[NLRS] 902 Update

[Mike King - KM0T]

Hi all, with a bit of scrambling and help of a local ham and you hams on the
reflector and others via email, I can say that the 902/903 station is up and
running.  Thanks to all for your help and guidance.

Heres the synopsis:

The gasfet front end was dead, associated with a leakey transfer relay.

WB0TEM - Marc in Akron Iowa and I got together at his shack and we put a new
one in, an ATF10136 I believe, biasing did not appear to be a problem.
Noise figure measured at 1.3 to 1.4 db.  Same as specified in the SSB
Electronics LT33LP rover transverter.  It was a real neat trip to Marc's, he
told me many stories of EME from the old days and showed me all the projects
he was working on.  The antenna farm for EME and tropo was very impressive,
most things homebrew there, very neat to see what one can do when they put
their mind to it.  His work bench of equipment and experience in building
things sure shows.

I took the old gasfet out myself in the home shack the night before, that
was kind of a chore, never having done it before.  But an experiece to be
had by all.  Putting the new one in was no sweat.  I am no longer afraid of
it!

The schematic showed that the gasfet was an old MGF1200, most from the
internet said it was a MGF1302.  Then alot said it could be replaced by
MFG1402 or the newer AFT devices.

Then off to solve the problem of the leakey relay.  A few suggested putting
a few relays together so to provide more isolation for the RX side.  Then
put a dummy load on the the good isolation relay to put the RX on the dummy
during the RX cycle.  This seemed like a good idea.

I then figured my old N type relay had the poor isolation, then use an SMA
for the RX dummy load transfer.  Or use just a SMA relay alone, however some
said that the power handling was low.  I could not find any hard data, but
Gary, W0GHZ said that specs he looked up said 200 watts at 900 Mhz.  K8ISK,
Terry indicated 250 watts is easily handled by an SMA relay.  Many other
emails indicated they said 10 watts to 50 watts, etc.  All across the board.
No hard data however.

My own experience with SMA relays at 900 Mhz indicated that it handeled 175
watts no problem using my old 902 amp.  It ran 3 contests full bore with no
relay issues.  However I fried the amp due to overdrive :)

The new amp puts out 250+ watts, so I was kinda torn about using just an
SMA, so I used the two relay scenario.

Chris, N0UK indicated that I needed to be aware about cascading the relays,
not directly together.  He suggested I look up in the VHF/UHF DX book from
the RSGB.  It indicated that one should use a 1/2 or 1/4 wavelenght lenght
coax inbetween depending on if your relay terminates the open positions to
ground or not.  So I went with that suggestion and made up a 1/4 wavelenght
coax.  Good article on it in that manual. explains it real well.

I put er all together and and the setup works just fine.  250+ watts an no
fried front ends!  So, we will see  how it holds up for the contest!  I hope
to post all this on my webpage after the contest for my webpage June contest
writeup then.

Anyway, Im receiving the local QRM again and hope to have an on the air test
asap to give it the final test.

Thanks again to all that helped, I truely appreciate it!

Oh, Im going to order a few spare gasfets.....ya never know!

73

Mike - KM0T en13vc
www.qsl.net/km0t