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Cavity TH327
Category: EME Equipment, Created: 2003-02-15 12:06:30

The dream of solid power in EME is either solid state but expensive or heavy, using tubes but affordable due to surplus equipment.
Rohde+Schwarz manufactured TV repeaters which can be modified to make useful power on 1296 MHz.
I have this kind of amplifier in use since 1990, prior to be on EME. Since, many have tried out the TH327 TH347 tubes for 1296 MHz. It has become the workhorse for the big guns in this field.
Credit for the drawing below goes to ON7UN. Followed by some pictures of the cavity etc.
Further down is the first set of modifications, ON7UN and his gang have made. The latest version can be fond on the website of Eddy, ON7UN here: TH347 Cavity Modifications

This cavity is what makes EME real fun

This cavity is what makes EME real fun
This cavity is what makes EME real fun Visible the  typical colouring by humidity and heat in remote mountain sites
German craftmanship allows very stable operation during decades

This is Eddy's and Marcs way to get the TH327 running on 1296. The cavity is R+S ULV273 or Plisch type.
Thank you both for this fine work and description!

Hi Folks,
Last thursday we modified an R&S Cavity. This cavity is slightly different from the 'older' Plisch cavities.
In my Plisch amplfier, this cavity is present, and it is exactly the same cavity as the R&S cavities Dominique aquired recently.

TH-327 Rohde & Schwarz cavity modifications:
Remove all screws of the filter part, and take out the filter parts from the cavity. You can use this nice 7/16 output
connector to make your outcoupling. Do not trow away ! fig 1

pic 1

In between the two cavities ( filter / amplifier ) a coupling has been made. Remove the two small screws. You can
now remove the coupling by pulling the prope inside the filter cavity part.
Remove all screws from the amplifier cavity, and remove the inside fig 2 . Watch out for the finger stock, at the ventliation holes. ( air inlet for the Cathode cooling ) In this picture you can see the plunger
is at his lowest point. We need to modify this part so the plunger can go 10 mm lower. Also clean the inside of
the amplifier cavity using silver polish.

pic 2

Remove the blower inlet, also disconnect the HV to the anode ring. Remove the anode ring and the Capton capacitor.
You now have the base plate and the two cavities left. Cut off the filter part as in fig 3 .

pic 3

Now your are ready to make a new outcoupling ā la OE9PMJ . Drill a 15 mm diameter hole at 51 mm from the top of the cavity. This is
your new outcoupling point. fig 4

pic 4

Close the old outcoupling hole, by turning down a small piece of brass, and soder this part in the old outcoupling hole. fig. 5

pic 5

After making a temporaly outcoupling probe for measurement ( ā la OE9PMJ, a disk diameter of 15 mm ) we found that the cavity was resonating at 1.320 MHz when
the plunger was at its lowest point. We needed to lower the pluger about 5 mm . Take out the 3 shafts that move the plunger in and out. You will see the shaft diameter
is enhanced, to prevent the plunger to go any lower in the cavity. This has been done to prevent the finger stock to move in the ventilation holes. A drawing of the
output coupling probe is on Dominique's website. ( circuits TH327 )
We turned down the shaft diameter at the enhancement by about 10 mm, so the plunger can go lower in the cavity part.Fig 6.

pic 6

Watch out for the finger stock when it goes inside the ventilation holes. We tried to mount the plunger upside down, fingerstock up in the cavity. Like this the finger stock is not in the vent holes. This gave us very unstable behaviour of the cavity. We did not yet figure out why. To re-insert the plunger, watch the finger stock. In fig 8 you we insert the plunger upside down, using a plastic foil.

pic 8

Re-assemble the cavity and insert a TH327 / 347 tube. You are now ready to make measurements . fig 7

pic 7

In my cavity design, the air inlet for the Cathode is made by 3 holes. The later versions of R&S have one bigger hole. Close this hole, and make a couple of smaler
holes at the lowest possible part of the amplifier cavity. Cooling the cathode is nessesary ! Fig 9 shows the finger stock trough the vent holes, when the amplifier
cavity is resonating @ 1296 .

pic 9

Cavity is completed now ( fig 10 ) make the 7/16 outcoupling now using the coaxial output of the removed filter. Turn a disk diamter 15 mm. The coax part has a 15 mm diameter also.

pic 10

Now its time for measurements. We have the 'luxery' having a network analyzer, but of course less sophisticated equipment can be used as well. ( signal generator with
a directional coupler and a diode detector ... ) Of course you can also try to tune the cavity to 'full trottle', but start with low power, to avoid any damage to the tube.
When aliging the cavity 'cold' using test equipment, you are very close to full trottle when tuning 'hot'.

Below is the network analyzer's screen when after a quick tunig. Return loss -20 dB, not yet enough ... fig 11

before tuning

After tweeking, return loss of > 35 dB can be achieved. fig 12

pic 12

And now, after hard work, it's time for a (Belgian) Beer ! Watch out ! This beer is called 'Duvel' or DEVIL in english !

pic 13

Best 73's

Eddy ON7UN

For further information visit Eddy's webpages TH347 Cavity Modifications

or download the .pdf file from my webpage (there is a list of various .pdf files) here

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