[TowerTalk] Re: narrowband filters

[Michael Tope]

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.

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.

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 :):)

73 de Mike, W4EF.............................................

----- Original Message -----
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: "Tower Talk List" <towertalk at contesting.com>
Sent: Monday, July 05, 2004 7:55 PM
Subject: RE: [TowerTalk] Re: narrowband filters


> On Mon, 5 Jul 2004 18:00:44 -0700, Richard (Rick) Karlquist (N6RK) wrote:
>
> >100 kHz is less than 1% BW on 20 meters.  If you can
> >manage an unloaded Q of (quite difficult, but doable),
> >you will lose about 1 dB per resonator.  Let's say this filter
> >covers 14000 to 14100.  At 14150 (one bandpass octave out),
> >you will get about 6 dB suppression per pole.
>
> >Let's review that:  you lose 1 dB of desired signal for
> >each 6 dB of undesired signal suppression, assuming a Chebyshev
> >response.
>
> >Of course, you could separate the passband and stopband
> >somewhat, which would make things better at the expense
> >of losing the top of the CW band and the bottom of the phone
> >band.
>
> The latter is more like what I had in mind. For most CW contests, one
would be pretty happy
> with the center of the filter at 14025. The lowest frequency of interest
on SSB is 14150, and
> perhaps you decide that you don't work that low, instead you move up the
band a bit (where
> the QRM is less anyway). Remember, this is Field Day, not a DX contest.
Now you've got a
> design problem that, given the same achievement in filter design gives you
appreciably
> more rejection simply by defining the problem in a manner more appropriate
to the use (or
> allows it to work with lower Q).  Now your stopband attenuation is more
like 10-12 dB for 1
> dB burned in the filter.
>
> Applying the same logic for 40 meters, again you tune the passband to 7025
and decide
> that the SSB station will stay above 7200. Not a DX contest, but a
reasonable set of
> parameters for Field Day.  On 40, the design problem is less demanding by
a factor of 2:1.
>
> Jim K9YC
>