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G0FTD > ATU 11.05.06 00:26l 47 Lines 1484 Bytes #999 (0) @ WW
BID : 1C4579G0FTD
Read: GUEST
Subj: Re: 3kw roller inductor tuners. (G
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<I0TVL<HS1LMV<ED1ZAC<GB7YKS<GB7SYP<
GB7BEN<GB7ESX<GB7NNA
Sent: 060510/1851z 06269@GB7NNA.#31.GBR.EU $:1C4579G0FTD [Witham, Esx]NNA V3.1
VK3ABK wrote:-
> To reply to VE1AEE, you could have said, 'AMERITRON' have made their ATUs
> with a heavy duty 'switched' inductor for many years. Others do the same,
> but MFJ seem to be staying with a roller inductor.
If MFJ made decent ATU's, or used decent components then I wouldn't
be sitting here now waiting to take delivery of some high voltage caps,
to ensure that I can run more than 25 watts on 160m when my antenna
is 1-j2000 ohms (ish).
Tha caps are rated in the tuner that I pinched the orginal cap from, is
rated around 250v.
At around 25/30w I get flashover.
The maths..?
Well lets take a look at the following:
Volts = SQRT(watts * impedance)
= SQRT 25w * 2000 ohms
(1-j2000 ohms is just 2000 ohms reactive).
= 223 volts
Considering the approx 25/30w and the approx antenna impedance and the
rating of 250v working then the flashover problem as I experience
seems to tally up.
If my antenna was (for example) 1-j400 ohms then:
Volts = SQRT (watts * impedance) =
25watts * 400 ohms = 118 volts.
In my real case my antenna on 80m is 12-j400 ohms therefor:
Volts = SQRT ( 100watts * 500 ohms) 12-j400 is 400 and a tiddly bit ohms!
= 200v and so I don't get any flashover, as proved in practice.
Just in case I have stumbled across one of those lucky examples where the
maths appear to back up the apparent results, I shall make a note to
investigate further as well as measure with an rf probe what's occuring -
when I get the time of course !
- &y -
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