Heathkit "Cantenna"

[Russlk]

If you put a 150 ohm, 20 watt resistor in parallel with the 77 ohms, in the oil in the can, the combination will be 51 ohms good to 60 watts. Since the 20 watt resistor is in oil, it will take more power, so probably 100 watts will be OK.

 

[Andy1845c]

If you put a 150 ohm, 20 watt resistor in parallel with the 77 ohms, in the oil in the can, the combination will be 51 ohms good to 60 watts. Since the 20 watt resistor is in oil, it will take more power, so probably 100 watts will be OK.

I though about that! lol I didn't know if it was a good idea or not though I might have to keep the option open. I also read somwhere that I could parallel several lower wattage 50hm: resistors and run them in the oil. My radio puts out 100w max, so I doubt i would ever need to come close to the 1kW.

Would this method affect impedance though? Is it "cleaner" to have just one resistor? As I metioned in my first post, impedance isn't somthing I understand well, although I am beginning to understand it better.

 

[Hero999]

Make sure the resistor is non-inductive, avoid all wire wound types, only carbon or metal film will do here.

 

[JimB]

JimB - Those look very much like my resistor! The ends of mine are kind of a sulfer yellow though, as opposed to the tarnished copper color. Would it hurt/could it help to take some fine sandpaper or steel wool and clean the ends of mine? Mabey they are corroded, but it dosn't look like they are to the naked eye.

I would be very wary of rubbing those resistors with anything.
The copper plate is quite thin, and the carbon on ceramic is very thin, it is very easy to change the value of those resistors by gently rubbing with fine emery paper. (The resistance increases).

If you are reliably measuring 77 ohm (ish), I would leave the ends alone.

JimB

 

[Dean Huster]

Actually, wirewound resistors can be made non-inductive. Tektronix did it for a long time for the collector/plate load resistors in their vertical output amplifiers. They're made by winding a resistance of twice the value needed onto a ceramic core and then wrapping on a second winding in parallel with the first but wound in the opposite direction. The magnetic fields cancel and the resistor is basically non-inductive. The most common failure was a display with a horrid amount of spike and ringing on a step waveform and an amplitude that was 'way high because one of those windings opened up making the resistor inductive (the spikes and ringing) and pushing the gain high (resistor doubling in value).

Dean

 

[Sceadwian]

That might reduce the effects of inductance, but what does it do for impedance? Magnetic fields are still created, they're just in opposition so they don't build up to extremes.

 

[Hero999]

Inductance only exists due to the feild around the wire, ia another winding is in the other direction the fields will cancel and so with the inductance.

For example, take a perfect isolation transformer with a 1H primary and secondary and connect the windiings anti-phase and in parallel with each other, you will find that the impedance will be 0hm: at and frequency.

In practice with the resistor Dean is talking about, the feilds won't perfectly cancel due to imperfect coupling so you will measure some inductance but it will be a lot less than a straightforward wirewound resistor.