BATC Forum

My Portsdown is now in it's box and driving my Mitsubishi 18Watt PA block however post filter I have about 2mwatts .using an Ebay amp this becomes 20mW which is just enough to drive the PA to 10watts ...with the programed attenuator on the horizon with an insertion loss or 2dB I really need a bigger intermediate amplifier of around 16dB @23cms ..I wonder what other Portsdown builders are using?

Adrian

Hi Adrian,

Out of interest what bias current are you running the RA18H1213G at and what spectral regrowth are you getting at 10W? I ask as the information I see for the 60W vhf/uhf modules it requires running at less than 10W to keep spectral regrowth reasonable. Is the 23cms module very much cleaner?

73 Dave

Hi Dave with 4A bias current @10watts o/p shoulders are a touch under 40dB down...dropping to 8watts they go the other side of 40dB
I had to go for the 18 watt amp as I was told that the 60watt version is now out of production
73 Adrian

Just measured the shoulders post filter @ the 2mW level on 1249 which are -45dB
Adrian

I'm running the same 23cm brick.
I run at 5 amps and also can get around 10W out with 40db shoulders.

I'm using a PGA103A as an intermediate amplifier on 23cm.
With that in circuit I have a 3db pad on it's input and on the brick a 7db pad to get the right drive levels for around 10w out.

A while ago I was given one of the EBAY 30db amplifiers that had 20W put up it, and it didn't like it!
I took out the mimic on this board and put a PGA in it's place.
This runs off 12v in my set up, and all I did was to change the feed resistor to get around 50mA current to the PGA.

I've since built a 3 band 2-70-23 PA with 3 bricks, a Portsdown RF switch board to select the right brick's input, and outputs switched with coax relays as a "compact" PA stage, With the 2m brick I needed a 4db pad on it's input, the 70cm brick is straight in, and the 23cm as mentioned above.
PTT lines for PA selection and the relays driven from the RPI via a PIC based sequencer that also provides power to a mast head amp.


Dave.

Dave...That sounds ideal! I have a similar system with a 2M and 23cm PA in a remote box.
I see that pcbs for the mmic are readily available on ebay ..but why are all the useful chips in the States and Australia??!! with enormous minimum orders and P&P charges!
A question ..it sounds as if you are running your intermediate amp between the mod board and the 'switch' I was told that the switch would only handle +16dBm ? I am guessing that your amp is putting quite a bit more through the switch?

Thanks for your reply
Adrian

This looked interesting ..https://www.ebay.com/itm/Wideband-LNA-w ... 2903994700
However I guess its a different flavour 103a as only 12dB gain at 1050MHz ?
A

Not wishing to downplay the Portsdown RF switch but you can always use a 50 / 144 / 432 / 23cms mobile duplexer - as described here https://wiki.batc.org.uk/Output_duplexers.

73 Noel - G8GTZ

I need the additional LO output to drive my 70MHz to 470MHz converter for 4m and 6m DATV