Input attenuator design

VK4BXI
Posts: 62
Joined: Sat May 22, 2021 11:44 am
Location: Brisbane

Input attenuator design

Postby VK4BXI » Sat Feb 05, 2022 12:18 am

Hi all, I'm about to purchase 1kW Lmos amp. The amp only requires 3~5 watts for full output so I want to use ~50 watts out of my 7000 mkII to achieve that. That requires a hi power attenuator of ~ 10 db. I have several 100 watt 50 ohm "chip" type load resistors available but no other high power values. So my thought was to use a tandem type power bridge with 10 -13 db of power to sensing ratio and use the forward power sensing output as the drive to the amp. I have suitable relays to do changeovers etc. But my main concern would be am I going to increase the distortion of the transmit signal ?
I wonder what others use to handle this ?

Regards Bob
w4bf
Posts: 25
Joined: Wed Mar 24, 2021 10:56 am

Re: Input attenuator design

Postby w4bf » Mon Feb 07, 2022 10:55 am

Sounds like this is a kit type amp, so I'd put the attn INSIDE the amp and avoid all the relay issues.

Optionally, You can set the max power out at full drive level on any band on the 7000 by Setup > PA Settings

Caution - Lower numbers mean HIGHER power. You want to INCREASE the values from default ones to lower the power.

Set this up into dummy load before you hook up amp. The values are stored in memory.

Once set up, you won't have the full 100 watts anymore with drive at maximum when not using the amp (unless you go in and change the settings) but you can run the anan - amp combo at reduced power just by dropping down your drive to get back to 100 watts or so total output of the combo.

You should be using an external coupler to feed sample of LDMOS amp back to the ANAN for pure signal, otherwise you give up one of its great features. Lots of posts on proper attenuator levels for this coupler.
VK4BXI
Posts: 62
Joined: Sat May 22, 2021 11:44 am
Location: Brisbane

Re: Input attenuator design

Postby VK4BXI » Thu Feb 10, 2022 9:00 am

Thanks for the reply. Yes the amp is available as a kit .....but I am buying it as a fully finished box so I need to investigate further. Clearly the best position is inside amp .....but the amp does have input power overload protection so not so easy. One thought is to have the 10db attenuator in line all of the time....even on recieve. The background noise at my qth is ~s7 most of the time so where I would normally have 10~15 db of input attenuation switched in I should be able to use 10db externally with out any issues.
I do already have the coupler for the amp output already made and tested and I will be running PS.
It was after making this coupler that I wondered if I could make a 10db coupler as well and use it to reduce the ~50watts of drive from the output of the rig into a 5watt drive by using the the forward sampling port of the coupler which should be 5 watts for a 10 db coupler. and dump the surplus watts into a dummy load (away from the amp on the end of some good coax). Anyway that was my original question to the group but perhaps not well stated.

Regards Bob
w4bf
Posts: 25
Joined: Wed Mar 24, 2021 10:56 am

Re: Input attenuator design

Postby w4bf » Thu Feb 10, 2022 11:09 am

Yes, you could make a coupler for -10 dB but you'd experience the same rx path loss as a 10 dB pad, so no real advantage, that I can see.

Yes, a fixed 10dB pad probably has no observed effect on usable rx, at least below 20 meters, etc. Simplest way to add some protection without putting it inside the amp.

The reduction of ANAN output even with drive at full level, via the PA settings in the SETUP menue does work and in my case, it powers up every time with the set values.
VK4BXI
Posts: 62
Joined: Sat May 22, 2021 11:44 am
Location: Brisbane

Re: Input attenuator design

Postby VK4BXI » Thu Feb 10, 2022 8:46 pm

Thanks, the main reason for looking at a 10 db coupler was that I have 100 watt 50 ohm chips already. Making a 100watt 10 db attenuator would be a challange and buying one would be ~ $ OZ 100 from the US. So the thought of a coupler was tempting.

Bob

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