Peter,
Looking at the 1 kW PEP Racal TX I have here, the approach the
designer(s) took was to have three (solenoid) chokes in series (and
at 90 degrees to one another , if I recall the physical layout
correctly) with one fixed and one switchable bypass capacitor, arranged as:
HT in ---25 uH---88 uH---25 uH--(valve anode)
Between the first 25 and 88 (HT side) there is a fixed 10nF down to
deck and between the 88 and 25 (valve side) there is another 10 nF
capacitor which goes to deck via a pair of contacts which are closed
for operation on frequencies above 6 MHz. HT is 3 kV DC and the
valve is operated per the Eimac data sheet. I've never had any choke
fires or trouble in the HT supply, so this particular combination
appears to work OK. 10 nF is a high value though (I can't recall
what type of capacitor is used for these).
In making a TX cover 1.6 to 30 MHz with no gaps, there were clearly
'issues' with the design 'behaving badly' as, above a certain
frequency, sections of the tune and load roller coasters get shorted
out by solenoids. Even with the shorts operating, performance still
tails off noticeably at 28 MHz due, I suspect, to the mere physical
size of the tank circuit components.
That doesn't answer any of your questions directly, and I won't claim
the design example represents anything other than something that, in
the early 1980s, was considered 'fit for purpose'. As someone once
told me: "There's all sorts of bad designs out there - some are by
amateurs and cost next to nothing and some are by professionals and
cost the earth!"
Regards.
Mark.
-----------------------------------------
At 10:23 17/07/2006, you wrote:
>So what inductance should the shunt feed RF Choke be?
>Take a 'classic' choke - the old National R-175, used at the kW DC
>input in AM days. It's 145 microhenries, so with something around
>2kV rms across it, and its reactance of about 3.2kohm at 3.5MHz,
>there's 625mA of RF in the choke. Plus the DC, of course. Assume
>that the choke Q is 100 (probably being generous here) and we have
>12.5 watts dissipation in the choke.
>I have a 4 'pie' choke, reputedly used in the NCL 2000. That's 950
>microhenry, and issipates a couple of watts. But the problem is the
>series resonance - in the way I have it mounted, that just happens
>to be at 21.5MHz.
>Some transmitters I've seen used under 100 microhenries, but had no
>series resonances in their operating range.
>So what's the best to use?
>The lower the value, the better the chance that there won't be a
>nasty series resonance, but there's more tuning capacity needed, and
>more current through the bypass cap at the HV end. Incidentally, I
>seem to remember that Rich has made some comments about the
>value of the bypass capacitor, which obviously has to handle the
>current. It seems that you need an extra choke between the bypass
>cap and the HV supply if you're really going to keep the RF out of
>the supply, and thus the mains. I know Alpha in some of their amps
>switch the choke, which is one approach. But especially when you're
>covering 160 as well as 10, it's obviously not straightforward as to
>what choke to use. I've tried the tack of a solenoid choke (possibly
>ex ART13 - solenoid, about 1/2 inch diameter, about 4 or 5 inches
>long, 95 microhenry) in series with the NCL 2000 choke. Series
>resonance for the pair is 24MHz.
>Question: how far away from the operating frequency is acceptable
>for a series resonance? The resonances seem to have quite high Q.
>Measurement method is to remove the tubes, use a scope probe as that
>gives about the same capacity as the tubes, connect a load resistor
>of the calculated laod value between plate and ground and feed a
>signal generator in (via a return loss bridge) at the pi output.
>Tune the pi for max volts across the load resistor consistent with
>max return loss. Vary the sig gen frequency, and you can see
>frequencies where there's very sharp dip in plate volts - down
>pretty well to nothing. Which I feel is a delicate test for the
>series resonance.
>It does seem, prima facie, that a lot of the chokes in use are
>rather low in inductance for ideal operation.
>73
>Peter G3RZP
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*******************************************
Mark Hill - G4FPH
E-mail: g4fph@mjha.co.uk
Current web pages at: www.g4fph.net
Old web pages at: www.qsl.net/g4fph
----------------------------------------------------------------------
Dipoles resonant on 1942 / 3720 / 7085 kHz
Three element wire beam coming soon on 14180 kHz!
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S witch off
I solate
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