Heathkit Cantenna Dummy Load ?

[Switch Kit]

Just wondering why this load would read 58 ohms ? The mineral oil is clean as can be , took the top off and it still reads 58 ohms ? I know there was never much more then 500 watts pep put through it. I just don't know that much about DL's other then there supposed to read 50 ohms . Thanks

 

[Dr_DX]

My Cantenna doesn't have a very good match either for a dummy load. I always get a 1.2~1.3 SWR using it. Haven't measured the ohms. My GoldLine 2KW dummy load (paint can style) does a lot better. You can try removing the tap on the Cantenna and see if that changes things.

 

[brandon7861]

Check it periodically to make sure it isn't getting progressively worse. If it stays at 58Ω, I wouldn't worry. Its a budget dummy load meant to take a ton of power, its not a calibration standard.

VA7JW has a nice paper on the HN-31. That paper mentions the resistor tolerance of the original was ±10%, and that alone gets you close to 58Ω. It also talks about how temperature can cause the resistance to vary from 37Ω to 83Ω. There is also mention about making sure the dummy load is at room temperature when measuring with a DC DVM because the dissimilar metals produce a thermoelectric effect that can influence the reading. Many DVM's use a small current, so small that the voltage produced across the dummy load is close to that of the thermoelectric effect, so if the cantenna is warm, you will get bogus readings. An AC measurement with a VNA or an old school VOM would avoid this problem.

There is a ton of good info here

https://www.orcadxcc.org/content/cantenna_va7jw.pdf

 

[Switch Kit]

Dr DX , check the ohms on it when you find the time ? also my SWR's are about the same as yours (maybe slightly higher but under 1.5) I guess my next question would be , if the dummy load reads 58.7 ohms would that or could that change power readings one way or the other ? Thank you Brandon for chiming in on this as well , I've never put much more then 500 watts pep into this DL over the years and I very seldom used it . I used a Vectronic's 300 watt dry dummy load for the most part . I got the Cantenna brand new in the box a dozen years ago but at that time I never gave it any thought to check the ohms .

 

[nomadradio]

If it's a Heathkit HN31, tighten the screws that clamp the resistor to the straps at each end. The temperature rises and falls each time it gets any decent wattage dumped into it. The expansion and contraction of the metal each heating cycle will progressively loosen those scerws.

73

 

[Cheech]

My MFJ paint can dummy load is showing 36 ohms and 1.5 swr at 27.205 Mhz.

 

[Switch Kit]

Heathkit HN31 , that's the one Nomad , the screws were pretty tight , the whole assembly looked as good as the day I bought it , much thanks for the idea all the same .

 

[Dr_DX]

Dr DX , check the ohms on it when you find the time ? also my SWR's are about the same as yours (maybe slightly higher but under 1.5) I guess my next question would be , if the dummy load reads 58.7 ohms would that or could that change power readings one way or the other ? Thank you Brandon for chiming in on this as well , I've never put much more then 500 watts pep into this DL over the years and I very seldom used it . I used a Vectronic's 300 watt dry dummy load for the most part . I got the Cantenna brand new in the box a dozen years ago but at that time I never gave it any thought to check the ohms .

Model : Cantenna HN-31
Impedance = 69 ohms
Temp = cold

I use it to counterbalance my mic boom flat plate stand- works good for that.