Carsten Groen's (OZ9AAR) personal place

1500W attenuator

Building on the experience of my 400W attenuator, I decided to make an even more capable one. The goal was at least 1KW and at the same time it should be usable to at least 1.3 GHz (23 cm band).

Although the return loss (SWR) is very good below 430 MHz, the coupling to the SMA connector gets very low. This means in practice, you can only use it as a conventional dummyload at lower frequencies.

I can produce a limited number of the attenuator for anyone interested (or you can have your own made from the data linked below),

Assembled and tested attenuators (WITHOUT heatsink) is sold for €300,- each

This is plus shipping.

When ordering, I will deliver sweeps from my R&S ZNLE3 VNA of the attenuator (S11 and S21 curves).

The design

The attenuator is based on a 1500W terminating resistor from Diconix, part number 17-0694.

Datasheet, PDF file (search for the part number in the PDF file).

I bought the resistors from PCS Electronics. 

Apart from the resistor, the attenuator uses:

1. DIN 7/16 female connector, Radiall part number R185403547
2. SMA female connector also from QAXIAL, part number SMA14-2H-15E .
3. 4 pcs M4 screws (+ washers), 10mm long (for 50 Ohm resistor)
4. 8 pcs M2.5 screws, 4mm long (for cover and SMA connector)
5. 4 pcs M3 button head screws, 8mm long (for DIN 7/16 male)

(plus heatsink/fan depending on usage, the heatsink needs to be LARGE!)

Both the connectors can be purchased from RF Microwave in Italy (as well as other places).

The coupling port is done very simple in this case. It is only the center conductor of the SMA connector that picks up RF at a low level. The coupling factor depends on the frequency, generally the higher frequency the higher the coupling factor is ("more power on the coupling port at high frequencies").

As the coupling port is made like this, the return loss on the coupling port is not good. The coupling port does not look (at all) like 50 Ohm. In most cases that does not matter too much, one way of helping with this if needed by the equipment connected to the coupled port, is to insert an SMA attenuator (f.ex 10 dB). This will make the connected device see a much better return loss (two times the attenuation).

Mechanical design

The design of the attenuator is pretty simple. The attenuator needs to be mounted on a suitable heatsink, 1500W is a LOT! even if only for a short period!

The temperature of the resistor must NOT exceed 100° C according to the datasheet!!!

Please observe the temperature of the heatsink/dummyload!

The aluminium box for the attenuator is 70 x 57 x 38 mm in size, it has four holes in the corners to fasten it to a heatsink (using M4 screws, 45mm long). The four holes in the corners are space 60 x 47 mm.

In order to get a good return loss at 1.3 GHz, I had to add a small "flap" to the center pin. This is a trick I have seen others do in order to get a better match (Mark, 9H1BN, did the same "trick" on a Diconix 1200W resistor).

The "flap" shown below is an experiment, I will try and do some more tweaks, but so far the measurements looks very promising. The sweeps shown below in the "Measurements" section is with this "flap" mounted as shown on the picture below.

Design files

Using the files below, you can make your own attenuator. 

The top cover can be ordered thru Scandcut.com in Sweden at very good prices. Check their website for what materials/thickness they currently support.

Files:

1. File for the aluminium box, STEP file
2. Description of the threads in the aluminium box, PNG file
3. File for the top cover, STEP file

Measurements

Return loss for the input port:

Input match on the DIN 7/16 shown as VSWR:
SWR on 70cm: 1:1.11

SWR on 23cm: 1:1.11

SWR on 13cm: 1:1.30

Coupling factor from input (DIN 7/16 connector) to coupled port (SMA connector). 

As an example, 1KW on the input port at 23cm (with -57.46 dB coupling) would result in approximately 1.8 mW on the coupling port (+2.5 dBm).