Guy What a terrific tutorial, I really appreciate you taking the time. I agree with you that this would be a good FAQ for others contemplating this idea.
I am content with making the toroid as specified as I have most of the materials to hand and will search for those tx caps. I will also look for big old variable caps to augment ones I have to hand (some WW2 brass butterfly and very thick-vaned, wide-spaced types). I have some time before my linear is ready, so, for a few months I will be at 100W while better types come to hand. I will also have a look at the Picatune to see what I might glean from that. Thanks again and I will let you know how things turn out. 73 David G3UNA/G6CP -----Original Message----- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Guy Olinger K2AV Sent: 20 November 2018 18:46 To: David Cutter Cc: TopBand List Subject: Re: Topband: FCP for lower power Hi, David, Regarding the FCP 160-80 meter dual banding project and circuit component sizing particulars ... I completely understand the urge to downsize. However, the devil is in the blasted details. This question faithfully recurs from time to time: Can we use smaller wire, use a smaller core, smaller relays, etc? That is why I have replied to the list instead of off-list. You have some company in your inquiry, and likely will have more. I'll also likely turn this into an FAQ entry. The question originates in the understandable but incorrect idea that the only or essential reason for our using larger components is to prevent burning them up while running QRO. Back in pre-publish days Jack and I were locally famous for burning stuff up and setting utility buildings on fire. And we DID work to have our creations not catch on fire. But ultimately we realized that sneaky accumulated miscellaneous losses were more sinister than fire risks. With a main and ultimate goal of helping people to get out and make QSO's, we had to refocus our thinking on eliminating loss to continue with new discovery and progress. In the FCP, and related projects in the loss list, the central design aim is to REDUCE LOSSES. Avoiding burning up at QRO power levels IS one, but ONLY ONE benefit from using larger components. And it is NOT the essential or controlling issue. Nearly all of the sizing decisions on k2av.com have to do with reducing dB loss, considerably more critical to results at QRP than it is at QRO. Loss reduces performance, and loss will affect QSO's far more often at QRP power levels. QRP operators are most likely to be in the noise at the distant end, even when QRO's extra 25 dB puts that QRO signal well out in the clear and armchair copy. When one's TX signal is in the noise at the far end on difficult propagation paths, all those recovered tenths of dB's and possibly a few entire dB's added together can enable copy and QSO at the far end. The subject of toroid core sizing can be difficult to understand and explain, but downsizing DOES have costs. In toroids with high coverage windings, all else held equal, eddy current loss in watts increases with the square of the inverse of the toroid volume, thus putting smaller toroids at a disadvantage. The smaller inside circumference of smaller toroids may force the use of smaller wire, with less insulation and more copper loss in the winding, to keep the same number of turns in the winding. These are the two main problems with using the T200 powdered iron cores for FCP isolation transformers. Again, this hurts results at QRP more than at QRO. Separately there are the issues of breakdown voltages and capacitor RF current handling. We can be tricked if we don't pay attention to some sneaky square law subtleties. Sizing of relays in this project involves breakdown voltage, not power dissipation. Unfortunately for expense scrimpers (count me in that group), voltages reduce only with the SQUARE ROOT of the reduction in power. At 1500 watts NEC4 shows the RF voltage at the FCP shorting relay to be 8 kV peak-to-peak. Adding a 50% safety margin for situations not in the model sets the required voltage rating at 12 kV. Dropping power from 1500 to 100 watts lowers voltage approximately by a factor of 4, to 3 kV which includes the safety margin. Dropping from USA 1500 to British 400 watts only cuts voltage numbers in half. On the bright side, I have seen Jennings RD5B vacuum relays, well-suited for the FCP shorting application, go for as little as 35 USD on eBay. I paid a surplus house 90 USD for the one I am using. Choice of a non-vacuum capacitor for the circuit does involve breakdown voltage. BUT far more importantly for solid capacitors, the choice requires knowing their internal resistance and/or their current carrying capacity. Here, as with voltage, the current reduces only with the square root of power reduction. Regarding vacuum vs. solid capacitors, there is a large gap between the current handling capacity of various film type capacitors and those using special material such as HEC-50 series transmitting capacitors. Three HEC 170 pF in parallel for a fixed 510 pF for the 80m network cap can reasonably handle 12 amps RF circulating current in the circuit. Other types of caps seemed physically large enough, but had no published manufacturer's ratings for RF current. They began overheating in only three minutes at QRO, easily seen in quickly increasing SWR following the heat-driven change in capacitance. In minutes, the caps were too hot to touch. At my location at 1500 watts, only the HEC's and vacuum caps have survived. More important, caps with runaway heat at QRO would have the same dB LOSS at QRP, but offering no loss warning from smoke and wild SWR's when TX. Alas, there seems no cheap or junkbox way to construct without breakdown or loss. Using the Deltrol relays with increased contact spacing seems to clearly cover voltage from only 100 or 200 watts, or possibly "no cheating" 400 watt limits. With a hi-pot meter that measured up to 15 kV we measured breakdown of a Deltrol modified for two 80 mil (2 mm) gaps series-connected. The relay started to arc across all four contacts at only 6000 VDC. Scientifically, getting only 6000 VDC breakdown from the Deltrol's two gaps in series, instead of double the single gap measurement of 4000, seems to involve something about reduced gap resistance once an arc is in progress, thus not allowing the two ratings to add in series. This *measured* DC breakdown result tells us that a single modified Deltrol relay with in-series gaps enlarged to 2 mm is at its properly engineered limit at 400 watts and higher power levels are eating the safety margin. I note, as one correspondent suggested, that using TWO modified Deltrol relays with their four 2 mm gaps in series would seem to handle QRO. But the second relay only adds 2 kV per additional gap or 4 kV additional for 10 kV. This at least exceeds the expected QRO RF voltage of 8 kV RF peak-to-peak at the FCP shorting point. But it reduces the safety margin to 25%. Maybe not so good an idea for a non-sealed relay used outdoors with connections through the weather enclosure. At this writing we only have good reports using non-vacuum relays with the modified Deltrol (or similarly modified equivalent relay) at stations operated at 100 watts. Note that the ubiquitous RJ1A genre of vacuum relay types, with their typical 3 kVDC ratings also DO NOT rate at QRO or even 400 w in this application, retaining a safety margin only at 100 watts. The modified Deltrol handles more at a fraction of the cost. Personally I am using Jennings vacuum relays RD5B (SPST NO) and R3B (DPDT) in my 160-80 setup. The R3B is a sentimental keepsake from the estate of W0UCE, used on his 160-80 switching setup, and now on mine. Aside from dropping it on a concrete floor or adjusting it with a sledge hammer, the R3B seems indestructible. I can assure you Jack never gave it any rest. Again, David, I do understand the urge to downsize, but would respectfully suggest you not do it. Personally, being able to do 160 very well, AND using the 160 wires to ALSO get the top-drawer performance of an 80 meter end-fed half-wave L, **in my only remaining place on the property adequate to erect such a wire**, would be worth a lot of money for relays, vacuum caps and whatever. I must also admit I did throw up some cheep stuff that didn't survive, just to see what happened. I really didn't expect them to survive, and they didn't. If you do try the cheep route, it would be useful to know for the record what you tried at what power, and which of them arced, smoked, incinerated, melted, etc, and which did not. Something like "A fellow from the UK already tried that and it went up in flames," is a very useful tidbit for some correspondents. (I don't give out source call signs unless I have permission.) Just remember that if it heats up, melts or breaks out in flames at QRO, that the now-demonstrated large dB loss will also be present running QRP and LP. 73 and good luck, Guy K2AV On Tue, Nov 13, 2018 at 5:25 PM David Cutter via Topband < topband@contesting.com> wrote: > I am planning an inverted L over a folded counterpoise as K2AV. > My power levels will likely be about one third of the design levels > for the K2AV design. I can imagine during tune up or adjustments that > I might hit half the power level for short periods, so my component > values need not be so extreme. I'm thinking mainly of the vacuum > variables and other caps. I intend to use the 80m option, so relays will also have a relaxed spec. > > Has anyone made a lower power version or can advise thereon? > > David G3UNA/G6CP > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector