Saturday 26 November 2022

8M ISM - Switching RF 20dB attenuator


When I was carrying out tests on 40MHz ISM I always had a attenuator inline which reduced the receive capabilities too. My first attenuator which I'm ashamed to admit used 1/2watt resistors but it was all I had at the time, I placed an ordered for 2watt rated ones but due to Royal mail strikes the post was delay and I wanted to get on with the local range tests. 


Quickly thrown together attenator : (

This morning the post arrived and in the bag of components I ordered were the 2 watt rated 100ohm resistors and I was able to rebuild and update the attenuator to a switchable one.

I had bought three old "non working" microwave modules converters mainly for the enclosures off ebay for £5 with the plans to repurpose them for other projects. New boxes this size are around £3-4 plus they had BNC's, other hardware and trimmers/parts that can be reused too.

I wanted the 20dB attenuator to only be inline when in transmit and bypass it on receive so decided I could use the the "TX GND" and "13.8v" pins from the ACC socket of the FT817.

Parts I used were:

MM Enclosure,
5v relay from the junk,
6 x 100 ohm resistors (2 Watt rated),
1 x 100 ohm resistor (1/2 Watt ratedfor 5v relay),
1 x 1N4001 diode,
2 x BNC connectors,
Pieces of wire,
FT-817 ACC mini din,
Copper clad board

The New 20dB attenuator with relay

 

All connected to the power meter/dummy load


Meter on 1000mW scale showing 10mW out with 1W in from FT817

So far the old microwave modules enclosures are getting used and this will allow me to monitor the without having to keep removing the fixed attenuator.


Update: 1452hrs

I decided to add a TX LED on the attenuator just so I know power is getting to the unit and it's changing over the relay.


Back in the loft, I can remote control it from the shack via VNC

 

Update: 1459hrs 

Thought I would leave it beaconing on WSPR for part of the day. Hoping to do a longer range test tomorrow (weather permitting).

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