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Dummy Load High SWR?

Posted: 22 May 2020, 09:48
by Dave C
Hi All,

I've just knocked together a 50 Ohm Dummy Load with 3 x 150 Ohm resistors in parallel as per my pic, it measures 49.8 Ohm so pretty much smack on tolerance. The patch lead that I used is about 12" long (30 cms), but when I key up the mic the SWR reads at about 1:7, does anyone have any suggestion where I can go from here to get it down please? Also tried and alternative patch lead to rule out that {bnghd}

Thanks in advance..



Dave

Re: Dummy Load High SWR?

Posted: 22 May 2020, 10:06
by ch25
I can't see photos, but aren't they wirewound?

Re: Dummy Load High SWR?

Posted: 22 May 2020, 10:37
by wa10
as above, are they inductive wirewound resistors?, or is the patch lead 75ohm coax.

Re: Dummy Load High SWR?

Posted: 22 May 2020, 10:37
by Dave C
Image
ch25 wrote:
22 May 2020, 10:06
I can't see photos, but aren't they wirewound?
Not sure I've posted the pictures properly, can you see these now Chris?

WH50 150R 5% is what they are also patch id 50 Ohm..

Dave

Re: Dummy Load High SWR?

Posted: 22 May 2020, 11:25
by ch25
404 now.
Attach photos to your first post.

Re: Dummy Load High SWR?

Posted: 22 May 2020, 14:07
by Dave C
Picture now on post 1...

Re: Dummy Load High SWR?

Posted: 22 May 2020, 14:21
by ch25
Wrong type of resistors. That's a reason.
You need carbon type, not wirewound (inductive).
Like: http://qso365.co.uk/2011/08/building-a-dummy-load/
Or: https://www.ebay.com/itm/A3RMK6-RFP-Dum ... 3654436218

Re: Dummy Load High SWR?

Posted: 22 May 2020, 15:11
by Dave C
Took the inspiration from he used inductive resistors, are they NOT wire wounds?

Re: Dummy Load High SWR?

Posted: 22 May 2020, 15:29
by ch25
YT is full of shiĦy advices. You just found one :)

Buy that resistor https://www.ebay.co.uk/itm/100w-50ohm-D ... SwX9Bc-hYF or https://www.ebay.co.uk/itm/RF-terminati ... Sw0~pcEz~V or similar.
Chris

Re: Dummy Load High SWR?

Posted: 22 May 2020, 15:51
by wa10
The video is using dale none inductive resistors, they have two windings in opposite directions to cancel inductance, it sorta works at low frequencies,

your welwyns are single winding inductive resistors, neither type are suitable for rf dummyload at higher frequencies,
you may be able to cancell most of it out with a parallel capacitor but that aint a rf dummyload anymore, its a very lossy tuned circuit,

good loads have flat vswr over a very wide bandwidth, anything else can screw with your rf power measurements if the transmitter/amplifier is not as clean as a whistle on harmonics,

easy option that will work good enough is one of the flat pack DC-daylight resistors ch25 posted link to & stick it in an old cb amplifier heatsink,

don't use skinny wires, use flat wide strap or braid or copper clad board for wiring.

Re: Dummy Load High SWR?

Posted: 22 May 2020, 16:12
by Dave C
Cheers gentlemen for your input, I'll get onto making some alterations....

Best Regards

Dave