Reducing station - station interference in close proximity

Reducing station - station interference in close proximity

Post by Bill Bressle » Wed, 28 Jun 1995 04:00:00


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
 - using resonnant antennas
 - 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)
 - a mix of vertical and horizontal antenna polarization
 - any other ideas ?

Bill Bressler

Nortel/BNR RTP, NC

(919) 991-4854  Voice
(919) 991-4722  FAX

 
 
 

Reducing station - station interference in close proximity

Post by Scott Richard Rosenfe » Wed, 28 Jun 1995 04:00:00


Quote:>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

The antennas shouldn't make much of a difference.  In fact, POORLY
matched antennas would theoretically cause LESS of a problem because
a portion of the power to the antenna is dissipated as heat in the
feedline.  Beams at right angles and different altitudes would be OK,
too.

Quote:>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

Wouldn't make too much of a difference

Quote:> - using resonnant antennas

This could make the problem WORSE, not better, or possibly have no
effect.  

Quote:> - using bandpass filters (anbody know of good sources or designs?)

This is a decent idea; unless your transmitters are REALLY dirty,
you should essentially be messing up harmonics of your xmit freq.
For instance, transmitting on 7.012 will mess up 14.024, 21.036, and
28.048.  Every wonder why the ham bands were set up like this :-)

Quote:> - 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)

Sounds like a single-frequency, single-time, non-tuneable solution...
but who knows?

Quote:> - a mix of vertical and horizontal antenna polarization

This would help, too, in theory.

Quote:> - any other ideas ?

We just try to stay away from each others' harmonics.  If the guy on
80 stays 20 kHz up from the edge, this equates to 40 kHz on 40, 80 kHz
on 20, 120 kHz on 15, and 160 kHz on 10.  Of course, this means that the
harmonics fall such that there's a fair amount of band left on all the
other bands.

This, unfortunately, ALSO means that someone doing CW on 3.550 will show
up on 14.200, right in the middle of the SSB band...basically, it requ-
ires a good deal of freq. coordination.

>Bill Bressler

>Nortel/BNR RTP, NC

>(919) 991-4854  Voice
>(919) 991-4722  FAX


--
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301-549-1022 Talk to me eves&weekends, else lv. message \ /  Long   Original
Scott Rosenfeld  Amateur Radio NF3I  Burtonsville, MD    |   Live    $5.00
  WAC-CW/SSB  WAS  DXCC - 136 QSLed on dipoles __________| Dipoles! Antenna!
 
 
 

Reducing station - station interference in close proximity

Post by Gary Coffm » Sat, 01 Jul 1995 04:00:00


>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
that much.

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
--
Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | emory!kd4nc!ke4zv!gary

Lawrenceville, GA 30244     |                     |

 
 
 

Reducing station - station interference in close proximity

Post by DB Wilhel » Sun, 02 Jul 1995 04:00:00

 > SNIP (part of original)...
 > > - 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.
 >
   MORE SNIP...

Gary, as usual, makes some very good points.  I also would recommend
a good high Q antenna tuner, and I have found that link coupling and
a parallel tuned resonant circuit is usually more effective than
either a PI section (actually a lo-pass filter) or a T section (hi-pass)
for this type of "harsh" environment.

On the notch circuit side, I haven't tried it, but I would think that
a shorted length of lo-loss coax less than 1/4 wavelength could be used
for the high Q inductor part of the series tuned notch filter.  A
good quality variable capacitor could be used in series with this
coax inductor to easily tune the notch.  My thinking is that hi-Q
capacitors are relatively easy to acquire, but hi-Q inductors are
more a rarity.

73,
Don W3FPR