V Arcadian
>
>
>
>  

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[Jerry Flanders]

I would put a high-value resistor - 10K or more - from the vertical
to ground. That should dissipate the static buildup AOK.

Jerry W4UK

At 01:48 PM 8/19/2008, [hidden email] wrote:

>Is there any reason to choose one specification over another for the
>1N5711's which are found in the KPA100 and KAT100?  The ones
>currently  being shipped
>appear to be GTE's vs whatever the kits came with in 2006.
>
>I had thought that the frequent frying of these diodes on my k2 was due to
>lightning strikes but it looks more like the product of static discharges via
>my  23 foot vertical - you can hear the snaps as the potential arcs
>across the
>coax  connection at my Balun.  I had thought that this only happened
>in storms
>  but it turns out to start as the storm is coming.  Clearly the
> answer is to
>disconnect the radio from the antenna unless I'm using it.  It's also been
>suggested that I put an inductor shunt between the lead in and
>ground, but  I'm
>at a loss to size it.
>
>It also occurred to me to bring a wire up from the grounding system to  about
>the height of the antenna and put one of those wirebrush static wicks on  it.
>
>Is what I'm considering crazy?  If you think so, it will be ok to tell  me.
>
>
>73   AI4TO       john  ferguson    M/V Arcadian

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[AD6XY]

A proper static discharge path to ground is essential in all antennas likely to see a high E field, which in areas with thunderstorms really means all antennas.

The RF choke is probably the easiest way to ensure a low DC impedance - connect it across the centre of a dipole or from a vertical element to ground - at the lowest impedance point you can - usually the feed point. A high power high value resistor will work but not for higher fields like nearby lightning. I use an air wound coil on my higher frequency antennas 4m/6m and a high power choke using an RF rated toroid for the lower bands. As long as the impedance is 10 time higher than the antenna it should be OK. That means 500 Ohms which at 7MHz is 12uH or more. Make sure you use a large torroid able to handle the high RF currents that may occur on some bands where the antenna match is poor.

If your antenna is multiband but not trapped it will have a high impedance at the second harmonic, e.g. 40m for an 80m antenna, 20m for a 40m antenna. Apart from the obvious point that this antenna is not going to work well if fed via a coax and balun, it does make things harder because you need a higher impedance. A possible alternative is to use a 1:1 transformer to couple the coax to the antenna, act as a balun and also isolate the static. As high voltages would arc remember to ground the centre tap on the the antenna side. If it is balanced, this should be a voltage minimum. Of course, if you are already feeding it against real ground, then the vertical will be already by grounded in the transformer fed configuration you will not need a centre tap.

Mike