Dummy Load High SWR?

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Dave C
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Dummy Load High SWR?

Post 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
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Last edited by Dave C on 22 May 2020, 14:07, edited 1 time in total.
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ch25
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Re: Dummy Load High SWR?

Post by ch25 »

I can't see photos, but aren't they wirewound?
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Re: Dummy Load High SWR?

Post by wa10 »

as above, are they inductive wirewound resistors?, or is the patch lead 75ohm coax.
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Re: Dummy Load High SWR?

Post 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..

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Re: Dummy Load High SWR?

Post by ch25 »

404 now.
Attach photos to your first post.
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Re: Dummy Load High SWR?

Post by Dave C »

Picture now on post 1...
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ch25
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Re: Dummy Load High SWR?

Post 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
Last edited by ch25 on 22 May 2020, 15:31, edited 1 time in total.
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Dave C
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Re: Dummy Load High SWR?

Post by Dave C »

Took the inspiration from he used inductive resistors, are they NOT wire wounds?
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Rigs: Major M588 AM FM SSB (Std), President Grant II Premium (Std), Midland Alan 78 Plus (Std), Lowe TX 40 (Std).

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ch25
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Re: Dummy Load High SWR?

Post 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.
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Re: Dummy Load High SWR?

Post 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.
W8JI
"An antenna with a poor ground using few radials cannot have a support mast grounded to the radial common point (at least it shouldn't if designed properly) There is no exception to this!

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Dave C
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Re: Dummy Load High SWR?

Post by Dave C »

Cheers gentlemen for your input, I'll get onto making some alterations....

Best Regards

Dave
Life is like a box of chocolates.... Usually empty when I get there.... :crazy:

Rigs: Major M588 AM FM SSB (Std), President Grant II Premium (Std), Midland Alan 78 Plus (Std), Lowe TX 40 (Std).

Twigs: Antron 99 (Std), K40 Mag, Firestick 4', Cherokee Dial A Match 4', Sirio 1000 PL Mag.

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