TopBand: Bad spurs/parasitics

[W8JITom@aol.com]

In a message dated 96-12-08 23:20:05 EST, you write:

>Can someone who knows Vic, VE7UZ tell him about his problem?
>
>He has a bad spur/parasitic on about 1822 when he is transmitting
>on 1832.  It is well over S9.  Maybe another Yaesu?
>
>Tree N6TR

Hi Tree,

I discovered this problem with my IC-751A when using external antennas. 

They use a PIN diode with only a few volts of bias to hold off the external
antenna connection. If the RF voltage on the external antenna exceeds the
voltage of the diode, it turns back on and runs the RX antenna back into the
low level TX RFchain (since the tuned filters are used for both TX and RX.

I discovered this while testing a 600 watt PA on an outside antenna,
fortunately I had a spectrum analyzer fired up and saw the exra pip two MHz
outside the 14 MHz band.
When I checked 160, my rig did the same thing but IN band. I had to do
extensive mods to cure the 751A. 

This last contest, I heard about five stations with spurs outside the band.
One K8 in Michigan was ten over S9 with a buzzy signal on 2.040 MHz while he
was on 1.830 with a 25 over nine signal.

I think manufacturers need to pay more attention to switching methods, and
not depend on PIN diodes biased with three volts to hold off external
antennas.

Another problem with the 751A and other rigs is keyclicks. In the 751A, they
are caused by the VCO still stabilizing while the TX is akready on. This is a
problem when using QSK with split VFO's or the RIT active. It goes away when
the QSK is turned off becasue the TX VCO has a chance to come up and
stabilize. 

If I RX with my 751A on 1912, and TX on 1805, the VCO sweeps the rig  from
about 1905 to 1805 with a sweep signal at the leading edge of every
character. I do NOT use full QSK because of the trash the VCO puts out in the
751A.

I quite telling other people about garbage on their signals long ago, because
they usually get pissed off no matter how it's worded. But these have to be
common problems, since my Yaesu and Kenwood rigs also have spurs and trash
visable under some operating conditions. My 950SDX, when operated at 80 watts
output to drive a linear, puts out a leading edge spike on CW or a first
syllable spike on SSB of 250 watts!!!!!  If that spike drives a PA, it
flattops the PA and sends out broadband garbage. My IC-706 does the same
thing, but the spike is only ~150 watts.

No wonder life is rough in crowded areas!

73  Tom

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