Return-Path: Received: from rly-mg01.mx.aol.com (rly-mg01.mail.aol.com [172.20.83.107]) by air-mg01.mail.aol.com (v125.7) with ESMTP id MAILINMG014-9f44b0c41e9229; Tue, 24 Nov 2009 15:28:40 -0500 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-mg01.mx.aol.com (v125.7) with ESMTP id MAILRELAYINMG016-9f44b0c41e9229; Tue, 24 Nov 2009 15:28:27 -0500 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1ND1yz-0007IN-VL for rs_out_1@blacksheep.org; Tue, 24 Nov 2009 20:27:29 +0000 Received: from [83.244.159.144] (helo=relay3.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1ND1yz-0007IE-6N for rsgb_lf_group@blacksheep.org; Tue, 24 Nov 2009 20:27:29 +0000 Received: from mail-bw0-f221.google.com ([209.85.218.221]) by relay3.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1ND1yw-00057C-CX for rsgb_lf_group@blacksheep.org; Tue, 24 Nov 2009 20:27:29 +0000 Received: by bwz21 with SMTP id 21so6818815bwz.4 for ; Tue, 24 Nov 2009 12:27:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:mime-version:received:in-reply-to:references :date:message-id:subject:from:to:content-type; bh=R2oCmk+rRylONApOIWqDspiHvqOfnQVo+MUt2ynKJjY=; b=Ws63yTpeoN20otuYsmFGsh++Pj6Wh0+JXBxjdj93aah3OxQLIcPZVcLNLYb/C8/iaw wKE34zFZS5LESbTRljz9EJQnke+FqSIdQt48JquL6M7uRzExFEzBUR33sE4oUKda5tMj WRdKqcZMZJhYYN9lPd+OYOKoVoCbPMYODqq2c= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; b=SYw98b75jUe+osWP5Diril1qYg2xXN2u3gU5nLzAFBWNKJPGXVv5Aro0tzs6XaPwSQ A92eZ7uPnQyLyEJH28UzbhhocCOMAka2XycaWOeB9N5/p6zvWTkngCO4A8EsJ4tCwMwV 9p3guI0ooUL3TnklRCbWynBp+UblT3y5qHx7U= MIME-Version: 1.0 Received: by 10.204.35.142 with SMTP id p14mr2437742bkd.21.1259094440661; Tue, 24 Nov 2009 12:27:20 -0800 (PST) In-Reply-To: References: <9afca2640911241140j230515e9kcbcaffd3911ccb82@mail.gmail.com> Date: Tue, 24 Nov 2009 20:27:20 +0000 Message-ID: <9afca2640911241227o2150e5ddn71496f2423f50633@mail.gmail.com> From: Roger Lapthorn To: rsgb_lf_group@blacksheep.org DomainKey-Status: good (testing) X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,HTML_MESSAGE=0.001 Subject: Re: LF: IRF510 500kHz PA stability issues - advice please Content-Type: multipart/alternative; boundary=00032555aa16dfd2da047923c42d X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.0 required=5.0 tests=HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-AOL-IP: 193.82.116.20 X-AOL-SCOLL-AUTHENTICATION: mail_rly_antispam_dkim-d300.1 ; domain : gmail.com DKIM : pass X-Mailer: Unknown (No Version) --00032555aa16dfd2da047923c42d Content-Type: text/plain; charset=ISO-8859-1 Andy, Thanks for the quick feedback. The 500kHz antenna is a 5m long vertical with a spiral top hat. It is fed via a (thickish) ferrite rod wound with the required inductance to resonate and this is tapped at the 50 ohm point (very low down - just a couple of turns). Well that is the general idea! Yes the IRF510 is rated at 100V. 73s Roger 2009/11/24 Andy Talbot > If you've studied the drain waveform when operating inrto a correct > load, and assuming its all within design guidelines, then the trouble > is because of nasty impedances presented by the antenna during the > tuning process. > > What sort of antenna matching are you using? The often suggested idea > of an an adjustable tap onto the loading coil can lead to damage as, > when the antenna is detuned, the small number of turns of the link, > with an untuned coil, appear not far different from a short circuit. > A Class E PA, when presented with a slightly inductive short circuit > can get a bit stressed. > > On my antenna over the years on 73, 137 now 500kHz the system has > always been fed in series, using a ferrite tapped transformer to match > load impedance. I kept well away from tapped or link windings as > they are not easy to calculate or predict reliably. WIth a series fed > system, a detuned antenna results in a high impedance. This may not > always good news either to some class-E designs, but all my LF/MF PAs > are completely open-circuit-proof voltage fed designs. The only > true class-E one here was for 1.8MHz, and can be seen at > http://www.g4jnt.com/TopbandPA.pdf . This was completely imune to > open and short circuits and seemed very robust; but I did use the > meatier and more robust higher voltage IRF540. Doesn't the IRF510 only > have a 100V rating? > > Andy > www.g4jnt.com > > This email has been scanned for damaging side-effects by the health > and safety police > > > > 2009/11/24 Roger Lapthorn : > > I've been using a single IRF510 FET PA stage on 500kHz and it produces > > around 6-7W of RF. But, when adjusting my antenna I've managed to damage > > several devices now. I think the stage is running somewhere towards > class-E > > looking at the PA drain waveform on the scope and I believe either the > stage > > is going unstable or it is running in a condition in which too much > current > > (and heat) is taken and the device fails.500kHz teh ant > > > > Are there any simple "rules of thumb" or design/biasing/decoupling > > guidelines to ensure that my devices don't destroy themselves with a less > > than perfect load? > > > > Any simple advice would be appreciated. > > > > 73s > > Roger G3XBM > > > > > > > > > > > > > > -- > > ------------------------------------------------------------- > > http://g3xbm-qrp.blogspot.com/ > > http://www.g3xbm.co.uk > > G3XBM GQRP 1678 ISWL G11088 > > > > -- ------------------------------------------------------------- http://g3xbm-qrp.blogspot.com/ http://www.g3xbm.co.uk G3XBM GQRP 1678 ISWL G11088 --00032555aa16dfd2da047923c42d Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Andy,

Thanks for the quick feedback. The 500kHz antenna is a 5m lon= g vertical with a spiral top hat. It is fed via a (thickish) ferrite rod= wound with the required inductance to resonate and this is tapped at the= 50 ohm point (very low down - just a couple of turns). Well that is the= general idea!

Yes the IRF510 is rated at 100V.

73s
Roger


2009/11/24 Andy Talbot <andy.g4jnt@googlemail.com>
If you've= studied the drain waveform when operating inrto a correct
load, and assuming its all within design guidelines, then the trouble
is because of nasty impedances presented by the antenna during the
tuning process.

What sort of antenna matching are you using? =A0The often suggested idea of an an adjustable tap onto the loading coil can lead to damage as,
when the antenna is detuned, the small number of turns of the link,
with an untuned coil, appear not far different from a short circuit.
=A0A Class E PA, when presented with a slightly inductive short circuit can get a bit stressed.

On my antenna over the years on 73, 137 now 500kHz the system has
always been fed in series, using a ferrite tapped transformer to match
load impedance. =A0 I kept well away from tapped or link windings as
they are not easy to calculate or predict reliably. =A0WIth a series fed system, =A0a detuned antenna results in a high impedance. =A0This may not<= br> always good news either to some class-E designs, but all my LF/MF PAs
are completely open-circuit-proof voltage fed designs. =A0 =A0The only
true class-E one here was for 1.8MHz, and can be seen at
http://ww= w.g4jnt.com/TopbandPA.pdf =A0 . =A0 =A0This was completely imune to open and short circuits and seemed very robust; but I did use the
meatier and more robust higher voltage IRF540. Doesn't the IRF510 only=
have a 100V rating?

Andy
www.g4jnt.com

This email has been scanned for damaging side-effects by the health
and safety police



2009/11/24 Roger Lapthorn <r= ogerlapthorn@gmail.com>:
> I've been using a single IRF510 FET PA stage on= 500kHz and it produces
> around 6-7W of RF. But, when adjusting my antenna I've managed to= damage
> several devices now.=A0 I think the stage is running somewhere toward= s class-E
> looking at the PA drain waveform on the scope and I believe either th= e stage
> is going unstable or it is running in a condition in which too much= current
> (and heat) is taken and the device fails.500kHz teh ant
>
> Are there any simple "rules of thumb" or design/biasing/dec= oupling
> guidelines to ensure that my devices don't destroy themselves wit= h a less
> than perfect load?
>
> Any simple advice would be appreciated.
>
> 73s
> Roger G3XBM
>
>
>
>
>
>
> --
> -------------------------------------------------------------
> http://g= 3xbm-qrp.blogspot.com/
> http://www.g3xbm= .co.uk
> G3XBM =A0 =A0GQRP 1678 =A0 =A0 =A0ISWL G11088
>




--
----------= ---------------------------------------------------
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
G3XBM =A0 =A0GQRP 1678 =A0 =A0 =A0ISWL G11088
--00032555aa16dfd2da047923c42d--