[Wswss] Spurs & Mixing: Men in Black visit K7FL/B

[Scott Townley]

I'm curious about the 156 MHz product.

The product is probably 4th-order IM (2x28.2985+2x50.060=156.7170).  Or 
it could be a straight mix of both 2nd harmonics...but that would 
require both your 2nds to be really high in amplitude.

Anyway, if it is locally-generated 4th IM, it seems unlikely that it 
would propagate too far.  You'd see it locally, of course, but unless 
it's forming within the 2m system (and the 2m antenna is a decent 
match/radiator at 156 MHz), the product's "ERP" would be awfully low.  
I'm betting the stacked loops aren't terribly good at 28 or 50 MHz, 
though (not much pickup signal), so if there is any mixing happening in 
the 2m system it's direct radiation into the PA, some sort of 
common-mode ingress, or something like that.

It might be an interesting test to keep the 28 and 50 MHz beacons on, 
and terminate the 2m antenna in 50 ohms, and see if the product goes 
away.  If it does, then you know (1) it is being formed somewhere in the 
2m system and (2) it probably has a decent-enough ERP to be detectable 
at 10 miles.

Scott NX7U

On 10/19/2013 10:17 AM, n6ze at aol.com wrote:
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> -----Original Message-----
> From: Dennis Ashworth<dennis at ashworth.org>
> To: pnwvhfs<pnwvhfs at googlegroups.com>
> Sent: Fri, Oct 18, 2013 6:54 pm
> Subject: [PNWVHFS] Men in Black visit K7FL/B
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> Catchy title, huh?
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> Around 8 PM one evening in early October, we had a knock on the door that my XYL answered. After a brief introduction, Viv told the people at the door that "Oh, you are looking for my husband!"
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> I went downstairs to meet the visitors. They introduced themselves as RF Engineers from General Dynamics in Phoenix and have been tracking a signal interfering with a Coast Guard repeater on Elkhorn Mountain, Washington. Elkhorn is about 10 miles ESE and line of sight from K7FL/B. They have been working this interference issue since June (3 months!), being contracted by the Coast Guard since earlier attempts by the Guard and the FCC (!!!) had not located the source. Triangulation and extensive local tracking finally brought them to my QTH.
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> I invited the guys to Beacon Haus, the building in the back 40 which houses all 5 of my beacons. They had their spectrum analyzer in tow. As I deactivated the beacons one-by-one, we identified a culprit ... the 222 MHz beacon. Looking on the spectrum analyzer, the 222.050 beacon had a spur +/- 8.95 MHz. and only 5 dB below the fundamental signal strength. I volunteered to leave the 222 beacon off until fixed (a pretty easy decision) :-) I had my accommodating hat on!
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> I should mention here that the two General Dynamics guys were great! Although quite well versed on ham radio, they had not heard of ham radio beacons so I explained the function the beacons serve. We swapped noise tracking stories, hatred for wall warts and plasma TV's, etc ... They travel worldwide and their stories were pretty entertaining.
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> Since the guys had brought a mountain of test equipment, I suggested we look for additional abnormalities which could potentially cause interference to other services. It was getting late and they had spent a long day searching for .... ME! However, they offered to come back (with coffee and donuts) in the morning since their flight didn't leave until late the next day.
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> The next morning, we took spectrum plots of 3 beacons. The 222 beacon definitely has huge spurs 8.95 MHz each side of the fundamental. We took screenshots and I also confirmed that the spurs were detectable on my SDR receiver dongle. That's a good reference to use should I want to test the effectiveness of any fix. The beacon will stay off until I get a chance to move it into the house and troubleshoot.
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> We also found an interaction between the 28 MHz and 50 MHz beacons. As it turned out, this was the more significant issue affecting the Coast Guard. It's a good thing they had came back. When both the 28 MHz&  50 MHz beacons were activated, a signal appears on 156.718 MHz ... almost spot on with the Coast Guard emergency coordination frequency (Channel 16). I actually figured out what was happening when I recognized two conflicting Morse signals at 156.718. This led me to the 28 MHz beacon which has a different automated CW message than the 4 VHF/UHF beacons. Shutting off either the 28 MHz or 50 MHz, the image disappears completely ... it took the product of both 28 MHz and 50 MHz to generate the image. The antennas are quite close together and I'm sure the mixing is occurring from signal overload.
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> As mentioned on my website, I am looking long term (1-3 years) for a new QTH for all my beacons (once they are certified as 'clean'). However, I will make a point of getting the 28 MHz beacon relocated ASAP. The 28 MHz beacon doesn't have the line-of-sight requirements of the other beacons and I have several non-hams willing to share barn space. Separating the 28 MHz and 50 MHz beacons will solve the image issue. Until then, the 50 MHz beacon will remain QRT.
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> The 222 beacon will also be QRT until I have time to troubleshoot the spur (ideas? The 222 board info is at http://www.hamtronics.com/pdf/TA51.pdf  ). I do have access to a spectrum analyzer and will scan all the beacon transmitters for spurs, etc., just to make sure nothing significant is detectable outside the ham bands.
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> My visitors assured me that neither the Coast Guard, Homeland Security nor General Dynamics was going to bill me for the time, travel, etc incurred since June. Thank God.
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> Embarrassing, entertaining and enlightening!
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> I'll keep my beacon website updated:
> http://ashworth.org/dennis/beacon
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> Dennis, K7FL
> Battle Ground, WA
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-- 
Scott Townley
http://www.nx7u.net