[TowerTalk] Re: narrowband filters

[Jim Lux]

At 11:05 PM 7/5/2004 -0700, Michael Tope wrote:
>Its interesting to see how antenna spacing plays into this Jim.
>I just ran a couple test cases in EZNEC using two 20 meter
>1/2 wave horizontal dipoles at 40ft above average ground.
>The antennas were placed tip-to-tip. In case 1, I used 550 ft
>spacing (center-to-center), and then I re-ran the simulation
>with 100 ft spacing. Huge difference. In case 1, the isolation
>was around 70dB. In case 2 the isolation dropped to 30dB.
>Doing a link budget assuming Ten-Tec Orion like phase
>noise (-140dBc/Hz) performance and 550 ft spacing would
>give a broadband noise floor in a 3 KHz SSB bandwidth of
>-125dBm. That's going to be below the background noise
>level in all but the most quite locations. By contrast, go to
>100 ft spacing and a noiser radio (-120dBc/Hz) and you are
>talking about S9+ QRM between stations.

I was setting up a model to look at coupling between a 3 el yagi at 35 ft 
and a vertical.. You've jumped me there..
I was thinking about setting up an overnight run to make a map of 
"cochannel interference level" for an idealized scenario of antenna on 
tower and other antenna moving around, say, on a 1 meter grid.  Sure, 
there's a lot of assumptions involved, but I'll bet the "quasi near field" 
plot will be interesting.





>These numbers are consistent with anecdotal observations
>that say with good radios and wide antenna spacings
>stations can coexist on the same band just fine at the
>100 watt  level  without the need for elaborate filtering or
>phasing schemes.

Yes, I suspect so..



>Of course this analysis doesn't factor in operator psychology.
>This year at our Field Day we kept our GOTA station closer
>to the main stations because the GOTA station operators
>complained about feeling isolated from the main group in
>a prior year when we put them at maximum distance from
>the main stations :):)

Nothing says the GOTA station can't be close, it's the antenna that needs 
to be in the "right" place, and coax is cheap. (or, at least, cheaper than 
filters and DSP)