[Lowfer] Re: TX output protection.

Sun, 31 Aug 2003 09:05:09 -0600

The thing to remember about MOVs is that, for purposes of final 
protection, they could considered to be just large capacitors.

This is fine, if you don't mind a capacitor with pretty lousy tempco and 
being stuck with the value(s) that you might find kicking around in your 
MOV collection (having measured them yourself, of course...)

The two things that I have used for protection of my MedFER and LowFER 
transmitters are:

- Bleeder resistor.  Because there was no DC ground intrinsic to the RF 
output, I used a coupling cap.  A 100k to ground is "invisible" to the 
output and keeps charges from accumulating due to wind static, 
gradients, etc.
- A gas discharge tube.  I tend to use the "NE-2" models (although I 
acquired a pile of NE-2G's and have been using those recently...)  
Little neon lights are really cheap and fire in the 60-90 volt region - 
generally below the breakdown voltage of the final transistor and well 
below typical operating voltages.  These have little capacitance, react 
pretty quickly, and won't get "stuck on" between zero-crossings of the 
RF waveform.  [I just measured a couple of these and found that they had 
a typical capacitance of between 0.65 and 0.75 pf at about 1 MHz, as 
measured on my AADE LC meter...  I've seen these used in the front ends 
of VHF and UHF radios, too...]
- Don't forget that the majority of the energy in an EMP due to nearby 
lightning strike will also be eaten/reflected by the matching network:  
Only that amount of energy in the vicinity of your output matching 
passband is likely to find its way into the finals, anyway - and that 's 
a small percentage, typically, for a LowFER or MedFER TX - leaving only 
a relatively small amount of the actual energy to be taken up by the 
flash of light from the NE-2 - or smoke from your finals.

I've never lost a MedFER or LowFER final to lightning, yet - but I've 
blown plenty of them up with "oopses" in the past.

Of course, in the event of a direct hit, the protection that you get has 
to do with how well your grounding is done, how the antenna is mounted 
and where, the sincerity of recent genuflection, etc...

Clint
KA7OEI

>
>Message: 17
>From: "KD7JYK" <[email protected]>
>To: <[email protected]>
>Date: Sat, 30 Aug 2003 22:06:44 -0700
>Subject: [Lowfer] TX output protection.
>Reply-To: [email protected]
>
>I hear the horror stories of zapped finals due to lightning and ESD, has
>anyone considered MOVs on the output?  I have seen them available from just
>a few volts to several hundred.  If not these, what is used?
>
>Kurt
>
>---
>Ross Technologies Signals Intelligence Division
>Rosetta Proving Grounds
>
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