Bringing open wire feeder into the shack

[VK7DR]

Wayne, getting back to your actual question.... let us be practical. It doesn't matter how you bring the line in - a slot in the mortar for eg is a as good as any. Just sip a bit of insulation (if needed) over the wire. Any disturbance to the uniformity of the feeder over that small distance will have no (practical) effect at all. Just keep the tuner close to the point of entry and no probs. Since its a block wall, the notion of "RF in the shack" is moot, since, in or out, the feeder is right there. What you get is what you get. For eg, I cannot use my OWL fed multiband antenna on 20m without getting RF into the mic. But that' life with OWL and modern equipment. Back in the days of the carbon and Crystal mic we never had RF problems.

[VK4WDM]

Back in the days of the carbon and Crystal mic we never had RF problems.

And we have the scarred lips to prove it

Now I have another problem. I can't find my old balanced tuner. Its the size of a shoe box so should be easy to spot. Maybe I lent it to somebody and never got it back. A 4:1 balun and coax is looking good.

73

Wayne VK4WDM

[VK2OMD]

An exercise on coax and currents...

The following diagram (by DK7ZB) contains an error.
...

As you know, I1=I2, but there is an error in the markup of currents on this diagram, can you spot it?

AS mentioned, coax imposes the constraint that I1=I2.

There is an implicity assumption that all currents (including on the dipole) are uniform, and that is a reasonable assumption if the region portrayed is very small.

dk7zbdiag_ver001.gif (5.7 KiB) Viewed 1518 times

So, cropping the diagram a little, it is reasonalby correct understanding that it relates to a very small region at the end of the coax.

We all know that the dipole current (horizontal conductors in the diagram) forms a standing wave and is certainly not uniform along the dipole.

In the larger context:
- current I1 and I2 inside the coax is uniform on a lossless coax with no standing waves (ie perfectly matched);
- Current I3 is most unlikely to be uniform, common mode current almost always forms a standing wave.

So with the qualification that the cropped diagram above represents a very small region around the end of the coax, it is a good illustration of what happens.

dk7zbdiag_ver002.gif (2.16 KiB) Viewed 1518 times

The bit I cropped off implies that I3 is the same at the grounded end as it is at the feed point and as mentioned, current I3 is most unlikely to be uniform, common mode current almost always forms a standing wave.

The I3 arrow across the circuit node (dotted join of conductors) implies that the current into the node from above is I3 and out of the node to ground is I3, implying that the current from the source (generator) shown is zero.

Owen

[VK2OMD]

Now I have another problem. I can't find my old balanced tuner.

That might be a blessing.

A 4:1 balun and coax is looking good.

An effective 4:1 current balun on the outside of the wall, and very short length of RG213 or better to the ATU sounds like a good solution for a full wave loop on fundamental and even harmonics.

Hint: don't follow the flawed designs that pile a Guanella 4:1 balun onto one core (eg as described in the Foundation manual)... it doesn't work properly. Proponents often cite Jerry Sevick as their authority, but Sevick qualifies his design as working on isolated or floating loads only... BH, if the load is isolated, curent into one terminal MUST equal the current out of the other terminal, you don't need a current balun!

Owen

[VK3LU]

Owen,
If one was to construct a 4:1 current balun, what is the minimum spacing between the cores from a capacitive coupling standpoint.

Cheers
Nev

[VK2OMD]

If one was to construct a 4:1 current balun, what is the minimum spacing between the cores from a capacitive coupling standpoint.

Nev, the first thing I would do is to prompt you to question whether you really need or should prefer a 4:1 balun. (Remember that 4:1 voltage baluns were the popular choice probably as it was the simplest construction and low in cost. The same benefits are associated with a 1:1 current balun.)

You are right to recognise the stray capacitance as significant.

Basically, a 4:1 Guanella balun made from two 1:1 baluns has lower choking impedance that each of the component baluns, and the stray capacitance futher exacerbates that.

Nevertheless, if you really need 4:1, you have to work with what you have and try and optimise the result.

I have not done extensive measurements on 4:1 Guanella baluns, but I would consider using something to space the two cores perhaps 10mm if you can achieve that in a reasonable enclosure. Make sure the material you use for that purpose is durable, will withstand the temperature, and is low deielctric loss.

One method of separating the cores might be to anchor them to opposite walls of the enclosure.

Good luck with your project.

Owen

[VK3LU]

Thanks Owen
At present I am using a 4:1 voltage balun. Given the visually obvious unbalance of the antenna, I have often wondered about using a 4:1 current balun to minimise feeder radiation.
Another possibilty is to add a 1:1 current balun after the voltage balun... perhaps.
I suppose I am pipe dreaming... the system is working well as is. But it is good brain excercise to contemplate what could be.

Cheers
Nev

[VK3LU]

Thanks Owen
At present I am using a 4:1 voltage balun. Given the visually obvious unbalance of the antenna, I have often wondered about using a 4:1 current balun to minimise feeder radiation.
Another possibilty is to add a 1:1 current balun after the voltage balun... perhaps.
I suppose I am pipe dreaming... the system is working well as is. But it is good brain excercise to contemplate what could be.

Cheers
Nev