LPF wire diameter

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OgreVorbis
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LPF wire diameter

Post by OgreVorbis » Wed Aug 15, 2018 3:18 am

I can't seem to find this information anywhere including ham radio sites.

I want to know how to calculate the power handling of an LPF given the wire diameter. Does the spacing or loop diameter also matter?

I have an LPF that handles about 100-150W (it is on one of those MRF9180 boards). Bypassing the LPF gives me 300W. And no, it is not from the harmonics; I measured it. The LPF is clearly lowering the output power. What are the requirements for the inductors on a 300W LPF?

Thanks in advance!

nrgkits.nz
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Re: LPF wire diameter

Post by nrgkits.nz » Wed Aug 15, 2018 11:18 am

The nH and pF values all remain the same no matter what power. Power handling is defined by wire diameter (as you're aware) and also voltage rating of the capacitors. There's no real formula required for calculating wire thickness for a specific power, as long as you use wire that's thick enough you'll be ok. It all comes down to the heat generated, and things can get very hot at high power if there's a mismatch. Using thicker wire though will alter the inductance of the coils, so you need to compensate so they remain the same value in nH, the thickness, loop spacing and inside diameter all affect the inductors nH value.

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Re: LPF wire diameter

Post by OgreVorbis » Wed Aug 15, 2018 9:13 pm

Thanks for the reply!

What typically would be the diameter required for 300W? There must be some kind of formula for this. I understand that RF travels on the surface of the wire.

Anyway, it is important that I have enough thickness, but not too much because the PCB that I'm using was already designed and has small spaces for the LPF coils. So I want to know what I can get away with in terms of size. I can't just make it as big as I want.

MC Spanner
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Re: LPF wire diameter

Post by MC Spanner » Fri Aug 17, 2018 1:03 am

14SWG (just over 2mm) silver plated copper wire should do you for 300W. I've just got rid of a BLF278 rig off my bench with this in the LPF and the capacitance neutralisation inductor between the drains of the PA device. Go a notch thicker if you really want to be sure but 12SWG (2.6mm) is getting overkill. Check out the 300W boards on http://3-mtr.info/sales/ for ideas - pretty sure the filters in some of the 300W boards are using 14 gauge. As for formulae, maybe you could derive one? But I wouldn't be sure what premise to even base it on as this is not DC we're dealing with!

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Re: LPF wire diameter

Post by OgreVorbis » Sat Aug 25, 2018 11:09 pm

OK, so I just installed the larger coils today. The MRF9180 seems to perform a little better than with the smaller coils, but it is still not nearly as good as without any filter. I'm a little confused because I know that the additional output power w/o coils was not from harmonics. And I know that the amp is still working at good efficiency (produce 150W draw 210W) and I tested by moving the frequency up and it didn't drop power. So this means the cutoff of the LPF is not harming the output power either. So what is it then???? I get max 180W with LPF and 300W without. Why?

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Re: LPF wire diameter

Post by sinus trouble » Sun Aug 26, 2018 10:27 pm

Maybe not the best advice? But I try to go for the biggest gauge that will physically fit the PCB sensibly!

Also I could be wrong? But I believe the LPF will be factored into the overall output impedance matching?
As a crude example, an 8ohm audio amplifier loaded with a 4ohm speaker would drastically increase in power delivery at the cost of damaging the output transistors!
I am as stupid as I look! :|

nrgkits.nz
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Re: LPF wire diameter

Post by nrgkits.nz » Sun Aug 26, 2018 11:05 pm

It sounds to me as if the LPF is changing the impedance matching. How many inductors/capacitors are you using? There's some calculators online you can use to calculate the inductor and cap values. Also what kind of power meter are you using to measure?

Electronically
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Re: LPF wire diameter

Post by Electronically » Sat Sep 08, 2018 9:35 am

I use 16 or 18 awg wire as it's stronger .If you use anything upwards from that it tends to be to springy .so 16 or 18 is fine for coils .

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nrgkits.nz
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Re: LPF wire diameter

Post by nrgkits.nz » Sat Sep 08, 2018 11:26 am

Electronically wrote:
Sat Sep 08, 2018 9:35 am
I use 16 or 18 awg wire as it's stronger .If you use anything upwards from that it tends to be to springy .so 16 or 18 is fine for coils .

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The thin springy wire you can wind around the insulation of some speaker or mains cable with the inside copper removed then dip it in glue. L2 on the PLL Pro3 was done like this.

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Re: LPF wire diameter

Post by Electronically » Sat Sep 08, 2018 11:36 am

yup your right nrg .This was also used on 1/2 vertical with a 7mm diameter winding for his primary and secondary coil .But on his his nrg pro 3 board I think it was 6mm diameter winding .

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Re: LPF wire diameter

Post by Electronically » Sat Sep 08, 2018 11:41 am

I must also note that on the board it was silver steel wire used .But on vertical it was copper enamelled wire .

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