[BL1] Seems lossy in all useful configurations
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[BL1] Seems lossy in all useful configurations
 
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I tried using a BL1 (4:1) with my K2 and KAT2 to allow earth free
operation using an improvised antenna in a first floor (US = second floor) flat, but I've come to the conclusion that under almost all circumstances, even though I'm getting reasonable SWRs, most of the power is going into heating balun. I'm sure that this is partly through not originally understanding the limitations of baluns, in particular that they are only really usable on the equipment side of a balanced antenna coupler that presents the design impedance to the balun, or an antenna that does this without a coupler, so don't really make sense when impedance matching is done on the equipment side of the balun, as in the KAT2. In particular, if the load is significantly reactive, with the 4:1 configuration, the DC short that exists will also be a relative RF short. However, some other concerns are: - although the core type is not specified, so I cannot actually calculate the value, I'm concerned that two turns isn't going to be enough to get 10 times the load impedance across the whole frequency range (10 times is the typical guideline for transformers); - flat line, rather than co-axial is used for the windings, which means that a significant part of the differential mode flux is carried in the core, and so the core acts a conventional transformer. However, my understanding is that common mode choking is normally done in a frequency range at which the core is lossy, which means that some of the differential mode signal is subject to core losses; - although the cores used are marketed as balun cores, their intended use as balun cores is as voltage baluns, with all windings going through both holes. The BL1 configuration means that not all the flux around the winding is actually in the core, and that there will be some magnetic coupling between the two windings (which would, conventionally, be one two separate cores). Can anyone: - find flaws in the above; - indicate the range of operating conditions over which most of the forward power actually reaches the load (you can get low SWRs without the load receiving power, if there are transmission losses) and over which there is also effective common mode rejection; - suggest a source of a design or kit for an unbalanced to balanced antenna tuner that will work with the basic K2, and if not too much more expensive, will work with the 100W option. -- David Woolley Emails are not formal business letters, whatever businesses may want. RFC1855 says there should be an address here, but, in a world of spam, that is no longer good advice, as archive address hiding may not work. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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David Woolley wrote:
> I tried using a BL1 (4:1) with my K2 and KAT2 to allow earth free > operation using an improvised antenna in a first floor (US = second > floor) flat, but I've come to the conclusion that under almost all > circumstances, even though I'm getting reasonable SWRs, most of the > power is going into heating balun. > > I'm sure that this is partly through not originally understanding the > limitations of baluns, in particular that they are only really usable on > the equipment side of a balanced antenna coupler that presents the > design impedance to the balun, or an antenna that does this without a > coupler, so don't really make sense when impedance matching is done on > the equipment side of the balun, as in the KAT2. There is no difference if the balun is on the input or output side of an unbalanced tuner. See <http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00050000000000000000> I assume that you are using the balun because the antenna is balanced. If not, just use the KAT2 directly. I just recently set up a similar system, a balanced antenna with a radio that has an unbalanced tuner built in. What I did was measure the r and x presented by the feedline with an antenna analyzer. It turned out that on the two bands that I was concerned about, r values were between 15 and 150 ohms. But there was significant reactance (in my case, inductive). When I tried loading the antenna directly through a 1:1 balun there was significant heating. And the SWR was very high, making it hard for the internal tuner to find a match. I then added some capacitors in series with the feeders to cancel the reactance. I used a compromise value which did not reduce it to zero on both bands, but made a significant dent in it. As a result, it was possible to load the antenna on both bands with no detectable heating of the balun. I suggest that you start by determining what you are dealing with by measuring the r and x. You may be lucky and have a situation that can be handled without any switching, as I did. Or you may have a super-low resistance on some bands, large reactances of opposite signs on different bands, etc. In that case you might be able to improve the situation by changing the feedline length. I would not try to fix this by changing the balun. Knowledge is power: find out what you are trying to match! -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: [BL1] Seems lossy in all useful configurations
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Vic K2VCO wrote:
> > There is no difference if the balun is on the input or output side of an > unbalanced tuner. See That's only true of an ideal balun. A real current balun has a finite common mode impedance, which is typically designed to be about 10 times the nominal operating impedance. If you put it between the ATU and the antenna, it will be working into the actual antenna impedance, which off resonance will quite high and reactive. Under those circumstances, the ratio between the common mode and differential mode paths may not be very large. > > I assume that you are using the balun because the antenna is balanced. > If not, just use the KAT2 directly. The requirement that I'm trying to meet is that the K2 chassis not be RF hot and that, if I connect the mains ground, there is no significant RF in that ground. RF choking the ground has the same problem that the choke impedance will not be large compared with an off resonant antenna. That requires driving an antenna that is, at least approximately, balanced - in reality it's not possible for me to get true balance, because the antenna has to be indoors. A choked earth doesn't remove the need for something to carry the return current, and whilst it may get called an artificial earth, it is really part of a semi-balanced antenna, and unless it is massive compared with the "antenna", will probably be more balanced that unbalanced. > internal tuner to find a match. I then added some capacitors in series > with the feeders to cancel the reactance. I used a compromise value In other words, you added a balanced tuner on the antenna side of the balun! I'll accept that there may be solutions that do coarse tuning on the antenna side, to get the impedance into the right ball park, then use the unbalanced tuner, on the equipment side, to do the fine tuning, but what you are describing is still doing a lot of the tuning antenna side. -- David Woolley Emails are not formal business letters, whatever businesses may want. RFC1855 says there should be an address here, but, in a world of spam, that is no longer good advice, as archive address hiding may not work. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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David Woolley wrote:
> The requirement that I'm trying to meet is that the K2 chassis not be RF > hot and that, if I connect the mains ground, there is no significant RF > in that ground. RF choking the ground has the same problem that the > choke impedance will not be large compared with an off resonant antenna. > That requires driving an antenna that is, at least approximately, > balanced - in reality it's not possible for me to get true balance, > because the antenna has to be indoors. The usual solutions to RF on the chassis are 1) changing the feedline length (so that it is not close to an odd multiple of a quarter wavelength) and 2) connecting a quarter wavelength radial to the chassis to decouple it. Of course these solutions can be more complicated in multiband situations. > In other words, you added a balanced tuner on the antenna side of the > balun! I'll accept that there may be solutions that do coarse tuning on > the antenna side, to get the impedance into the right ball park, then > use the unbalanced tuner, on the equipment side, to do the fine tuning, > but what you are describing is still doing a lot of the tuning antenna > side. OK, I'll accept that. I wasn't trying to prove a point about where the tuning takes place, but rather to suggest that reactance has a lot to do with balun efficiency and that the place to start when trying to match an antenna is by measuring the r and x so you know what the problem is. -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: [BL1] Seems lossy in all useful configurations
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In reply to this post by David Woolley (E.L)
David,
Most any indoor antenna is in close proximity to the transceiver, and it is quite likely (even with the most carefully balanced antennas) to have RF floating around most everywhere, including the transceiver chassis and any earthing wires. I would suggest that you may be better off trying to create an RF ground condition at the antenna side of the tuner and not worry about the rest of the antenna system. An RF ground condition can be created (actually forced) by attaching 1/4 (or odd multiple) wavelength wires to the ground terminal of the tuner. The far end of the wire should be insulated because even with QRP power levels, a high RF voltage can appear at the far end. If the quaterwave wire does not create an RF Ground having an impedance of 20 ohms or less at the shack end, then it is not an electrical quarterwave - antenna and transmission line principles prevail. Yes these 'counterpoise' wires may radiate as part of the antenna system and cause pattern distortion, but with an indoor antenna, the main concern is usually that a signal is radiated - most situations where an indoor antenna is necessary, the amateur cannot be 'antenna pattern picky'. 73, Don W3FPR David Woolley wrote: > > The requirement that I'm trying to meet is that the K2 chassis not be > RF hot and that, if I connect the mains ground, there is no > significant RF in that ground. RF choking the ground has the same > problem that the choke impedance will not be large compared with an > off resonant antenna. That requires driving an antenna that is, at > least approximately, balanced - in reality it's not possible for me to > get true balance, because the antenna has to be indoors. > > A choked earth doesn't remove the need for something to carry the > return current, and whilst it may get called an artificial earth, it > is really part of a semi-balanced antenna, and unless it is massive > compared with the "antenna", will probably be more balanced that > unbalanced. > Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by David Woolley (E.L)
In a message dated 10/13/07 4:58:04 PM Eastern Daylight Time, [hidden email]
writes: > There is no difference if the balun is on the input or output side of an > unbalanced tuner. In theory, no. In practice, there can be a big difference. See > > > < > http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00050000000000000000> > He's not looking at the big picture. If the balun is ideal, or close enough to ideal, or if the shack-end impedance of the balanced load are within a certain range, the unbalanced-tuner-with-balun-at-the-antenna-end idea works fine. Thousands of hams use it with no problems and good results. But in some cases the shack-end impedance of the balanced line can be very high, very low, and/or highly reactive. Under those conditions some baluns don't work well, and all sorts of odd things happen. Sometimes the end result works well enough that the ham doesn't notice anything wrong, particularly if s/he has nothing else for comparison. Sometimes the problems can be fixed by things like changes to the line length or adding reactive elements in parallel or series with the line. You can't just blindly increase the number of turns on a wound-core balun to increase the impedance because you may set up self-resonances that cause all kinds of fun. The best approach IMHO is to model the antenna-feedline system and see what the actual shack-end impedances are. Or measure them. Then decide what tuner setup is needed to do the matching job. 73 de Jim, N2EY ************************************** See what's new at http://www.aol.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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[hidden email] wrote:
> In a message dated 10/13/07 4:58:04 PM Eastern Daylight Time, [hidden email] > writes: > > >> There is no difference if the balun is on the input or output side of an >> unbalanced tuner. > > In theory, no. > > In practice, there can be a big difference. > >> See >> <http://fermi.la.asu.edu/w9cf/articles/balun/balun.html#SECTION00050000000000000000> >> > > He's not looking at the big picture. His point is that if you use an unbalanced tuner with the balun on the input, then there is a common-mode reactance that is equivalent to the differential-mode reactance that is being canceled by the tuner which is reflected back to the balun. So the problems that the balun has on the output do not go away when you move it to the input. Using a *balanced* tuner followed by a balun or a link-coupled tuner like the Matchbox solves this problem. But what many of us want to do is to feed a balanced line from a radio with a built-in automatic (but unbalanced) tuner, like the KX1-K3, without adding an additional tuner. > If the balun is ideal, or close enough to ideal, or if the shack-end > impedance of the balanced load are within a certain range, the > unbalanced-tuner-with-balun-at-the-antenna-end idea works fine. Thousands of hams use it with no > problems and good results. > > But in some cases the shack-end impedance of the balanced line can be very > high, very low, and/or highly reactive. Under those conditions some baluns don't > work well, and all sorts of odd things happen. Sometimes the end result works > well enough that the ham doesn't notice anything wrong, particularly if s/he > has nothing else for comparison. Absolutely correct, and I recommended exactly what you do: measure the r and x on the balanced antenna and then either change the line length, add reactance to compensate for the x, and choose an appropriate balun ratio to get as close as possible to the r. Of course, you can't always do this easily for all bands! > You can't just blindly increase the number of turns on a wound-core balun to > increase the impedance because you may set up self-resonances that cause all > kinds of fun. Although if you are only interested in a few bands it's easier. > The best approach IMHO is to model the antenna-feedline system and see what > the actual shack-end impedances are. Or measure them. Then decide what tuner > setup is needed to do the matching job. Modeling it is harder than it looks because all kinds of things affect the real antenna, and sometimes the impedance at a particular distance along the line from the antenna changes rapidly. My antenna was an 88' inverted V with the apex at 40 feet, so the ends were not that far from the ground, causing incorrect modeling results on the lower bands. I have an Autek VA-1 antenna analyzer which is small and has a self-contained battery, so I just put it on a large carton and hooked it directly to my parallel feeders. An MFJ with an AC supply got incorrect results because of capacity to ground. I was lucky, and on the bands of interest, 80 and 40 meters, I got r ranging from 15 to 160 ohms and x was inductive in both cases. So I chose a value of capacitance which canceled the inductance on 80 (the band on which tuning was the sharpest) and which reduced the reactance on 40 significantly (and also incidentally changed the sign). Once I did this I got good results from a 1:1 balun. I tested it by attaching a thermocouple to the core and measuring the temperature rise before and after adding the reactance compensation. Without the capacitors, the temperature went up several degrees when I transmitted at 100 watts for one minute. With the capacitors, there was no measurable temperature rise. The SWR seen by the radio was below 8 to 1 throughout the CW bands on both 80 and 40 (as low as 2:1 at one point on 80) so the internal tuner had no trouble matching it. I also checked for balance by using an RF sniffer on the feedlines at the exit of the balun and got good results. Performance seems to be no worse than with the Johnson Matchbox -- and now I can operate on 80 and 40 meters with this antenna without tuning anything, something that makes life easier in contests. The balun I used was a huge home-made choke with six (!) ft-240-31 cores wound with 8 turns of RG-58/u. This is only intended for the low bands and would probably be poor on the high bands, but the Elecraft balun should work well on all HF bands. There's an article in the latest QEX that describes a method of using a combination of a voltage balun and a current balun to get better performance in situations characterized by wide ranges of impedance. I would have tried this if I hadn't been able to meet my needs as described. The Elecraft balun is a current balun, and one could add a simple voltage balun. Someone should try this! -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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