[Elecraft] KPA500 receive loss

[hb9cvq at hispeed.ch]

Even (sub) nano second (ns) fast acting gas tubes are primarily not a safe solution to RX front end burnout protection.

Semiconductors:
Issues: Energy Protection level, RX Intermodulation prevention.
There are however some fast semiconductor diodes (ESD protection) available with low capacitance. 
They have safe enough voltage protection level even for sensitive RX electronics, but limited energy absorption.

Spark gaps:
A several kV, ns-fast, transient (ESD, EMP, high dV/dt) voltage RX input spike may be better limited in energy but more voltage amplitude; still around 800 to 1000V left over for a short time.
Suitable spark gap can trigger quickly (dV/dt dependent).
There is always some time delay in gap breakdown. Sensitive RX components need to survive that safely, without being pre-damaged .

This low pressure gas ionization process is also governed by HV breakdown Paschen law  https://en.wikipedia.org/wiki/Paschen%27s_law .
The number of first available free gas electrons, needed for a quick avalanche breakdown, are important.
By gap fast overvolting (field emission process from electrodes-> source of free electrons, without radioactive , ionizing material ) the spark gap is triggered.
The gas tube would fire at much lower DC voltage levels ( e.g. <100.. 200V+ ) 

Fast pulse triggering creates a very broadband spectrum of the residual onset pulse  (Time -Frequency Domain Fourier transformation ) . 
This triggering is different from a finally, time retarded, development of low resistance breakdown plasma channel.
Here the residual voltage may be 30 to 50V.

Too bad, the first spike may have already killed the RX. 

Therefore , if capacitance is not an issue (no HF RX) , one uses low pass filters after the spark gap.
To control the energy reduction one can use additionally multi-stage protection modules ( 1.) Gap tube-full energy ,2.) elevated energy semiconductor, 3.) low energy semiconductor )
The triggering of the stages must be well time coordinated. If AC voltages are continuously present in PS Networks , the spark gap (low R Plasma) network follow on currents must be limited to lead to well controlled extinguishing of the gap.

PS: 
One other trick to avoid high capacitances (high VSWR) is to arrange protection diodes in a bridge configuration  -cancelling out C
One other trick to avoid intermodulation is to DC bias diodes appropriately.

Experience furthermore teaches: Even 4 unbiased Schottky Diodes ( ca. 0.3V each )  in series/anti parallel  at RX front end are often critical regarding INTERMOD , e.g.in  multi TX contest site scenarios )

End of lecture ...if considered too long and irrelevant just ignore it Hi Hi 😉 



-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On Behalf Of Dr. William J. Schmidt
Sent: Samstag, 5. Juni 2021 18:02
To: 'Elecraft Mail List' <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] KPA500 receive loss

I'm sure you could invent many ways to protect these parts (gas tubes, sacrificial TVS diodes, fusing, etc.).  The best way, of course, is to disconnect it (which of course I did not do).  These are low voltage parts on the receive side path so I don't know if a gas tube would be appropriate here...  Someone with more experience with them can chime in here.


Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ 

email:  bill at wjschmidt.com


-----Original Message-----
From: CUTTER DAVID <d.cutter at ntlworld.com>
Sent: Saturday, June 5, 2021 10:46 AM
To: bill at wjschmidt.com; Elecraft Mail List <elecraft at mailman.qth.net>
Subject: Re: [Elecraft] KPA500 receive loss

Would a gas discharge tube save the sensitive parts?

David G3UNA


> On 05 June 2021 at 16:20 "Dr. William J. Schmidt" <bill at wjschmidt.com> wrote:
> 
> 
> This happened with my backup amp at the contest station on J6.  I left 
> it connected by accident overnight (too much rum) and we had some 
> static storms.  Took out the pin diodes (well, more accurately the 
> diodes that act like PIN diodes).  When put in operate, the receive 
> drops by 30+ dB but still transmits fine.  A simple replacement of the 
> four diodes and three (?) transistors solved the problem.  Dirt cheap.
> You can make these measurements at the test-points to confirm:
> 
> 
> TP2= RX 13.1, TX 0.8
> TP4= RX 3.9, TX 13
> 
> 
> Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ 
> PJ2/K9HZ
> 
> email:  bill at wjschmidt.com
>


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