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

Mon Oct 21 17:11:55 EDT 2013

What a great story. Reminds me of my ole repeater days on top of radio 
towers.
Good to know there are a few folks out there who still know how to find, fix 
and address RFI problems without throwing a big fuss.
Thanks for sharing!
73,
Dale
K9ZPY
SF, CA

----- Original Message ----- 
From: "Scott Townley" <nx7u at nx7u.net>
To: <n6ze at aol.com>
Cc: <k1fjm at aol.com>; <wswss at mailman.qth.net>
Sent: Saturday, October 19, 2013 10:57 AM
Subject: Re: [Wswss] Spurs & Mixing: Men in Black visit K7FL/B

> 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:
>>
>> -----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
>>
>>
>>
>>
>>
>>
>> Catchy title, huh?
>>
>>
>> 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!"
>>
>>
>> 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.
>>
>>
>> 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!
>>
>>
>> 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.
>>
>>
>>
>>
>>
>> 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.
>>
>>
>> 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.
>>
>>
>> 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.
>>
>>
>> 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.
>>
>>
>>
>>
>>
>> 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.
>>
>>
>>
>>
>>
>> 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.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Embarrassing, entertaining and enlightening!
>>
>>
>>
>>
>>
>> I'll keep my beacon website updated:
>> http://ashworth.org/dennis/beacon
>>
>>
>> Dennis, K7FL
>> Battle Ground, WA
>>
>>
>>
>>
>
>
> -- 
> Scott Townley
> http://www.nx7u.net
>
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