Every multi-transmitter environment suffers to some extent from harmonic interferences. In some cases, coupling due to closeness of antennas is sufficiently high that potentially damaging voltages may be present at the receiver terminals. Harmonics can be so strong that operation within 25 or 50 kHz is not possible due to receiver overload. Application of band reject coaxial stubs will reduce transmitted harmonics from the final amplifier and reduce fundamental pickup as well. This translates to fewer complaints from the operators, higher contest scores, and reduced risk of receiver front end damage.
Approximately 30 dB of harmonic attenuation can be expected from a single stub. Lumped constant filters can also provide the mentioned benefits, but none are available for amateur use unless they are home built. Top Ten Devices uses high quality RG-213 coax with a silver plated, teflon PL259 and heat shrink tubing on the shorted or open end. Rated ultra-conservatively at 1.5 kW continuous duty operation, these stubs will last a lifetime.
Stubs are manufactured to order and are individually trimmed with a Hewlett Packard network analyzer to 10kHz. When ordering, please specify CW or SSB, and the stub will be cut to plus 25 kHz from the sub band edge.
A single stub can be connected to the output of an amplifier with a UHF tee connector. For multi band operation, the stub(s) may be connected after the antenna switch. There is no requirement for a specific length of coax between the amplifier and the stub. Well shielded line should be used to prevent radiation of harmonics before they get to the stub. Two or more stubs for a given band may be used together. For example, a CS-4 and a CS-5 together will null 40,15 and 10. They would be connected with two UHF tee connectors close to the amplifier, if possible, to minimize pick up or radiation on the connecting lead. For stations where the amplifier is used on more than one band, automatic stub switching can be accomplished using a Six Way Relay Box controlled by a Band Decoder. The station antennas can be switched at the same time.
Detailed information on an automatic band switching system with performance data and graphs was published in the Sept/October 1996 NCJ.
Band Reject Coaxial Stubs Pricing
Part Number Bands Passed Bands Nulled Cost
CS-1 80 40,20,15,10 $76
CS-2 40 80 $76
CS-3 40/15 20,10 $45
CS-4 20/10 40,15 $45
CS-5 20 10 $30
CS-6 10 20 $30
CS-7 40 15 $35
CS-8 40 15 $22
CS-9 160 80,40,20,15,10 $140
CS-10 80 160 $140
CS-11 10 Channel 2 TVI $22
CC-80 Use w/2 CS-1 or CS-1x stubs $55
CC-40 Use w/2 CS-2 or CS-2x stubs $35
CC-20 Use w/2 CS-5 or CS-5x stubs $31
CC-10 Use w/2 CS-11 stubs $25
1. Specify desired frequency as either the CW or SSB portion of the band. Custom frequency $5 extra.
2. Stubs designated CC-xx are coupling cables for multiple stubs and have connectors on each end. They are 1/4 wave at the second harmonic of the band designated by the "xx." Two 1/4 wave shorted stubs coupled with these cables will give about 70 dB attenuation at the second harmonic.
4. CS-8 is an open circuit reactance compensator intended to be used with the CS-7 to correct the VSWR on 40. CS-7/CS-8 may be used alone or in combination with the CS-2 and CS-3.
5. Approximate shipping cost is $3 per stub. Shipping will be billed at actual cost, plus handling.
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