From: Sverre Holm
Sent: 20. mai 2003 10:51
To: QRP-L (qrp-l@Lehigh.EDU)
Subject: AT Sprint: Crossmodulation finally solved!

My AT Sprint is completed and has now run QSOs on all three bands, but
at night here in Scandinavia, there is a severe overload problem,
especially on 40 meters.

I use an BLT antenna tuner, with a built-in resistive SWR meter, which
when left on, attenuates by something like 8-9 dB. The signals were
much easier to copy with this attenuation. The hard part was to
remember to flip the switch during transmission!
...
Finally I started measuring voltages in the input circuit and noticed
that the voltage across the two limiting diodes was close to the
clipping value. I disconnected them, and most of the intermodulation
disappeared! 

After consultation with Steve and also a check of "Experimental Methods
in RF Design" page 6.70, it turns out that this is not uncommon. EMRFD
recommends two or even three diodes in series for the input clipper.
So, tonight I will modify my boards: six diodes for the 40 m board and
four diodes for each of the 30 and 20 m boards. 1N4148 diodes are cheap
and one can use through hole types here.


73, 
Sverre

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Sverre Holm, LA3ZA
www.qsl.net/la3za