50pF 15 kV DoorKnob caps for 50MHz LPF

[VK4UH]

Hi all

Im looking for a PAIR of 50pF 15 kV doorkonb type caps to build a legal-limit 50 MHz Low Pass Filter to cmplete my Larkin Panel based solid state amplifier project
(using the K1WHS design).
Alternatively I would be interested in a complete filter if one was surplus to requirement?

Can anyone help?

Kevin VK4UH Brisbane
vk4uh@wia.org.au

[VK2AAH]

Hi Kevin,

On EBay right now...

http://www.ebay.com.au/itm/Factory-New- ... 43b8035a1b

The seller has plenty it seems.

Cheers


Richard
VK2AAH

[ZL2WHO]

Make contact with Bob ZL1RS.. I know he has a few but not sure on values.

[VK4CZ]

KJ I sourced mine from, Fair Radio Sales in the USA... although 2nd hand, they are g'teed and for Type 850 substantially cheaper!
https://www.fairradio.com/catalog.php?m ... goryid=965

[YU7EF]

Kevin, few months a go I have built that 6m LPF based on K1WHS data but used 7.5kV NPO door knob capacitors

LPF is installed right at the RF deck output of my GS35b amplifier and tested at full 1.5kW power without any problems with excellent results

However, original coils were not producing correct performance so I took some time and re-designed it, getting even better suppression ...
With new coils I got perfect result and measuring with spectrum analyzer confirmed calculated results 100% ...

Attached is picture of my LPF (used 60x60 square tubing to house it) and building information for revised design for those interested to build it

GL, Pop, YU7EF

[VK3AUU]

I am wondering, considering the impedances involved and therefore the voltages and currents generated, what is the necessity for using such high voltage capacitors? Are there not lower voltage capacitors available which are capable of handling the currents flowing through them?

David

[ZL1RS]

by ZL2WHO » Sat Feb 09, 2013 1:20 pm
Make contact with Bob ZL1RS.. I know he has a few but not sure on values.

Some 50pfd 7.5kV 850 series "door knob" capacitors are on their way to Dr Kevin.

If anyone else wants 850 series "door knob capacitors" I have the following values available:

25 pfd 7.5kV Type 850 or 850S NPO
50 pfd 7.5kV Type 850 NPO
68 pfd not labelled, probably 7.5kV (new)
75 pfd 7.5kV Type 850 "N750"
100 pfd 5kV Type 850 "N750"

Unless noted these are used capacitors in good condition as salvaged from 5kW HF transmitters (txers were in going condition when removed from service) and have been tested with a capacitance meter. Complete with screws/bolts for the end connections. 2 x "door knob" caps including airmail postage to VK = AUD15.00. Quantities are limited. Contact me off the forum to my_callsign:at:yahoo:dot:com.

Heads up: Possible 6m Filter "Parts Kit" Some diecast aluminium boxes and very good quality silver plated/Teflon insulated SO-239 connectors are on the way here to try Pop YU7EF's 6m filter. If successful I will offer a small number of 6m filter "parts kits" for AUD40.00 including postage to VK. "Parts kit" = diecast aluminium box (undrilled), 2 x SO-239 silver/Teflon sockets, 8 x nuts and bolts to mount the SO-239's, 2 x 50pfd "door knob" capacitors (as above), sufficient enameled copper wire to wind the coils, and a couple of copper tabs to go on the capacitors to solder the coil ends to (see Pop's photo). I'll re-post here later in February after the parts have arrived, I've tried the filter and am happy that the "parts kits" will be satisfactory .............

73

[YU7EF]

That 50pF capacitor is the same I used

Since Bob listed 25pF capacitors as well , it might be of interest to some of you to use those for a 2m high power LPF

73, Pop, YU7EF

[ADMIN]

Great information and charts there Pop and Bob.
I will move this topic into the 50MHz forum, so this doesn't get swept away with a For Sale/Wanted "dead ads" cleanout.

[ZL1RS]

Heads up: Possible 6m Filter "Parts Kit"

Wow, Pop's 50MHz LPF design is spot on The parts finally arrived and one was assembled in a couple of hours.

YU7EF 6m LPF as built at ZL1RS

The plot below is from my Vector Impedance Antenna Analyser after a quick tweeking of the coils to bring the SWR dip to 50.1

YU7EF 6m LPF SWR curve

The insertion loss while transmitting at 50.1 MHz is unmeasurable on my Bird 43 Wattmeter (1100W indicated with or without the filter in line) ... maybe a needle width less with the filter in-line if I look hard

The Insertion Loss above the 6m band follows the curve in Pop's posting (I can measure it with my HP 8592B SpecAn but can't plot it). The LPF's suppression at spot frequencies of interest is as follows:
100MHz (6m 2nd harmonic) - 33dB
150MHz (6m 3rd harmonic) - unmeasurable in the SpecAn "grass" so >50dB down from 6m.

If anyone is interested, I have a few "parts kits" available as specified in my earlier post. Contact me off the Forum via e-mail ... zl1rs:at:yahoo:dot:com and we'll make the arrangements.

73

[YU7EF]

Great job Bob, I got same result right away and it really showed it's value - neighbor's TV antenna
for analog TV is less than 20m away but absolutely no problem with TVI at full 1.5k output ...

See you soon on my MS for EME QSO and a new DXCC ... hope to be QRV March 25-30 for at least
few days if weather doesn't mess it up even though forecast looks promising ....

73, Pop, YU7EF

[ZL1RS]

Continuing on with the 6m LPF saga ...

Ref: http://www.packratvhf.com/Cheese%20Bits ... 202010.pdf (8.84MB about converting Harris low band TV modules to 6m). Page 8 says:

Harmonics:
... The 2nd harmonic is suppressed (-30 dBc) due to the push pull configuration. (Splitting
and combining at 180deg suppresses even harmonics). The circuit does not provide any
suppression for the 3rd harmonic. The 3rd harmonic of a single module is only -18 dBc at 400 Watts ...

Say what? The harmonics from a 6m solid state PA might only be 18dB down from the fundamental carrier output? Consider the effect of a third harmonic 18dB down from 400W ... that is about 6W of 150MHz energy going into a 6m Yagi driven element which can be a fairly efficient radiator at 150MHz (ever used a 7MHz dipole on the 21MHz band?)

Computer simulations of 50MHz Low Pass Filters show harmonic attenuation as follows:
5 "pole" LPF - 2nd harmonic 33dB down, 3rd harmonic 53dB down (see Pop YU7EF's earlier post)
7 "pole" LPF - 2nd harmonic 55dB down, 3rd harmonic around 80dB down
The 5 "pole" filter is quite adequate for a valve PA, but the 7 "pole" filter with better harmonic attenuation is worth considering if using a solid state PA.

After building Pop's 5 "pole" version, we exchanged a few e-mails on 7 "pole" LPFs worthy of investigation. One design was of a similar form with an inductor as the first "pole" in the filter. Consequently that 7 "pole" LPF used 4 coils and 3 capacitors but tweaking 4 coils with their interactions was a very frustrating process ... so we moved to a 4 capacitor/3 coil version which was a dream to adjust by comparison. The schematic is:

6m LPF_7_capin.GIF (7.99 KiB) Viewed 2320 times

... and the resulting simulation plot:

I built this into the same sized die-cast aluminium box as the earlier 5 "pole" LPF by rearranging the capacitor and coil positions to fit. Alignment with an antenna analyser having a sweep function was fairly straight forward (but a little more touchy than the earlier 5 "pole" LPF). Here is the actual sweep of the 7 "pole" LPF:

6m LPF_7_plot.GIF (9.8 KiB) Viewed 2320 times

While the 50MHz feed through loss was negligible (about 0.1dB) and the 2nd harmonic rejection was near spec at 54dB, the attenuation of the third harmonic initially fell well short of the anticipated 80dB. This appeared to be coupling directly from input to output.

Adding two tin-plate shields into the box fixed that and the third harmonic attenuation then measured at just on 90dB. (FWIW the suppression at 100MHz (FM broadcast band) also improved to 60dB).

The coil dimensions are:
217nH : 6T, 2mm dia wire, 10mm ID, 18mm long
332nH : 7T, 2mm dia wire, 12mm ID, 21mm long

With this size wire the coils are OK at 400W CW/SSB but get quite warm after a few minutes of continuous carrier (QRO FM rag chewers and JT65a digital mode ops take note!) The coils start to "smell hot" at 1200W ... This issue was not apparent with the earlier 5 "pole" filter. No wonder continuous duty QRO commercial transmitters use copper tube with water cooling in their Low Pass Filters.

73

[VK6OX]

Very nice work Bob!

I think you have too much idle time.....but when I think again.....nah, you're running "point" for the rest of us who are interested!

BTW, the 5-pole job here has come up very well (tnx for the coil "pre-wind")...shows an SWR of 1.05 and the parameters are close to what Pop and you have produced.

Plastic shekels are in transit.

73 Andy VK6OX