RF-specific 'doorknob' capacitors Q-factor, suitability for tuned circuits

[Opcom]

May be a stupid question, but where a capacitors is needed to block DC and just pass RF, these seem to be the ticket.

How suitable are they to be part of a tuned circuit, say, as a bulk capacitance allowing a smaller and more economical variable capacitor for loading?

My presumption is that as long as the current and voltage ratings are not exceeded there should be no reason against using an RF doorknob.

One circuit calls for a 2500pF vacuum variable. It would be great to use two of them and a 500pF vacuum variable.
One circuit calls for a 1000pF fixed vacuum cap. The value is not critical for tuning as long as it is stable.

I'm specifically asking about the Strontium Titanate types here:
http://qro-parts.com/product_info.php?products_id=104
http://qro-parts.com/product_info.php?products_id=95

[WD8BIL]

I don't see a problem, Op! Many have been using doorknob padding for years. Those should work fine.

[KD6VXI]

Opcom,

Those Russkie caps are really good.  I've used them in a few applications.

What you will need to watch out for is:  When you use them as a padder, the higher value ones will hog more of the current.  So, you're going to see 2/5 of the current in each doorknob and a fifth in the vac variable.

This won't be a problem depending on how much power you're running.  In a qrp tuna, not going to be an issue. 

Scale accordingly.  Might be better to go with 4 x 500s.

I've also had GREAT luck with Chinese doorknobs.  They hold up about as good as a Draloric.


--Shane
KD6VXI

[K1JJ]

Yes, as Shane said, much will depend on where in the circuit the cap is placed, the amount of RF current it will hog based on the ratio between the vac cap and the doorknob - and the circulating resonant currents involved.

I have seen door knobs explode right away when used as the only capacitor in a plate tank circuit. And I have seen them last forever in antenna tuners as padders.  One ham used a string of doorknobs stacked with a band switch in a tank circuit - and used a roller inductor as the tuning element - BANG!   He changed the fixed caps to a big bread slicer and all was well.

Those look to be very good quality capacitors, so use as padders in rigs less than 1.5 KW should be fine.  When placed in a RF tank circuit, the circulating current will depend on the Q and could get very high on some bands and tune up tests.

As you can see, many variables are involved.  But at least you are starting with high quality caps which will give you the best chance of success. (compared to older 1950s/60s  fragile 500 pF  20KV TV door knobs)

T

[w4bfs]

good to hear all weigh in .... 160 mtrs can bring a new challenge ... I've seen a 3 or 4 section old trf variable used as a pi-net load work fb on 80 - 10 mars and fail badly on 160 .... all of each bank of rotor ground wipers looked fb and ohmed out low resistance ... a real pain to have to change

[Opcom]

Thanks much for the input on them. The last thing I want is to have one explode after going to all the trouble. 

[W2PFY]

Scrotium Titanate for dielectric. Perfectly for RF PA Up to 10kW !  Sounds like some powerful stuff!