Travelling Wave 3 wire dipole (Upgraded for 1KW)
- Thread starter VK2RK
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VK2RK
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The above solution suffered failures due to ingress of moisture in the resistors, several configuration had been adopted in the end those resistors with the ends incapsulated with polyurethane solved the failure problem but not the observed reactive components.
Image shows the four 1K wire wound resistors with the ends sealed against moisture and the thin film carbon resistors
The originally used resistors, are a non inductive wire wound type but did exhibit some inductance when used in series parallel to achieve the target power handling capacity the resulting inductance made the antenna system slightly reactive. In a travelling wave antenna this stray reactance must be kept at a minimum.
The best solution was to use a singular pure carbon resistor with the required power handling capacity, such a resistor is difficult to source and very expensive to purchase. The only option was to use readily available resistors and wire them in such a way to keep reactive components to a minimum.
Thin film carbon resistors are a good solution, but the maximum power handling is 5Watts in air,
The other option was to use flange resistors but it requires a metal heatsink to disperse the heat.
As the required power handling target of the termination resistor had to be 300 to 400 Watts, the only other solution was to use 5 Watt thin film resistors with some method to dissipate the heat besides air.
The only option was to use mineral oil, this presents containment issues, I decided to use a PVC tube so built to allow for oil expansion and contraction.
The hole at the end cap is a grove on the inside of the cap and tube forming a breather passage, this allows pressure to equalise as the oil expands or contracts.
The use of oil considerably increases the heat dissipation of the resistors the chart below shows to what extent over 7 times that of in air
Image shows the four 1K wire wound resistors with the ends sealed against moisture and the thin film carbon resistors
The originally used resistors, are a non inductive wire wound type but did exhibit some inductance when used in series parallel to achieve the target power handling capacity the resulting inductance made the antenna system slightly reactive. In a travelling wave antenna this stray reactance must be kept at a minimum.
The best solution was to use a singular pure carbon resistor with the required power handling capacity, such a resistor is difficult to source and very expensive to purchase. The only option was to use readily available resistors and wire them in such a way to keep reactive components to a minimum.
Thin film carbon resistors are a good solution, but the maximum power handling is 5Watts in air,
The other option was to use flange resistors but it requires a metal heatsink to disperse the heat.
As the required power handling target of the termination resistor had to be 300 to 400 Watts, the only other solution was to use 5 Watt thin film resistors with some method to dissipate the heat besides air.
The only option was to use mineral oil, this presents containment issues, I decided to use a PVC tube so built to allow for oil expansion and contraction.
The hole at the end cap is a grove on the inside of the cap and tube forming a breather passage, this allows pressure to equalise as the oil expands or contracts.
The use of oil considerably increases the heat dissipation of the resistors the chart below shows to what extent over 7 times that of in air
Last edited:
VK2RK
Active member
So far no sign that the PVC reached a temperature that softened the material, besides the duty cycle is not such that a heat rise becomes critical to the container. Further I have used high pressure PVC that is a lot thicker than normal.
I feel confident that it will last, I have pushed more than 400 for several minutes and no heat effect was observed, I think the thermal mass of the oil is large and determining the heat rise, the posted chart gives a good indication of this.