Topband: Rotator creating spur on 1820

[K4SAV]

Thanks George.  I'm pretty sure that the mixer in my case is the rotator 
or mast contact.  All I have to do to knock the spur down is just barely 
move the rotator.  It will stay down for a few minutes, then return.  
The mechanism causing that is unknown at this point. I see some 
possibilities. It could be due to intermittent contact in the rotator 
bearings, or intermittent contact at the tower top bearing, or due to 
the diodes in the rotator power lines (inside the rotator).  Looks like 
those diodes have no RF bypassing.  I can guess why Yaesu didn't do 
that.  Caps can easily fail when they are in circuits near lightning.  
If that happens the rotator would have to come down for repair. That 
would not be a popular feature. I'm hoping it's not those diodes.  The 
fact that the spur drops for a few minutes after moving the rotator, is 
a vote in favor of something mechanical.

The suggestion of a strap around the rotator is easy to do, so I will 
try that (after the storms get out of our area).  If the mixer is the 
rotator bearings or that top bearing that strap should help.  If the 
mixer is the diodes, the strap may do nothing.

That rotator may be the mixer for both my 1820 and 3510 spurs. My 80 
meter problem is from stations on 1310 and 890.  (1310 + 890) + 1310 = 
3510.  Not sure about the contributors for the 160 spur, other than one 
of the stations is on 1730.

If you have a problem and the mixer is at some distance from your 
station you may never find it.  I was originally worried that my 80 
meter problem was due to a rusty fence (lots of those around here), but 
it could turn out to be that rotator.

My old MFJ analyzer was completely worthless on 160 due to a very close 
AM station.  My new AA-600 is affected too but to a much lesser degree.  
I have a good homebrew high pass filter for my receiving antennas, 60 dB 
minimum. I put a 90 dB notch on the frequency of that AM station nearest me.

Jerry, K4SAV

On 11/3/2017 8:46 PM, k8gg at voyager.net wrote:
> Jerry, et. al.,
>
> I didn't have the problems from my own installation, but I used to live
> about 1/2 mile from an AM 930 station and there would be a 2nd harmonic
> spur at 1860 and a mixer spur of 2 x 1400 - 930 = 1870 KHz.  W0CD helped
> me find & build hi-pass filters in old ARRL Handbooks for my RX antennas.
> The only way I could check the SWR was to turn the transceiver down to
> about 2 watts and feed the beverages or pennants and scan the band to make
> sure the terminations, etc., were OK.
>
> I have repeatedly missed a couple of 160 contest stations in the former
> Yugoslavic area because they would run on 1870.  Even 4+ miles away at
> W8TOP in the CQ160 CW Contest there was no way for us to listen thru the
> harmonic mixer.  There was always some diode rectifications off site so to
> speak.  We suspect old support towers for FM stations and a nearly
> abandoned AM station between the AM930 and W8TOP, but, since the spur is
> not on site, we just live with it.
>
> The suggestion of a jumper across the rotator makes good sense if some
> dissimilar metals in the unit are making the diode.
>
> GL,  George,  K8GG
>
>
>
>
>
>> Likely all the spurs you see on 160 that are caused by AM stations will
>> land on 10 kHz increments because the stations are spaced on 10 kHz
>> increments.  I identified one of the stations that was a big contributor
>> to my problem and that was on 1730.  I haven't found the other station
>> yet.  I expected the station on 890 to be a contributor since it is only
>> 1700 ft from my antenna but I don't think it is the one.  (It is a
>> contributor to the spur that happens on 3510).  Regardless of which
>> stations are involved, the main problem is that the mixing is being
>> created near my rotator, and probably inside it somewhere.  I have to
>> kill the mixer somehow. More than likely the stations are clean.  It
>> will be interesting to see if the same mixer is causing both spurs, 1820
>> and 3510. Fortunately the 3510 spur goes away at night because one of
>> the contributor stations is daytime only.
>>
>> Interesting video by N8PR, but he is fighting a different problem from
>> what I have,
>>
>> I do not have a strap around the rotator.  That's worth a try. Thanks
>> Lloyd and Hank.
>>
>> Jerry, K4SAV
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