[AMPS] Re: 6m amp blocking caps

[Ian White, G3SEK]

2 Wrote:

I had written:
>>But dipping the two capacitors on their own tells you *nothing* about
>>their behavior in-circuit.
>>
>//  The issue at hand is using two, paralleled capacitors as a DC
>blocker.  My position is one DC blocking cap with more current capability
>is a better choice
>
Not unless the current capacity is more than two times greater than each 
of the two paralleled caps.

>>However, you can make some real measurements using an MFJ or Autek SWR
>>analyzer:
>>* To look for a series resonance, connect the components in series with
>>a 50 ohm resistor and sweep the frequency - SWR dips to 1 wherever the
>>total reactance is very low compared with 50 ohms.
>
>//  So Mr. Rauch was in error about the MFJ analyzer's dipper
>disfunction?

Since I don't know exactly what you mean, I wouldn't care to comment.

>>* To look for a parallel resonance, connect the components in parallel
>>with a 50 ohm resistor and sweep the frequency - SWR dips to 1 wherever
>>the total reactance is very high compared with 50 ohms.
>>
>//  The hat-trick is finding an elusive zero-L 50-ohm resistor.
>
Not at all - any physically small carbon-film or metal-oxide resistor 
will have a low enough inductance for all but the most accurate 
measurements at HF.

Most modern CF/MO resistors have a spiral element, but if you scrape off 
the paint, measure the diameter and length, and count the turns, you can 
estimate the inductance using the normal coil formula. You'll find that 
at low impedance levels the inductance is generally too small to matter 
below 30MHz.

The inductance of these resistors only begins to matter at VHF. For even 
lower inductance you can switch to chip resistors, and they are good for 
accurate SWR and return loss measurements up to at least 1GHz.

>>A network analyzer it isn't, but it does show what kind of resonances
>>you have, and where they are.
>>
>//  Dipmeters don't show where resonances are?

Of course dipmeters show where resonances are - but that is all. They 
don't show what those resonances mean.

-- 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)
                            http://www.ifwtech.com/g3sek

--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com