First thing I would say it to literally model your back yard, including all your antennas and feedlines. In the same model. That may cost you some for serious models that will handle that much wire. Or you can visit a friend who has the software and do it on his.
In our head, among all the miscellaneous mental simplification devices we have to understand the world, are some often expressed, like coax laying on the ground is out of play and can be forgotten. It's not true of course, and it's hardly the only common misnomer. The truth comes up when you uncover a couple dozen surprises about what is really going on and places where common mode current on conductors is playing havoc. There is no simple rule, but the site model of your yard is worth it. One may get away with a lot of stuff most of the time, even nearly all of the time, but the typical backyard is so random compared to others' backyards that no inviolable rules really exist. One problem with an elevated long run of 450 ohm line placed ~ 900 volts of common mode voltage across a blocking point running QRO and was destroying ferrite devices. Literally modeling the transmission line in the site model identified the reason why, it was just an unlucky length in an unlucky place. It was also dissipating a lot of power. Some redesign, and transformer winding, and problem gone. It was obvious in the model. My "intuitive" mental simplification device about how that worked was just plain wrong. Do a site model. There was real increase in TX radiated power after that was fixed. There is much about antennas that simply is NOT intuitive. The model is not fooled. I realize this suggestion has all the attractiveness of being told to take out the trash, mow the grass, or plow the garden, paint the house, or complete putting down and burying the radials to have a truly dense field. But if you have one of those surprises.... Heh, heh. IF you have the site model, you can also model insertion of common mode blocks, adding grounds, discovering where they have effect. LIke I said, taking out the trash is neither elegant nor entertaining, but highly useful overall. 73, Guy. On Fri, Sep 21, 2012 at 1:24 AM, <zen...@netspace.net.au> wrote: > > > Hi Rich, > > This DXE balun is just a hefty 1:1 choke balun It's a high grade > coax (50ohm) wrapped through ferrite cores. The balanced terminals > are floating and the unbalanced end is connected to the aluminium > housing and the shield of the 6 foot length of RG213 coax which > connects the balun and my tuner. I have a balun inside my tuner too, > but I don't use it: it's a 4:1 current balun and in this application > with a multi-band loop, I prefer to use the external 1:1 balun. > > I agree - with my loop , anything that changes the feedline length, > even by smallish amounts, alters the impedance seen by the tuner at > the end of the line and the manual tuner settings change on (usually) > multiple bands and the KAT3 has to reconfigure its LC settings. I > expected that, as a function of the impedance transforming effect of > the feedline, but what I didn't expect was the very significant > difference that different shack "grounding" arrangements would make > to the whole process of matching the antenna system at the shack end > of the feedline. > > About the common mode current that I'm measuring on the coax between > the balun and the tuner: Yes, I assume the 1:1 balun has sufficient > common mode impedance to be preventing common mode current standing > waves on unbalanced side of it. But I think what I'm measuring is > current that's being induced in the coax shield because it's lying in > the near field of radiation of the loop. Again, what is a mystery to > me is how changing the grounding arrangement on the balun could affect > the magnitude of that. > > I find this intruguing, but for the moment, I've put the clip on > ammeter away. I'm going to forget it, while i digest this, and go > and play on 20m instead. > > Thanks for all the comments I've rec'd, on and off the list. > > 73 > > John > > VK7JB > > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Richard Fjeld > Sent: Friday, 21 September 2012 1:00 PM > To: elecraft posting > Subject: [Elecraft] Fw: Balun Ground Connection > > > > John, > > It is unfortunate that we can't see a schematic of how the balun is > wired. > > I have thought of doing that same thing to be able to use an ATU. > > Presently, I have an 80 meter loop fed with homebrew 450 ohm ladder > line to > > a tuner with a 1:1 balun within. It works beautifully on all > bands. I wish > > I had done it many years ago. > > > > As for your grounding the coax at both ends, I effectively do that > here also > > on my coax switch with coax to my K3 which is also grounded. So > that is why > > it would be necessary to see the schematic to see what is different > about > > your situation. > > > > I can tell you that I have everything grounded to a 'ground tree', or > earth > > tree as our friends call it. I buried a 6 ga. wire from the central > point > > over to my tower. I grounded it at 2 or 3 places along the way, and > the > > tower is grounded at each leg with a separate ground rod. When I > did make > > the final connection to the tower ground, I saw a change in the swr > of my > > beams which are at DC ground, and coax fed again similar to what you > are > > doing. The swr got higher, but not bad. Maybe it went up point > five or > > less. From my experience, any time I change ANYTHING relating to my > antenna > > system, the SWR changes. > > > > As for #2, the common mode current, I thought a balun is supposed to > stop > > that from happening. > > > > Rich, n0ce > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
