Topband: alternative to vacuum variables

[ZR]

----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: <topband at contesting.com>
Sent: Friday, January 25, 2013 1:00 PM
Subject: Re: Topband: alternative to vacuum variables


>> Yes, there is a great solution, I'm using it for almost 10 years. The 
>> issue
>> with capacitors in 160m is the dielectric an most capacitors get hot and
>> change the capacitance. I used 19 x 62 pFNPO 3KV capacitor  to get 
>> 1200pF.
>> Never had a failure. Just keep them apart , don't let two capacitor 
>> touch
>
> This is an age old problem highlighted with amplifiers. The problem with 
> higher value ceramic doorknob capacitors is temperature coefficient, not 
> so much the heat. It is impossible to find NP0 (negative positive zero) in 
> a thick dielectric ceramic. Typically the transmitting doorknob cap has to 
> be less than 100pF to get an NP0.  A 200 pF 5 or 7.5 kV was typically 
> measured in the N1000-2000 range, more often than not worse than 
> advertised. 170 pF 5 kV's of better selection could get into the N150-300 
> range.
>
> Most of the reason I used to parallel multiple transmitting doorknobs was 
> to keep the temperature coefficient reasonable, not for heat.
>
> Multi-layer transmitting chip ceramics and some disk capacitors are 
> available in NP0 or the equivalent.
>
> Mica capacitors are generally very stable, so the old surplus WWII screw 
> terminal block micas work pretty well and are often just a few bucks.
>
> We should remember coaxial cables are transmission lines. Because of that, 
> they have pronounced standing waves. With any significant electrical 
> length in degrees the capacitance will NOT be what we calculate for 
> capacitance per foot. At just a few degrees they deviate from calculated 
> because they start to have significant open stub effect. This also results 
> in higher voltage at the open end than we have applied across the feed 
> end. With longer stubs (even though we might imagine them capacitors) the 
> voltage step up can be significant. This was a problem in more than one 
> amateur antenna using coax to make cheap capacitors, the Unihat vertical 
> being one of them that comes to mind.
>
> There are many cheap alternatives, but there are few cheap universally 
> good alternatives except perhaps a multitude of lower value temperature 
> stable caps in parallel, or mica or air insulated caps.
>
> 73 Tom

A problem with old TX micas is leakage as they have deteriorated with age. 
Ive had to toss about half of mine since they wont even pass at 1000V from 
one of my vintage cap testers. Id still have resevations about the remainder 
in HV RF situations.

Carl
KM1H