Switched LNA

[VK3XDK]

Gday everyone,
heres some pictures of an LNA that Ive been working on.
Uses a NE42484 super low noise fet (source biased) and has on-board relays (tohtsu)

I have both 2M and 70cm in service and they are working very well, I have chosen to tune mine for selectivity at the expensive of noise figure (which should still be very good)

[VK3XDK]

and another

[VK3XDK]

aaaand another

[VK3DXE]

Looking good Graham Any idea of cost yet?

For those interested, I've tested one of Graham's earlier preamps (sans relays) and found the performance at my QTH to be similar to my SSB Electronics SP-2000 masthead unit. From the look of this one, the frequency response should be a little tighter, which means less likelihood of overload from strong out of band signals (pagers, broadcast TV and radio, etc.)

Even if you only ever run FM, you should find a decent preamp will make a huge improvement to what you hear.

[VK5ACY]

I have some of those TOHTSU CX-120Ps in my junkROOM - I would like to try such a preamp to add to my collection.

The CX-120Ps were used in the DSE Kit amplifiers for 2m etc., and more such relays should be in junk boxes....

I can compare preamps for fun and entertainment, just the board and components...please !

Bill VK5ACY
BOWHILL SA
PF95sb

[VK3XDK]

I had a few in the junkbox as well (relays), I made them up mainly for myself but I always get a few spare pcb's (just in case they work )

they will work with most of the microwave gaasfet type devices, it's the old series capacitor then shunt inductor and capacitor (parallel tuned) input. (nothing real new here)
I have 10 PCB's here($4 each (below cost)),

If there's enough demand- I'll get more made up.

(i'll put a couple aside for vk5acy and vk3dxe) (send me an email)

[VK3XDK]

Quick update,
The unit has been very successful with my EME re-visiting in the last couple of weeks.
i have been running 200w through it (as much as my 300w amp puts out ). I did pop one ne42484 not sure if it was operator sequencing error or lack of relay isolation? Anyway i have incorporated a 4db pad on the devices output and have seen no further issues.

Being incredibly noisy in the middle of Melbourne, it has been performing very well with no obvious signs of inter-mixing

In the future might look at a higher IP3 device and some output bandpass filtering anyway!

[VK3QI]

Graham,

You are pushing the limits on those CX120P relays.

They are specified at 150Mhz for 200 watts CW and 50dB isolation.

50dB isolation at 200 watts is 2 milliwatts - not sure what the NE42484 is specified to?

Cheers

Peter VK3QI

[VK3XDK]

thanks Peter, we know
I am in a position to do it here (easy to fix), specs and real life are often seperated (ive been told the relays can handle 500W as long as swr is OK)

im looking at 300-400w real soon, keep you guys informed

[VK3XDK]

theTX pcb tracks will need to be beefed up too! Ive got one ready (beefed up with silver solder)

Im pushing limits, they were designed for typical 100w use.

[VK3XDK]

Graham,

You are pushing the limits on those CX120P relays.

They are specified at 150Mhz for 200 watts CW and 50dB isolation.

50dB isolation at 200 watts is 2 milliwatts - not sure what the NE42484 is specified to?

Cheers

Peter VK3QI

According to the data sheet, a maximum input of 15dbm (but prefer to never go even close)

[VK3XDK]

There is a very nice looking LNA available on ebay, it also incorporates switching (he rates it as 100w if remote switching is used)

http://www.ebay.com/itm/Low-Noise-Ampli ... 43c2d125b0

They are using a good device that I have thought about using in the past.
Out of curiosity, I looked up the relays that the ebay seller is using
He is using G6Z relays and they are only rated at 10W !! (900MHz) (with better than 1.2SWR)
http://vhfdesign.com/en/category/lna

Relays will usually take a lot more than their rating and I dont know at 144MHz, but i think he's pushing it! Something to be very wary of.

otherwise it looks like a very good/reasonably inexpensive option.

[VK3XDK]

AND,
All of my original switched LNA PCB's are now gone.
I doubt that i will get more made up unless "severely pressured"
I will continue to add to this post for interest reasons.

[VK3XDK]

AAAnd, here's the current layout (including component values for 144MHz)

[VK4CRO]

Hi , If you do more boards Put me Down for 1 please
de Ron

[VK4REX]

It's not hard to build your own, use a PGA-103 MMIC, use a PCB (SOT89 development PCB) and milled box (essential for keeping down induced noise) from RF Basic Store and walla you have a good 2Mx preamp. I built this one months ago and have real noise figure measurements for the unit. Use good low loss caps in the input and the rest is simple.

Hope this helps

Rex

[VK3XDK]

Tnx Rex,
not a device that i'd looked at before, it has very good ip3 (dynamic range) and i'm impressed with your NF results
shows how NF can be better than stated on the data sheets (the reputable manufacturers normally give "worst case" type figures)
Only disadvantage that i can see is that it is very broad-band when use on the generic boards.

The "RF Basic store" is very handy and i've found him very reliable.

[VK4REX]

Hi Graham

They are an excellent device and still give very reasonable results up to 1296.
As the device has a wide ip3 range I keep the input loss as low as possible and apply a filter on the output to limit the response.
Have attached a picture of the two units together with the frequency response of the overall unit

Rex

[VK3XDK]

Gday everyone,
I keep getting requests for these boards (they had all gone)
I am looking to get some more made up (PCB's) so hopefully i will be able to supply again in the next week or two.

I have also had a similar board made up using the PGA103 (Thanks Rex) , they have a VERY tight output filter system, high dynamic range and work very well! I still think the original ones are a bit quieter though and i am still using the original for my EME work. The PGA variety may be better for terrestrial with their higher dynamic range? (i need to more testing)

The original switched LNA boards have got me 50 EME calls (digital QSO's) since Christmas!! 2X10ELE 200W

Re. the new PGA boards- I hope to put some details/pictures here in the coming days. (i am supposed to be winding down! )

[I2NDT]

Hi Graham an Rex (an the others of course!),
give a look at this AN by minicircuits: http://www.minicircuits.com/app/AN60-064.pdf
many guys here in Italy and in France have reported failures of the PGAs due to their instability at "lower" frequencies.

best DX
de
i2NDT Claudio