The problem with doorknobs capacitor is the dielectric. Class one or NPO
does not change capacitance with temperature and has low loss on 1.8 MHz,
however the max capacitance you can find is 170pF. Most 500pf doorknobs are
N750 and can be used up to 3 amp on 80m, but only 1.5 amp on 160m, with the
dissipation of loss in heat and they change capacitance.
A simple solution is to parallel several small capacitors NPO. The small
capacitor below cost US1.00/each.
CERA-MITE - 564CC0GAA302EL620J - Capacitor, ceramic. 62pF 3,000V
Capacitor, ceramic. 62pF 3000V. Type: NPO, epoxy coated. Tolerance: 5%.
Package: radial disc. Dimensions: 16.9mm D x 3.8mm with 5.8mm lead spacing
http://www.electronicsurplus.com/cera-mite-564cc0gaa302el620j-capacitor-cera
mic-62pf-3-000v
I used 20 in parallel for several years tuning my Folded Unipole with no
problems, very stable, just do not let one disc to touch each other.
Regards
JC
N4IS
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Lennart m
Sent: Monday, March 13, 2017 1:29 PM
To: k2av.guy@gmail.com; 'Gary Schafer' <garyschafer@largeriver.net>; 'Jim
Kennedy' <kennedyjp@cableone.net>; 'Steve London' <n2ic@arrl.net>
Cc: 'Topband' <topband@contesting.com>
Subject: Re: Topband: [TowerTalk] What's heating up ?
Hi all,
There are some good, fixed EU caps that were used in coast to ship radio
transmitters.
One of the outstanding onses were manufactured by "Rosenthal", also famous
for the production of extremely high Q porcelain used in dishes.
Otherwise I would follow the advice from Guy.
73
Len
SM7BIC
-----Ursprungligt meddelande-----
Från: Topband [mailto:topband-bounces@contesting.com] För Guy Olinger
Skickat: den 13 mars 2017 17:41
Till: Gary Schafer <garyschafer@largeriver.net>; Jim Kennedy
<kennedyjp@cableone.net>; Steve London <n2ic@arrl.net>
Kopia: Topband <topband@contesting.com>
Ämne: Re: Topband: [TowerTalk] What's heating up ?
Once you know the problem is the cap, then you need to get really serious
about the cap. Caps in RF tuning networks are stressed applications.
Decide if you ever want to use it for anything except short and separated
cycles, like calling DX. If you get into contests, or ragchew for hours with
relatives or friends, you need to beef up.
Use your junkbox to tune up the network at 25 watts so you know what the
values are, and then go get something serious that will take what your model
says is needed for 5 kW, something with dissipation, etc, way in excess of
"ideal conditions" predicted by a network model. Do **not** just barely
cover 1.5 kW in your calculating.
Caps in tuning networks can wind up carrying many times the current
specified in models, as you tune away from "center" frequencies, or
experience wandering environment, like Z moving with wind, or rain saturated
ground, or trying to move up the band by switching a tuner in the shack.
If you do QRO, get a vacuum cap or create the value with three or four
parallel ceramic doorknobs of the HEC HT50 variety for values of 500 pF or
less. The Russian flat doorknobs are probably the best for 3300 or 2200 pf
specifications. Most of the pictures have kBap (kVA) numbers on them.
If you can't locate **manufacturer** current ratings or Russian kBap
numbers, then don't use or don't buy. Invest in caps you KNOW are rated.
A contest will find you out if you go cheep, and heat often is run-away,
where increasing heat increases resistance. And then you're toast, because
the heat has quite possibly changed the value and characteristics of the
cap.
Prices have been going up on caps, and you may easily spend $100 or more to
get fixed caps that will do the job without heating up.
Don't ask me how I know this. :>) But I won't ever go cheep on tuner caps
again.
73, Guy K2AV
On Mon, Mar 13, 2017 at 9:42 AM Steve London <n2icarrl@gmail.com> wrote:
> The characteristics of the doorknob are unknown. It's a junk box
> special, with all of the lettering faded. Found an interesting article
> on the web by I0IJ on RF vs. HF capacitors. I'll have to try
> experimenting with more caps from the junk box.
>
> Thanks for the suggestions.
>
> 73,
> Steve, N2IC
>
>
> On 03/12/2017 11:01 PM, Jim Kennedy wrote:
> > Garys suggestion is right on. I assume from the the cap you describe
> that its a doorknob type. If so be sure it is RF rated and not a HV
> type used in power supplies, Total different characteristics.
> >
> > Jim
> > W7ouu
> >
> > ----- Original Message -----
> > From: "Gary Schafer" <garyschafer@largeriver.net>
> > To: n2ic@arrl.net, "Topband" <topband@contesting.com>,
> towertalk@contesting.com
> > Sent: Monday, March 13, 2017 4:00:42 AM
> > Subject: Re: [TowerTalk] What's heating up ?
> >
> > Probably the capacitor. Try 2 or 3 smaller values in parallel to
> > make
> your
> > 2200pf. Then see if the drift is the same.
> >
> > Or the coil wire size is too small.
> >
> > 73
> > Gary K4FMX
> >
> >> -----Original Message-----
> >> From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
> >> Of Steve London
> >> Sent: Sunday, March 12, 2017 10:42 PM
> >> To: Topband; towertalk@contesting.com
> >> Subject: [TowerTalk] What's heating up ?
> >>
> >> I am experimenting with the N4KG reverse feed method for 160 meters
> >> on a tower with a lot of stuff already on it. I like the
> >> possibility that elevated radials will be an improvement over my
> >> terrible, dry, mountaintop ground characteristics.
> >>
> >> As I transmit, the SWR creeps up, pretty significantly. Something
> >> is clearly getting warm.
> >>
> >> Possibilities:
> >>
> >> - I am matching the 160 feedline with an L-network (series L, shunt C).
> >> TLW says 7 amps through the capacitor, with 3 watts of dissipation.
> >> The capacitor measures 2200 pf, the usual transmitting capacitor,
> >> cylindrical, about 3/4" in diameter.
> >>
> >> - The M2 balun feeding the 40 meter beam on the same tower. I
> >> monitored the SWR on the 40 meter beam while transmitting on 160.
> >> No change in 40 meter SWR.
> >>
> >> - The M2 balun feeding the KT-36XA on the same tower. I monitored
> >> the SWR on the KT-36XA while transmitting on 160. No change in
> >> KT-36XA meter SWR.
> >>
> >> - Gamma match on homebrew 15 meter beam. Gamma match is just a
> >> piece of
> >> RG-8 dielectric/center conductor inside aluminum tubing.
> >>
> >> Suggestions on how to troubleshoot this ? Start by disconnecting
> >> the feedlines to the baluns ? And once I determine which component
> >> is heating, what then ?
> >>
> >> 73,
> >> Steve, N2IC
> >> _______________________________________________
> >>
> >>
> >>
> >> _______________________________________________
> >> TowerTalk mailing list
> >> TowerTalk@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/towertalk
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
> _______________________________________________
>
>
>
> _______________________________________________
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> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
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