HF Mobile Amplifier Design Help

[Transmitter Man]

I hope my first post is in the correct forum category as it is RF related.

I am looking for someone experienced in solid state RF amplifier design, ideally with MF or HF circuit experience.

This is for a personal project relating to a 12V HF (1.8-30Mhz) high power amplifier.

If there's someone out there that think they may be interested please feel free to contact me .

David G4ZOW

 

[JimB]

I am not sure that I have all the experience you need, but go ahead and ask the questions?

Jim GM3ZMA

 

[Transmitter Man]

12V HF Amp project

Hi Jim,

Thanks for replying to my very first post on the site.

I am interested in the first instance of evaluating a pair of these relatively new RF Mosfets:

https://www.electro-tech-online.com/custompdfs/2011/08/rd100hhf1.pdf

AFAIK they are used by several of the Japanese HF transceiver manufacturers so are hopefully fairly rugged.

I think as they are rated at 100W pep output that the HF rig manufacturers are under-running them for long term reliability or and best IMD.

I would like to have a prototype board designed to see if they can be run at a higher output level at reasonable efficiency and still acceptable IMD figures across all HF amateur bands.

This would be with a view of adding some output filters and protection circuitry further down the road.

I have a couple of other rather unique ideas to incorporate but they can be discussed later.

If this project is of possible interest maybe it's best we take this to private mail although I'm happy either way should anyone else be interested.

David G4ZOW

 

[JimB]

David

I have had a look at the datasheet, for one device there is a lot of gain compression when the output is up at 100watts.
To get a clean and linear 100watt output would require two devices.

Filtering on the output to suppress harmonics is essential.
From my experience with lower powered amplifiers, without a decent LPF the second harmonic can be a high as -20 to -15db below the carrier.
A very poor situation.

As far as your project goes, I am not the guy to help you with PCB design and construction, I avoid it as much as possible!
I much prefer "ugly" construction techniques.

JimB

 

[Transmitter Man]

Hi Jim,

I am more than happy with 'ugly' construction as I can always get a board laid out later on.

Output filters are not too much of a problem as I have a couple of QRO 'mobile' amp's that one can reverse engineer to a large degree. I have schematics and parts list.

I also don't mind dipping into compression although I appreciate performance spec may start to go south.

My idea if a pair of these look better than previous 100W bipolars is to combine several pairs to generate higher power with a suitable input splitter and output combiner.

What do you think?

David G4ZOW

 

[Mikebits]

Check out this app note. It shows a board layout.
https://www.electro-tech-online.com/custompdfs/2011/08/ANVHF027B28RD100HHF120PP29.pdf

 

[Transmitter Man]

Hi Mike,

I'd not seen that before.

I'll drop a line to the UK company showing that info.

As a further question I wonder if any of you have any ideas on a simple circuit to 'sniff' the frequency for automatic band switching of output filters?

Thanks.

David G4ZOW

 

[Mikebits]

Hi Mike,

I'd not seen that before.

I'll drop a line to the UK company showing that info.

As a further question I wonder if any of you have any ideas on a simple circuit to 'sniff' the frequency for automatic band switching of output filters?

Thanks.

David G4ZOW

For a sniffer, I always used a piece of unshielded coax looped and then connected to a spec analyzer. The loop acts as an antenna the analyzer gives frequency. If no analyzer is handy, a +20 dB amp to a counter can be used. I mean, put the sniffer probe on the amp input.

**broken link removed**
Was that what you were asking?