[TowerTalk] SO2R or Multiop Contesting without interference

Jim Brown jim at audiosystemsgroup.com
Tue May 29 00:45:42 EDT 2018

On 5/28/2018 7:32 PM, Timothy Coker via TowerTalk wrote:
> Is anyone out there operating high power SO2R or Multi-Op without interstation interference (besides being on or very near a harmonic)?

If you mean none, no. But there are LOTS of variables, some of which can 
be controlled and others that cannot.  Take a look at k9yc.com/7QP.pdf 
and see what we've done in W6GJB's contesting trailer.  With 20M Yagi, 
40M dipole, and 80M inverted Vee we can operate on harmonically related 
bands and hear only our harmonic. Same at home. Each station is a K3 
w/new synth, 87A This weekend, I was running SO2R on 20 and 15 with 3-el 
SteppIR and 4-el home brew with no interference. Also worked 40 and 20 
with 2-el 40M wire Yagi about five ft below the SteppIR. The 40M has 
double stubs per this app note.


In the trailer are double stubs on both 80 and 40, same careful 
placement. Each station is K3/P3/KPA500/KAT500.

My station has 5B4AGN bandpass filters, Glen's trailer has W3NQN filter 
sets bought from Array Solutions, maybe 10 years ago. They belong to 
another team member, N6RNO. In both stations there are serious ferrite 
common mode chokes at the feedpoint of every antenna.  NO 
offbrand/unbranded junk connectors or adapters. Serious attention to 
bonding in the shack and everywhere else.

BUT -- no matter how clean your filtering and stubs and rigs are, 
passive intermod outside the shack can and will generate and reradiate 
"growly" harmonics.  Two years ago, we had the trailer in a VERY 
isolated place Trinity Co spot for CQP on Saturday. TOTALLY clean.  Next 
day parked alongside I-5 in Colusa Co, we had growlies galore. The 
nearest homes/biz were probably a quarter to a half mile away.  Last 
year WAY off the beaten path in Tuolemne Co we had bad growlies. We 
suspect a cell site perhaps a half mile away, or one of several RVs 
parked within a quarter to a half mile away.

W3LPL has identified one common growly generator as the internal 
junction between boom and mast, which N6RK confirms. The fix is to bond 
around it. W3LPL also lists SMPS as a common cuplrit.

AND -- SERIOUS attention to every piece of coax, every connector, every 
piece of gear in the signal chain matters. Several years ago, I wasn't 
getting the suppression from stubs that SimSmith told me I should, so I 
bought a spool of BuryFlex and a bag of 100 83-1SP connectors and 
replaced every patch cable in the shack. I didn't measure the 
difference, but it wasn't subtle.  Hard line has at least important 
benefits  -- 1) low loss, 2) varmint proof, and 3) spectacularly good 
shielding. #3 minimizes coupling of RF picked up on the shield to the 
center conductor by a mechanism quantified as the "transfer impedance" 
of the shield, which is defined as the differential voltage INSIDE the 
coax divided by the common mode current on the shield. A low number is 
better, and the lower limit is the resistance of the shield at the 
frequency of interest.  It also depends on the uniformity of the shield.

A few years ago, I did a search and destroy for SMPS around my home, 
mostly replacing with linear supplies. For a few built into equipment I 
added serious common mode chokes to the AC cable.  Every audio and video 
cable in our living room has chokes on it.  This is everything from 
modems to Ethernet to refrigerators. :)  In addition to the "growlies" 
they also generate noise.

> I read reviews of various antennas, new filters, etc of having non-existant issues. However, in my limited experience it seems that tribanders will have these issues even with the latest high power triplexors and band pass filters. Even problems will arise at multi-tower stations with monoband yagi stacks and/or different direction facing yagi's.
> Has anyone got the ability to say that they are trouble free on their various band combinations?

> For instance, at home I have lots of interference when receiving on 15m and while transmitting on 20m. Same goes when receiving on 20m and transmitting on 40m. For me, it can be both direction and/or antenna limited depending on what band combination and which tribander that I take out of the stack.

I have no tribanders but I do have a tower with 3-el 20M about 15 ft 
above 4-el on 15M. My SteppIR at 120 ft is my best antenna, and when 
working 15/20, put it where I think it will do the most good. Sometimes 
though I'll point that stack in a direction from where the Yagi is 
pointed to run two directions at the same time. For example, this 
weekend, I was running northern EU on 20 for hours at a time with the 
SteppIR aimed to about 10 degrees, and the Yagi at 305-315 degrees to 
copy weak JAs. And sometimes I'd be running on one of the monobanders, 
then switch to the other one so I was TX on one of them and RX on the 
other. I heard some small transients, but no serious interference 
between them.

I DO have crossband issues with Beverages. They have chokes, but no BPF. 
They're on my wish list. :)

> I'm curious about the true confessions that other operators have about what they are dealing with when transmitting on multiple bands at once? Have some of the stations got this under control? If so is it mainly the big property owning stations with many towers or have guys on 1-2 tower setups quelled this also?

One of the benefits of property is putting space between you and noise 
sources. :)  My two towers are about 200 ft apart. I have a pair of 
dipoles for 80M at 90 degrees to each other, and one end of both is 
about 140 ft up in a redwood. One those two dipoles, broadside to JA, 
has a fan element for 40M. They don't play well together. :)

Bottom line -- when we're trying to get 100 dB below your 1500W TX 
signal, EVERY tiny detail adds up. It's like peeling an onion -- big 
problems obscure little ones, you fix those and then you can chase the 
smaller ones.

N6DQ lost his fight with cancer about a year ago, and his widow, K6YL, 
wants to sell what I think is a complete set of 403A high power filters 
and high power triplexer. They were used in a trailer that they built 
and used together.  Joanna is a pillar of NCCC, serving several terms an 
officer and handling awards for both CQP and club operating.

73, Jim K9YC

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