[Top] [All Lists]


To: <>
From: "Jim Thomson" <>
Date: Fri, 6 Dec 2013 05:47:07 -0800
List-post: <">>
Date: Thu, 5 Dec 2013 03:03:44 +0100
From: "Peter Voelpel" <>
To: <>

It?s quite simple without measurements.

Lets assume the amp is tuned for full legal output into a low SWR on 80m
which needed 1000pf of load capacitance.
Then suddenly the SWR goes sky high because the coax connector went open.

How can the voltage across the load capacitor become 5KV when the reactance
of the load capacitor is just 45 ohms on 3.5MHz?
That would require about 110 Amperes of RF current through that capacitor.
And where is that big current coming from in a 1.5KW amp, not in a 100KW?


##  back in the late 1970s.....with my single 4-1000 GG did aprx 
1900w pep out. 
It used a clunky 3PDT mech relay.  One day the output side didn’t quite make 
contact, from
all the hot switching etc.  130w of drive applied.....and NO ant connected for 
a split second.
The result was the 1500 pf  cardwell  air variable load cap  was welded shut.  
It had aprx 1500 V
spacing.   I used a flat ignition file to grind the weld free.   It happened a 
2nd time months later..same deal.

##  I found the safest way to ensure the expensive  vac load cap never opens up 
is to install an adjustable 
spark gap across it.  Then adjust it for a break down V that is less than what 
the peak V rating is on the
vac load cap.   In normal operation, the amp will shut down when ant SWR  
exceeds 2:1  or whatever
threshold I desire.  The spark gap is adjusted for the eq of say a 4:1 SWR.  If 
the ant fails...and coax goes open,
and IF the high swr protection doesn’t shut down the amp 1st.... the simple 
adjustable spark gap will ensure
the vac load cap will survive. 

##  If you have seen how the mech screw adjustments are on any vibroplex keyer, 
the adjustable spark gap
is very similar.  Just take a pair of 10-32 machine screws....and grind the 
ends to a fine point.  Make one fixed,
and the other adjustable, complete with lock nuts to maintain a precise gap.  
Some bar stock etc, can serve to
support the machine screws.....just tap threads in em. 

##  For folks with PI-L networks in their amps.... the load cap is typ at a 
200-400 ohm point already..and peak
V across the load cap is already a helluva lot higher than the  load cap in a 
PI net. 

##  For folks with a PI net, make sure you use a piece of 50 ohm coax between 
the load cap and the TR relay. 
And with shield bonded to chassis at BOTH ends.   If wire or cu strap is used 
instead...then u end up with a huge Z bump.
That alone will put the load cap at something higher than a 50 ohm point... 
increasing the peak V.  On 10m band, the 
stray L  of a piece of wire instead of coax  will make the PI  now appear as a 
PI-L.... putting the load cap at a much
higher Z point.   same deal peak V increases.     Now with a wide open on the 
ant, coax cable etc, the load cap
will weld shut.   

##  as far as the current ratings on vac caps go.... read the various graphs 
very carefully.   They only handle max
current when FULLY meshed.   Most graphs will depict a peak at some 
4 mhz.  Below 4 mhz, the current ratings
drop way off.   At lower freqs, the XC  will increase  for a given cap value.   
Current drops off because of either the 
cap is being used at less than max C. and or the freq is below 4 mhz.  Both 
cases will cause an increase in XC.
Below 4 mhz the cap is said to be V limited. IE: the V would have to be 
increased well above max V ratings to
increase the current.      Above 4 mhz, the vac cap is now said to be current 
limited...again when fully meshed. 
IE:  you cant run max rated V on freqs higher than the peak depicted in the 
various charts.  

Jim  VE7RF

Amps mailing list
<Prev in Thread] Current Thread [Next in Thread>