Yes, certain doorknob caps are fine for QRO filters, at least at HF.
I have used low-cost Russian mil-surplus NOS ceramic NPO doorknob capacitors
in LPF circuits for QRO solid-state amplifiers at the 2.5 kW level with
complete success. These filters have been both the traditional multi-section
low-pass, and the harmonic terminating diplexer low-pass/high-pass filter
topologies.
The K15U-1/M1500 dielectric style has proven bullet-proof at this HF power
level where substantial amperes of RF circulating currents flow.
They are a fraction of the price of the gold-standard HEC HT-57 caps.
There are also the KVI-3 style Russian doorknobs. These capacitors were
designed for RF and pulse applications, but they are not suitable for high
power, in high-Q tuned circuits with large circulating currents. However,
they are very suitable for RF coupling, de-coupling and pulse work. We note
that a capacitor used in pulse circuitry, will for short pulse durations
handle at least ten times the current it was originally designed for. These
high-k capacitors of larger capacitance value are usually 10 or 20 kV rated
devices. These are fine capacitors as a DC block / plate coupling capacitor,
where due to the relatively high impedance, and not being part of the plate
tuned circuit (some people might argue about this), the RF current is
relatively low.
The ATC E-series of high-Q / low-ESR porcelain chip caps also do a fine job
in RF filter circuits up to quite high power levels to circa 1 kW. They are
expensive and availability to ham experimenters is hampered by the vendor
MOQ quantities.
At this power level in a 50 Ohm system the RMS voltage would be circa 230V
but that could well rise up higher at certain nodes within the filter
network due to Q. One can of course readily model all the filter node
voltages and circulating currents with a circuit simulator like SPICE, etc.
When marginal, placing several chip capacitors in parallel to split the RF
current path can mitigate risk of failure due to internal heating by I2R
dissipation in their ESR.
For the small C value, low-loss capacitors required for a VHF 2m filter at
QRO power level, I would fabricate bespoke homebrew caps made from a sheet
of Teflon between two parallel plates.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Tuesday, 29 April 2014 7:04 AM
To: amps@contesting.com
Subject: [Amps] low pass filter fail ;)
Date: Mon, 28 Apr 2014 08:32:04 +0200
From: peter chadwick <g8on@fsmail.net>
To: Bill Turner <dezrat1242@wildblue.net>, amps@contesting.com
Subject: Re: [Amps] low pass filter fail ;)
With a VSWR of only 1.2:1, I wouldn't have thought it was too close to the
cut off frequency. Passive LC filters (other than the absorptive types) work
by having a high VSWR in the stop-band. According to the table in Geffe's
book (Simplified Modern Filter Design), a VSWR of 1.22:1 represents a ripple
of 0.04dB. I would suspect a lossy capacitor - we don't know the details of
the filter designs and exactly what the capacitor was, nor the power level.
You can have some high currents at quite low power levels.
vy 73
Peter G3RZP
## 2m is 144-148 mhz. So where is the optimum cutoff freq? Heck, even with
a 200 mhz cutoff freq.... it will still kill the 2nd and up harmonics.
Most of the problems I have had with LP filters for HF is the caps...and
their current ratings. U are better off to use say 2 x 100 pf cap vs a
single 200 pf cap.
## I have one of last built henry radio 10 kw HF LP filters..with 7-16 dins
on each end. They used a mess of 25 + 50 pf NPO caps...and all are HT-57
types. They used a single 50 pf at each connector...and in the middle of the
filter, between the 2 x .25" OD tubing coils, they used 8 x 25 pf doorknobs
in parallel. Cut off was at aprx 33 MHz.
## HT-57 caps are rated for 15 kv...and in low values..are not only
NPO..they also handle one helluva lot of current. LP Box was aprox 5.5 inch
square x 10-12 inches long. Of course on the lower freqs, like 160-12m..the
size of the LP box becomes huge.
## Those 5 kw band pass filters made in the UK are slick. Band pass is
better than a LP. They were band pass on all HF bands except 160m.
160m is a LP.
## LP filters for VHF is not that big a deal. You can see the various LP
filters available for 88-108 mhz FM broadcast. Those filters are available
with any cut off freq u want..and any power rating too...like 25 watt up to
the sky is the limit.
Jim VE7RF
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