Return-Path: Received: from mtain-de02.r1000.mx.aol.com (mtain-de02.r1000.mx.aol.com [172.29.64.202]) by air-da03.mail.aol.com (v129.4) with ESMTP id MAILINDA033-863b4d874a75165; Mon, 21 Mar 2011 08:54:13 -0400 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by mtain-de02.r1000.mx.aol.com (Internet Inbound) with ESMTP id 26FF338000081; Mon, 21 Mar 2011 08:54:10 -0400 (EDT) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Q1ebS-000404-Dd for rs_out_1@blacksheep.org; Mon, 21 Mar 2011 12:52:58 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1Q1ebR-0003zv-Ct for rsgb_lf_group@blacksheep.org; Mon, 21 Mar 2011 12:52:57 +0000 Received: from relay02.pair.com ([209.68.5.16]) by relay1.thorcom.net with smtp (Exim 4.63) (envelope-from ) id 1Q1ebO-0003V4-9S for rsgb_lf_group@blacksheep.org; Mon, 21 Mar 2011 12:52:57 +0000 Received: (qmail 98267 invoked from network); 21 Mar 2011 12:46:11 -0000 Received: from 74.94.151.157 (HELO ?192.168.1.56?) (74.94.151.157) by relay02.pair.com with SMTP; 21 Mar 2011 12:46:11 -0000 X-pair-Authenticated: 74.94.151.157 Message-ID: <4D874AEC.7000204@charter.net> Date: Mon, 21 Mar 2011 08:56:12 -0400 From: John Andrews User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.15) Gecko/20110303 Thunderbird/3.1.9 MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <4D83D475.9090609@telus.net><000b01cbe622$6a23f8d0$8d01a8c0@JAYDELL>, ,<4D86BD6F.8020006@telus.net> <004101cbe7c1$5171ada0$4001a8c0@lark> In-Reply-To: <004101cbe7c1$5171ada0$4001a8c0@lark> X-Spam-Score: 1.7 (+) X-Spam-Report: autolearn=disabled,MAILTO_TO_SPAM_ADDR=0.276,RATWARE_GECKO_BUILD=1.426 Subject: Re: LF: Loop TX antennas at VLF? Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: * X-Spam-Status: No, hits=1.1 required=5.0 tests=MAILTO_TO_SPAM_ADDR 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-global-disposition: G x-aol-sid: 3039ac1d40ca4d874a724b7b X-AOL-IP: 195.171.43.25 X-AOL-SPF: domain : blacksheep.org SPF : none X-Mailer: Unknown (No Version) Just one caution - particularly for impedance measurements, and probably for operating: Treat the loop as balanced. If you use the capacitor method that Alan suggests to get to 50 Ohms (or whatever), follow it with a 1:1 matching transformer. From experience at 137 and above, it is very difficult to measure the impedance of a "large" (but electrically small) loop with mains-powered unbalanced test equipment. My usual setup is an unbalanced RF impedance bridge, a battery-powered receiver, and a battery-powered RF generator, all set up a meter or so above the ground. Back to the capacitor-matching approach: It's no more than a classic "L" network, where the loop supplies the inductance. The reactance of the series capacitor is smaller than necessary for tuning. John, W1TAG On 3/21/2011 8:11 AM, Alan Melia wrote: > I seem to remember from the 73kHz days and article in an early LF Handbook > (the blue cover ??) on matching a loop with a pair of capacitors at the feed > point and a program to calculate the values. It doesnt get round the voltage > rating problem but if you can crack that it may solve the ferrite saturation > ?? > > Alan G3NYK > > ----- Original Message ----- > From: "Rik Strobbe" > To: > Sent: Monday, March 21, 2011 8:30 AM > Subject: RE: LF: Loop TX antennas at VLF? > > > Hello Scott, > > the discussion is about a 10 by 10 meter loop and 100W RF power. > Using 4 x 1.5mm Cu wire (parallel) the DC resistance of a 10 by 10 meter > loop is about 0.1 Ohm. Ignoring other losses and tuning the loop the current > is about 32A. > I don't have an idea what addional (ground) losses tha loop will suffer from > at 9kHz. > The loop indictance was calculated 40uH (by Jim), so the loop has a > reactance of 2.3 Ohm at 9kHz. This means that the loop voltage is only about > 70V, so stray currents to the ground (or other grounded objects near the > loop) will be minimal and these losses can be ignored. > Other losses are due to induced currents (so called Eddy currents), but if I > remind well these losses are proportional to the square of the frequency > what means that at 9kHz these lossses are over 200 times less than on > 137kHz. > > In an ealier mail I suggested to use coax cable as loop wire. > But to my own surprise Belden (main coax cable manufacturer) gives rather > large DC resistanses: > RG58 = 4.2 Ohm/1000ft for the shield and 3.3 Ohm/1000ft for the conductor. > So 40m (133ft) of RG58 would be 0.56 Ohm for the shield and 0.25 Ohm for > shield and conductor in parallel. See > http://www.emaxit.asia/htx/document/P0002/7807A.pdf > Better would be RG8 with 1.2 Ohmm/1000ft for the shield and 1.9Ohm/1000ft > for the conductur. 40m of RG8 would be 0.16 Ohm for the shield and 0.1 Ohm > for shield and conductor in parallel. See > http://www.hytamerica.com/Download/Private/file/PDF/belden/8237.pdf > But even RG8 would be no better that 4 x 1.5mm Cu wire in parallel. > > 73, Rik ON7YD > > > > ________________________________ > Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] > namens Scott Tilley [sthed475@telus.net] > Verzonden: maandag 21 maart 2011 3:52 > Aan: rsgb_lf_group@blacksheep.org > Onderwerp: Re: LF: Loop TX antennas at VLF? > > Hi Rik > > I think I missed a message or two. What current level is the consensus for > 100W input at 9KHz into a reasonable loop? I missed how this was modeled > and wonder if my original assumptions where off base. > > Thanks for any clarification you can offer. > > 73 Scott > VE7TIL > > > > On 3/19/2011 9:27 AM, Rik Strobbe wrote: > Roger, > > the antenna reactance about 2.5 Ohm, so the antenna voltage (and thus > capacitor voltage) will be less than 100V. > I assume that polypropylene caps that work fine on 137 and 500 will also be > OK at 9kHz. > Farnell sells 1uF/275Vac at 0.44 Euro (10 QTY) and 0.1uF/305Vac at 0.32 Euro > (10 QTY), so for less than 10 Euro you should be able to tune the antenna in > 0.1uF steps. As the antenna Q is rather low (2.5/0.1 = 25) a 0.1uF step > should be OK for a first try. > > 73, Rik ON7YD - OR7T > > ________________________________ > Van: > owner-rsgb_lf_group@blacksheep.org> > [owner-rsgb_lf_group@blacksheep.org g>] namens Roger Lapthorn > [rogerlapthorn@gmail.com] > Verzonden: zaterdag 19 maart 2011 12:47 > Aan: rsgb_lf_group@blacksheep.org > Onderwerp: Re: LF: Loop TX antennas at VLF? > > Rik, et al > > Actually I am beginning to think that this small VLF TX loop is not such a > totally daft idea after all. The main issue seems to be with the capacitors > but these seem to be less onerous than winding a very big (and lossy) coil. > Certainly there sounds to be merit in a larger TX loop for /P operation. > > Thanks everyone for the constructive feedback on this thread. Most > interesting. > > 73s > Roger G3XBM > > On 19 March 2011 10:42, > > wrote: > Rik, Roger, Jim, Mal > > There are soil losses to consider with the loop as well. At 185 kHz (Part 15 > lowfer band), I ran a 50' X 50' transmitting loop made from mil spec RG-11 > (copper braid) and the soil losses were about equal to the wire losses. Not > sure what the soil loss would be at 9 kHz but it would be interesting to > know. > > Jay W1VD WD2XNS WE2XGR/2 > > > ----- Original Message ----- > From: Rik Strobbe > To: rsgb_lf_group@blacksheep.org > Sent: Saturday, March 19, 2011 6:07 AM > Subject: RE: LF: Loop TX antennas at VLF? > > Roger, > > as Jim calculated running 100W in a 10 x 10 m loop will give about 0.5uW ERP > (is you use 4 x 1.5mm wire in parallel instead of a single 3mm wire in order > to avoid skinn effect losses). Using more parallel wires of a coax cable > might pump up the ERP to 1 or 2uW. > > Looks pretty poor, but will a vertical antenna of a similar size do better ? > > At 9kHz the radiation resistance of a 10m high + 10m topload vertical > 75uOhm. > The antenna capacitance is 110pF, a reactance of 161kOhm. What means that > you will need a loading coil of 2.84H (yep Henry). Apart from the fact that > it will cost a lot on copper wire the coil losses will be high. You will > need a Q of 160 to reduce the losses to 1kOhm. In addition for such a small > antenna you can excpect several 100 Ohm ground loss, so let's assume a total > loss of 1500 Ohm. 100W TX power will result in about 0.25 A antenna current > and an ERP of about 8uW. > That's 6 to 10dB better than the loop, but instead of some cheap cap's you > will need a monster coil. I ran it one on the online coil calculators and it > came up to a 2.5m high and 1.8m diameter coil with amost 10km of 1.5mm Cu > wire (weight 150kg). And running 0.25A into the antenna will result in 40kV > ! > > So, despite the vertical could be 10dB better than the loop, the loop seems > much more easy (and cheap) to build. > It might be easier and cheaper to get the extra 10dB by usung mor wire in > the loop and pump up the power. > > 73, Rik ON7YD - OR7T > > > ________________________________ > Van: > owner-rsgb_lf_group@blacksheep.org> > [owner-rsgb_lf_group@blacksheep.org g>] namens Roger Lapthorn > [rogerlapthorn@gmail.com] > Verzonden: vrijdag 18 maart 2011 23:02 > Aan: rsgb_lf_group@blacksheep.org > Onderwerp: Re: LF: Loop TX antennas at VLF? > > Yep, I guess you're right Scott. And you know better than most. Ah well, it > was an interesting idea to toss around. > > 73s > Roger G3XBM > > On 18 March 2011 21:53, Scott Tilley > > wrote: > Hi Roger > > The practicality of pumping 35A into a loop is not an easy task! Couple > this with the stability of most capacitors creates a real engineering > challenge for a loop on 9KHz, notE the BW and Q. Not to mention really low > ERP one would get. > > This will be an engineering challenge for sure! > > 73 Scott > VE7TIL > > > On 3/18/2011 2:39 PM, Roger Lapthorn wrote: > Hi All > > Just run Andy's spreadsheet for magnetic loops to see the sort of figures we > get at 8.9kHz. Assuming 100W and a loop diameter of 10m with 3mm wire the > efficiency works out at -87.4dB and the ERP -67.4dBW (0.2uW). There is also > the matter of the low loss 6211.7nF capacitor. With larger loop diameters, > thicker wire (or multiple paralleled wires) and maybe 200W then the ERPs are > starting to get more useful. > > The Marconi does seem a better bet, even with all the issues with losses in > the huge loading coil, but a VLF TX loop doesn't look a total "no-hope" > approach. Larger loops, with improved efficiencies, may be easier than kite > or balloon supported ones in a /P location. > > And then there is the widely spaced earthed electrode antenna..... but I > won't start a discussion on the merits or otherwise of this as I am about to > go on holiday this weekend and will not be able to respond to emails next > week. We know from work by DK7FC (and VLF professionals) that this does work > as a radiating structure. > > 73s > Roger G3XBM > > > > On 18 March 2011 14:09, Roger Lapthorn > > wrote: > Just wondering if anyone has done the maths to work out what sort of ERP > could be expected at 8.97kHz with, say, 100W to a smallish loop antenna in > the garden? > > It would certainly avoid the need for very very large matching coils and may > be easier to engineer than a Marconi. Even an efficiency of -80dB would > allow 1uW ERP and, judging by results from G3XIZ with around 2uW, this could > be useful with long stable carrier transmissions of several hours. Most of > us could run a loop with an area of 100sq m. with thickish wire in our > gardens. A loop might also be more practical for portable operations perhaps > with a triangle with one high support. > > Certainly my own results with WSPR at 136 and 500kHz with just a few watts > and quite thin wire and around 80sq m loop area were encouraging. Mind you, > 9kHz is very much lower than 136kHz, so the radiation resistance would be > tiny I assume. > > 73s > Roger G3XBM > > -- > http://g3xbm-qrp.blogspot.com/ > http://www.g3xbm.co.uk > http://www.youtube.com/user/g3xbm > https://sites.google.com/site/sub9khz/ > G3XBM GQRP 1678 ISWL G11088 > > > > -- > http://g3xbm-qrp.blogspot.com/ > http://www.g3xbm.co.uk > http://www.youtube.com/user/g3xbm > https://sites.google.com/site/sub9khz/ > G3XBM GQRP 1678 ISWL G11088 > > > > > -- > http://g3xbm-qrp.blogspot.com/ > http://www.g3xbm.co.uk > http://www.youtube.com/user/g3xbm > https://sites.google.com/site/sub9khz/ > G3XBM GQRP 1678 ISWL G11088 > > > > -- > http://g3xbm-qrp.blogspot.com/ > http://www.g3xbm.co.uk > http://www.youtube.com/user/g3xbm > https://sites.google.com/site/sub9khz/ > G3XBM GQRP 1678 ISWL G11088 > > > >