Determined Sleuth Solves Field Day Inter-Band Interference

By Bill Willcox-KF6JQO

During the last three or so ARRL Field Day events the phone stations have been plagued by interference between the 10 and 15M bands, and to a lesser extent the 20M band as well. 

The transceivers for all three bands were within 10-20 feet of each other and all three fed individual band pass filters by means of lengthy coaxial cables.  The band pass filters then fed a multiplexer that in turn fed a multi-band hex beam antenna by means of a single coaxial cable equipped with a choke at the antenna.  Note: the band pass filters and multiplexer were all mounted on the same aluminum base plate in a plastic box.

The symptoms included bleed from the 15M to the 10M station when the 15M station transmitted.  The same thing was noted between the 20 and 15M stations, but was not as prominent.

CVARC is not a contest club but operating a 10M station is hard enough without getting stepped-on by your club mates, even if it is unintentional.

Several experiments were conducted to try and trouble-shoot this problem using just 10 and 15M transceivers:

The Field Day configuration interference was duplicated with the same filter box, coax cables and antenna.  Turning on attenuators on each transceiver eliminated the interference, indicating the noise was locally generated.

Next, the entire filter box, feed line and antenna were eliminated and each transceiver led to individual dummy loads.  There was no interference even though the transceivers were in close proximity.

We then eliminated just the antenna and fed one dummy load with two transceivers in close proximity through the band pass filters and multiplexer.  The interference returned.

Eliminating the multiplexer and feeding separate dummy loads through just the band pass filters eliminated the interference.

Based on this series of tests we concluded the multiplexer was faulty (or had inadequate inter band isolation) and replaced it with a new one.  Both the old and new multiplexers were tested and found to be within isolation specifications

Alas, this did not solve the problem. Next, we tried feeding separate antennas.  The 10 and 20M antennas were both mounted on the same mast and the 15M antenna was located about 15-20 feet from the other two and was oriented 90 degrees to the 10 and 20M antennas.  Each antenna was fed separately through just the band pass filters with lengthy coax cable between the transceivers and filters and also lengthy cables from the filters to the antennas.  Each antenna had its own choke at the antenna feed point.  There was minimal interference between the 15M station and the other two stations.  However, there was noticeable interference between the 10 and 20M stations.

We then replaced the lengthy cables between the transceivers and the filters with short, low-loss cables and fed separate 10 and 40M antenna (tuned to 15M) each separated by about 30 feet.  There was minimal interference from the 15M antenna to the 10M antenna and none at all from the 10 to the 15M transceiver.

We concluded that 1) separate antennas greatly reduced the interference, but only if mounted on separate masts and 2) short low-loss coax cables between the transceiver and the filters also reduced the interference.

The latest test setup is shown below.

It consists of two transceivers, one for 10M and the other for 15M closely coupled to individual band pass filters by means of short, low-loss coaxial cables.  The band pass filters feed separate dipole antennas by means of ordinary coaxial cables.  The dipole antennas are oriented in line and the antenna ends are separated by about 20-30 feet.  The antennas are flat top configuration mounted about 10-15 feet above ground.  Note that the transceivers and filters are all within 18 inches of each other

Results: With the preamps off, no interference was noted between either transceiver regardless of which transceiver was transmitting on either 10 or 15M and which transceiver was receiving on the alternate frequency.  Turning on the preamp on one of the transceivers allowed a very faint interference between from 15 to 10M, but not from 10 to 15M.

Conclusions and Recommendations For future Field Days:

  1. Use single band antennas mounted on separate masts fed by separate feed lines.  Orient the dipoles in line.  Make sure there is a choke at each antenna feed point.
  2. Use short, low-loss coaxial cables between the transceivers and the band pass filters.  This may require removing the band pass filters from the ground plate.
  3. If interference is still noted, make sure the preamp on each transceiver is off.

If we still want to use multi-band antennas with multiplexers to save on the number of masts, be prepared to use attenuators on each transceiver or coordinated transmissions to avoid interference between 10, 15 and 20M phone.

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