>I need help in solving a problem. Our Field Day group has grown from only
>one station per band to two (CW & SSB). We had considerable interference
>problems this past field day. Contributing factors:
> - all stations in 1000 foot diameter (FD rule)
> - all stations running <= 150 Watts (FD rule)
> - some stations using non-resonnant antennas
> - some stations using Beam antennas
>I know all FD sites have the same problem(s). I would like to know
>how other FD groups have solved or bettered the situation.
>Possible solutions are:
> - running 50 Watts power
Lower power obviously helps.
Quote:> - using resonnant antennas
This won't help because the antenna will be close enough to resonance
across the entire band to prevent it acting as much of a filter. It
can help for stations operating on *different* bands, but often not
Quote:> - a mix of vertical and horizontal antenna polarization
Using cross-polarization can theoretically give you 30 dB of isolation,
but on lower HF inside a 1,000 foot circle, near field effects will
reduce this isolation a lot, so this won't help very much. You can
arrange horizontal antennas so that they are end on to each other.
This puts the other antenna in the null of the first, and vice versa.
For verticals, arrange them one over the other. Either of these methods
will give some isolation. To avoid near field coupling, the spacing
should be as many wavelengths as feasible from end to end.
Quote:> - using bandpass filters (anbody know of good sources or designs?)
> - using a coaxial stub at the transceiver to act as either a bandpass
> of band-reject (any source of information on this would be helpfull)
Now you're on to something. You want bandpass/bandreject filters so
that the CW and phone portions of the bands are separated at the
radios. This is similar in principle to the duplexers used on repeaters.
Unfortunately, the percentage bandwidth of the HF bands, except 80
meters, just isn't very much, so realizable bandpass/bandreject filters
are difficult. You may do better with just tunable notch filters.
You need tunable notches because everyone is going to want to be
frequency agile, so coax 1/4-wave notch filters aren't very good.
Just use a lumped series LC from a coax Tee to ground in the antenna
to rig line. Tune it to reject the other transmitter. Try for high
Q by using air core inductors and air variable capacitors. These
notches, especially if used with antenna orientation as mentioned
above, will virtually eliminate fundamental overload problems.
You are likely to still get broadband noise from one rig into the
other, especially with today's "no-tune" rigs. Using good high Q
antenna tuners on each rig in addition to the series notch circuit
will help a lot here.
We put up a huge low band horizontal loop at our Field Day site,
and it got into every other antenna there by near field effect,
but none of the other radios bothered its radio because the high
Q balanced tuner we used rejected their signals. Put the tune
back into those no-tune radios and a lot of your interference
problems will go away.
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244 | |