[CQ-Contest] Proximity to other contest stn

Michael Tope W4EF at pacbell.net
Sun Oct 12 11:54:12 EDT 1997


The following table gives line of sight pathloss in dB as a function
of distance in miles for each of the 6 HF contest bands. As you can
see from the table, at 5 miles separation, pathloss is only 56 dB at
1.8 MHz. If your neighbor is running 1500 Watts (+62dBm) and assuming
0dBi antenna gain at both ends, then you are looking at a signal level
of +14dBm at your receiver input terminals, contrast that with a MDS
(minimum discernable signal) for a modern receiver of -135 dBm and you
get a feel for the dynamic range challenge. Fortunately in practice,
the antenna gains that you will realize at the horizon should be much
lower than 0dBi - most HF antennas have a null right at the horizon,
so you might be okay. Also, for a fixed distance pathloss increases
proportional to the square of the frequency (6dB every time you double
the frequency). This should help compensate for the increased antenna
gains usually associated with the higher frequency bands.

  Dist	Line of Sight Pathloss(dB)				
(Miles)	1.8MC	3.5MC	7MC  14MC  21MC  28MC
  1		42	47	53	60	63	66
  2		48	53	60	66	69	72
  3		51	57	63	69	73	75
  4		54	60	66	72	75	78
  5		56	61	67	73	77	80
  6		57	63	69	75	79	81
  7		59	64	70	76	80	82
  8		60	66	72	78	81	84
  9		61	67	73	79	82	85
  10		62	67	73	80	83	86

My neighbor in Florida (~1/2 mile distant) and I coexisted pretty well
on 75 meter phone, but we were both using fairly low horizontal
antennas. He was extremely loud at my qth, but never gave me any
trouble with receiver blocking or IMD. On the other hand, the local AM
station (920 KHz) which was 1/2 mile distant running 5KW directional
with a 4 square towards my QTH, totally overloaded the front end of my
rig making parts of the HF spectrum totally unuseable without a BC
reject filter. The next nearest AM station (1560KHz) which was about 2
miles away running 1KW omni overloaded the preamp in my ANC-4 noise
cancellor a little, but it didn't bother the HF rig.

It's hard to say for sure how well you will fair, there are a ton of
factors involved - receiver quality, trasmitter phase noise
characteristics, antenna gains, and operating habits (how close in
frequency you two want to operate) . I understand that the folks who
did the VK0IR expedition were able to run phone, CW, and RTTY on the
same band simulaneously with antenna spacings of only a couple
thousand feet and careful siting to take advantage of antenna
front-to-side and front-to-back. Another anecedotal tidbit that was
related to me that is in your favor would be the story of a 160
enthusiast who at one time had his beverages 5 miles from his transmit
antenna. Allegedly, he was able to tune for multipliers within a few
KHz of his transmit frequency while CQing despite the fact that he
running an amplifier.

My guess is that with careful station engineering and cooperation you
guys should be able to coexist, but don't bet the farm on just my
advice. I don't have enough experience with this problem to feel
comfortable giving you the blessing on such a big investment. If it
were me, I would think about trying to arrange some on the air tests
with your prespective neighbor using a mobile or portable setup or
perhaps talking to some of the multi-multi guys about their experience
with operating multiplier stations along side run stations. Also,
noise cancellation boxes like the JPS ANC-4 might prove to be useful
in situations where you and your neighbor are having trouble with
co-band interference.

Good luck, it sounds like a great location if you can make it work.

73 de Mike, W4EF/6

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