Saturday, 15 December 2012

13cm update

I investigated a fault on my 13cm masthead preamp some months ago. I didn't find any fault, but the system sensitivity was definitely low. I put it all back a few weeks ago, meaning to look at it again sometime. It still worked ( sort of) and that is better than not having any 13cm capability at all.

Normally I can hear GB3ANT/B , located near Norwich, all the time. Since the summer reception of the beacon has been poor. G3LQR reports hearing the beacon as normal so I have to assume the worst, hence the tests on the masthead.

Today I decided to have another go. With the masthead preamp in the shack a visual investigation revealed the preamp (VLNA13 - original Issue 1 board but with the current build standard) input SMA connector was starting to break away from the tinplate box. I had obviously missed this the first time! The preamp still worked, but the noise match was totally shot.

I replaced the preamp input 2- hole female SMA connector with a four-hole male SMA. With this arrangement I was able to connect the input of the VLNA direct to a bulkhead SMA female-female connector.

Now, on test, the masthead preamp measures 0.5dB noise figure and 24dB gain. As this is with a short SMA to SMA lead from the bulkhead Connector to the relay receive port, N to SMA adapters on two ports and a home made bias Tee to extract the 28v from the receive coax to the 12V regulator in the box, I regard this as quite good and certainly far better than I managed previously.
I hope to put the masthead box back on the tower tomorrow. It will be interesting to see how well the system works now!


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1 comment:

  1. Hi Sam GB3ANT is -6dB and -4Khz with a lot oh hum! must fix it soon. Andy G8VLL