We have a big water tank at our Field Day site, Jos. We
were thinking it might make a good 20 meter cavity. Of
course, I don't know if the U.S. Forest Service would
appreciate it if we drained out all the water and silver
plated the inside :):)
73 de Mike, W4EF................................
>
> > Why not use a serie of grain silo cavities on HF bands .............
> > Some math head can simulate that ?
> > I just wonder what the outcome might be.
> > Last but not least, a beautifull snap shot for a QSL card.....
> >
> > Jos
> >
> > From: "Michael Tope" <W4EF@dellroy.com>
> > To: "Jim Brown" <jim@audiosystemsgroup.com>; "Tower Talk List"
> > <towertalk@contesting.com>
> > Sent: Tuesday, July 06, 2004 8:05 AM
> > Subject: Re: [TowerTalk] Re: narrowband filters
> >
> >
> > > 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@audiosystemsgroup.com>
> > > To: "Tower Talk List" <towertalk@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
> > > >
> > >
> > >
> > >
> > >
> > > _______________________________________________
> > >
> > > See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless
> > Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any
> > questions and ask for Sherman, W2FLA.
> > >
> > > _______________________________________________
> > > TowerTalk mailing list
> > > TowerTalk@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/towertalk
> > >
> >
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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