I wouldn't proceed further until you know the complex impedance you're seeing across the link.

I think your plot is mostly showing the inductive reactance of the link itself:

http://goo.gl/AS35b
http://goo.gl/79Eu5

You can put it at the feedpoint. I tuned my 60 foot vertical on 160m with a capacitor. I happened to have a 2000pF vacuum variable which is only 44 ohms reactance when fully meshed. That's a reactance that in my case was less than a 10% increase in the required loading inductance, so I felt it was a reasonable compromise.


That's not really balanced.

It's... complicated

I mean that the side of the transformer/mid-loading inductor connected physically to the antenna doesn't need to have as many turns as if you were assuming a transformer with perfect flux linkage and a certain turns ratio. We match to a magloop by having a big loop with one turn and a little loop with one turn; only a portion of the flux from the big loop couples to the small loop and you use that to make an impedance transformation. No reason why you can't use the same principle for a link coupling arrangement in the middle of a short dipole, if you  want to keep most of the loading inductance at the end and then only want to put a turn or three in the center. Adjusting the link size and even its orientation can change the impedance seen on the transmitter side. Gives you some extra adjustments beyond the turns ratio of inductor in the antenna and the link.


Yes, that's what I'm talking about.


Recent events have upended most aspects of my planned career path (to the extent that I had a plan) and my personal life. It's not a dire situation but it's an unbalancing and chaotic one, and I need to focus my energy away from hobbies.

I'm hoping to be a NYC apartment dweller as soon as possible but I need to find a job there first and it might be a while after that I get back around to ham radio    

Keep in mind that you COULD do this like you do with a magloop: get lower mutual coupling by making the coupling link loop smaller than the main loop. Then you don't need as many turns on the main.

In terms of how much inductance to put in the center, I would base that on radiation resistance considerations. Use as much as necessary out further in the radiator to make the current profile flat and raise the radiation resistance (I model this losslessly so that Rrad is what I'm adjusting, then add loss resistance in later). Put the rest in the middle.

A broadband transformer could work too.

I want to do some experiments with link coupling for attempted noise reduction but I might never get around to it. Reached a point where ham radio experiments are going to take a back seat to everything else for quite a while.

Your equivalent circuit looks good... the circuit can be visualized as a series LCR where C has the capacitive reactance of the antenna, R is the combined loss and radiation resistance, and L is the loading inductance.

In my experience the tapped-coil version can roughly be approximated by a two-inductance L network instead of your transformer. There is maybe also some autotransformer action but that didn't seem to be the case for a large loading coil I compared with calculations.

I would strongly recommend that you don't rely on inductive/resistive chokes for feedline decoupling if you feed near an end.

Ultra short off balance antennas are great feedline exciters. I've tried some models with physical connections to the bottom of a short hatted dipole and needed 100k ohms for effective choking. That's like a trap. It is possible to adjust off-center-fed short antennas so they simply overwhelm a modest inductive/resistive choke like a common ferrite. You can reduce the common mode CURRENT to a reasonable fraction of the radiator current with, say, 5000-10000 ohms impedance but then a good broadband choke can have a fair amount of loss. Every situation is different and I'm mainly conveying some things I've observed n models and some experiments that weren't really an antenna.

Link coupling is a good approach because then the common mode impedance you insert is quite high and purely capacitive.

Now, it's possible that you won't have any common mode PROBLEMS especially at QRP but there's a large class of asymmetrically fed "short antennas" that are really just coax-exciting tank circuits and adding ferrite chokes that would be effective on a big dipole would just add a bunch of loss. You will note that many of these antennas sold commercially will strongly advise against chokes.

If you really want excellent feed decoupling, link coupling at the center is probably the best way to go, but I haven't done the work to check different approaches there quantitatively.


A current meter is pretty easy to build. Just a transformer, diode, capacitor and meter.

http://www.w8ji.com/building_a_current_meter.htm

You need to make sure it's assigned to be in PARALLEL with the source. I haven't messed around with bare NEC-2 or 4nec2 enough to know how to do this but I think there's a different setting or flag or whatever to put two objects on the same segment in parallel instead of series.

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For modeling purposes you can also calculate the required shunt inductance exactly (or nearly so) from the modeled resistance to help you dial in the right values. The feedpoint impedance Zf = Rf+jXf with NO shunt should be adjusted so that it presents the following reactance Xf (this is usually easy enough to do as long as Rf doesn't change much at the desired operating frequency, otherwise it will take iteration), and then you should place a shunt reactance Xs also calculated below across the feedpoint.

For feedpoint resistance Rf:  
1)Calculate delta = sqrt(50/Rf-1)
2)Adjust the loading inductors/cap hats such that Xf = -delta*Rf
3)Then calculate the shunt reactance Xs = +50/delta

The parallel combination of Zf and Xs should then be 50+j0. If you want something other than 50 ohms, replace the "50"s above with the desired feed resistance when matched.

----------------

Example: say Zf = 9+jXf where Xf can be adjusted by tuning with no shunt.
Xf = -9*sqrt(50/9-1) = -19.2 ==> Zf = 9-j19.2
Xs = +50/sqrt(50/9-1) = +23.43

When in parallel, Zmatched = Xs||Zf

Zmatched = 1/(1/(9-j*19.2) + 1/(j*23.43)) = 49.96-j0.05 ( http://goo.gl/LemgY )

Sounds reasonable to me.

Oh, I don't know about that. There may be something to some of the approaches, especially if constraints force you toward wire.

I think my comment is more about the fact that efficiency comparisons if they're even present tend to have a lot of issues. There are a lot of S-meter readings, or just "works well, I worked Cuba from Ohio" sort of reports.  There are a lot of antennas that get tested against antennas that are probably strongly coupled to the loop (this happened during some of my tests when I first built a magloop and was significant).  There are a lot of tests I've seen, not just of magloops but of all manner of small antennas for the 40m band installed on the top of 20-25 foot high supports. No attention is paid to measuring feedline decoupling. Many of that class of antennas are probably best characterized as hard to tune top loaded verticals with no radials.

You know, like this: http://n3ox.net/files/tunerant/tuner_ant.jpg

None of that is STRICTLY a problem in a hobby context, but it contributes to a horrible signal-to-noise ratio for advice for hams operating from an apartment building with a couple meters of feedline, hams who have a backyard that would accommodate a 15 foot high loaded vertical with radials, or hams who like activities that need performance approaching a "big and good" antenna.

It doesn't count as a magnetic loop if stray currents give you an order of magnitude more radiation resistance, it doesn't add to good antenna information if it's actually a tenth as efficient as a simple approach that's comparable in complexity and cost, and so on.

My hunch is that kind of feed system will often have feedline decoupling problems anyway.

Yeah. I wonder why we always have to wonder

I bought a rebuilt HD-73 from Norm's Rotor Service a while back; I had a spare controller but no rotor. Beautiful condition, works great.

It's unlikely that I'll be using any beams in the near future but I haven't figured out what I will be using   Just took all my antennas down (including beams that worked on 20/17/15/12) for a move.

Pretty sure that time has come in this boy's life to set DXing aside to focus on other things, but we'll see. I'll make sure to put something up for Heard anyway.

Looks like I'm going back into apartment living soon, but it would be fun to try more of my tiny antenna ideas "in the field" with daily use.

I might start to do something different, though, like QRP digital.