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Old 1st Jan 2022, 1:40 pm   #1
Malcolm T
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Default Coil winding for low power / P, Z match unit

Having re-evaluated a crude home brew L match for HF with about 10 metres of end fed using a max of about 20 Watts with a rather dubious coil I made, I opted to make another coil to the value of approx 40uH on a 25mm grey plastic pipe which turned out to be about 72 turns and using a couple of 365pF VC's from old radios it kind of worked but most of the coil taps were not used. It seemed happier down at one end of the coil almost like the inductance is too much. On 2 or 3 bands it worked whilst the others were next to impossible or very twitchy on the SWR.
So the plan is to rewind for a lower inductance coil say 20 - 25uH on a 28.5mm cardboard former and seal it with epoxy. Again using it in Pi or L configuration.
However I'm a bit confused now having read so many stories and other people's designs, about the inductance, coil diameter and wire diameter.

Do any other members have experience with QRP / low power Z matches for portable use with end fed aerials, before I go any further with winding a new coil, I would be very interested to hear.

Note the prime criteria here are simplicity, portability and low cost.

Ta.
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Old 1st Jan 2022, 2:15 pm   #2
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Default Re: Coil winding for low power / P, Z match unit

I wouldn't worry about how the coil is made, getting the right inductance is what matters. For a 20W transmitter even a small coil on, say, a Biro will be fine.
 
Old 1st Jan 2022, 2:25 pm   #3
G6Tanuki
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Default Re: Coil winding for low power / P, Z match unit

I guess you're talking about the HF bands - my personal approach has been to try and use either an air-cored coil or a ribbed ceramic former, on the basis that if you're only starting with QRP you need to preserve every last Watt !!

Use the thickest wire you can for the coil, and ideally space the turns one wire-thickness apart. Equally, if fitting the matcher into a metal case, ensure there is _AT LEAST_ 1 coil-diameter's space between the coil and surrounding metalwork [this is a horrible failing of many commercially-produced antenna-matchers!]

Some plastics can be RF-lossy - be careful about using lengths of black plastic rainwater-pipe or waste-pipe as the 'black' filler is sometimes carbon/graphite. There's a well-established test for RF-lossiness using a short offcut of pipe, a mug of water and a microwave-oven...
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Old 1st Jan 2022, 5:37 pm   #4
Malcolm T
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Default Re: Coil winding for low power / P, Z match unit

Thanks for the help, yes HF only . I,ve heard of the microwave test . I,m not going to use plastic anymore , either air or well dried cardboard tube from the kitchen foil roll etc .
The coil i made just a few weeks ago was close wound 22swg , going to try 20swg with spacing next then, reason , i have tons of it !. . Yes being careful not to place the coil near to the chassis always a good move.
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Old 1st Jan 2022, 9:40 pm   #5
kan_turk
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Default Re: Coil winding for low power / P, Z match unit

White rainwater/waste pipe is usually OK but it is always prudent to do the ‘microwave’ test prior to using it in anger
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Old 2nd Jan 2022, 9:51 am   #6
Malcolm T
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Default Re: Coil winding for low power / P, Z match unit

Ok Turk thanks for the advice, you just never know whats in stuff !.
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