Shielded balanced feedline

[VK4YEH]

Information on this beastie is fairly hard to come by, so any assistance in clearing the fog would be appreciated. This is what I know/assume:
- a balanced feedline can be made from two pieces of coax using the centre conductors.
- if the outer braids are not connected then the impedance is the sum of the impedances of each of the coax lines.
- if the outer braids are connected then the impedance is calculated thus - excuse the unmatematical maths
z = 1/((1/z1 + 1/z2))

so 2 x 75 ohm will present as either 150 ohm or approx 38 ohm depending on connections.

Any corrections to my assumptions/calculations will be very welcome.

Next question: has anyone tried this as a feedline and how does it perform compared with other balanced lines?

I am particularly interested in using it with a G5RV ....

Thanks

Tim Roberts
VK4YEH

[ZL1RS]

My understanding from the text books and practical experience is:

Using the two center conductors of two lengths of coax side by side as a balanced line will give twice the impedance ... i.e. the center conductors of 2 x 75ohm coax lines used as a balanced line will give a line impedance of 150ohms. The braids should be joined at both ends of the line, and it won't matter if they are "earthed" ... in fact, if you want "shielded balanced line" I suspect that at least the "radio end" of the braids should be earthed. I believe the two coaxes should be taped closely together along their length??

Using two coax lines side by side with the centers in parallel and braids in parallel is the way to get 1/2 the impedance, but it is still an unbalanced coaxial line. I've used 2 x 1/4WL lengths of 75ohm coax this way to get a 37ohm unbalanced 1/4WL coax line to match a 25ohm antenna load (coax phasing lines from two 2M yagis in parallel) to a 50 ohm coax feedline.

I don't know why you would want to use coax as a balanced line for a G5RV when ladder line is specified by the original design? The ladder line is much lighter and will not cause the center of the G5RV to sag so much ... unless you are running the balanced line up a central metallic mast? In which case it sounds line a good idea to try.

73, Bob

[VK4YEH]

Thanks for the info Bob

I probably didn't explain myself very well. I would make a ladder line for the feed/stub of the G5RV, but was wondering about a shielded balanced line from the stub to the radio.

So I guess my real question is, is there any advantage to be had by making the balanced line or should I just be more conventional and go for the coax. By the way I use coax on a G5RV at the moment and it works as predicted in the various documents I have read.

I run a radio club at my school and we have been looking at various antenna types in a general way. Now it's time to do some construction with the lads and a G5RV would be a nice easy - and cheap - multiband for them to get their teeth into.

Tim R

[VK2ZRH]

Hi Tim,

If you're going with the G5RV, then I strongly recommend you read these two online articles before you start:

FEEDING A G5RV: http://www.vk1od.net/antenna/G5RV/index.htm

OPTIMISING A TYPICAL G5RV: http://www.vk1od.net/antenna/G5RV/optimising.htm

Reading costs nothing ! (Oh, alright - for the economists - the cost of being online and the opportunity cost of not doing something else productive in $ terms).

73, Roger Harrison VK2ZRH

[VK4QB]

Hi Tim,
While the G5RV is a good antenna , its a hard one to explain why it works. I would stick to something more convential such as the open wire fed 1/2 wave dipole with a tuner. Roughly the same length as G5RV but not as critical on length These work just as well and much easier to explain to kids why and how.
73
Brian

[VK4YEH]

Thanks for the input Guys.

Yes Brian I agree that the half wave dipole is an easier antenna to understand (and build), and so I did consider it/them as a construction project. My decision to go for the G5RV is based on the following:

1) Parent friendly. My boys are 14 and 15 years old. Most parents will want the cheapest most unobtrusive antenna, and most will want one only - unlike the back yard of my place that resembles a spider web crossed with an echidna.
2) very forgiving in terms of orientation, height etc (up to a point of course)
3) ease of construction. granted not as easy as a half wave dipole but still well within the grasp of a 14 year old with the minimum of tools.
4) bands . All my boys are working towards the Standard Call, so I need to accommodate their future licences. See point 1 above which is closely related to this one
5) cost. Yes, a dipole would be marginally cheaper but see point 4 above
6) construction. we will be making the twin line as well - measurement and cutting, twin line construction and theory, coax connectors, coax theory - lots of things to learn and do with this one.

Having said that, I'd be very happy to hear about suitable other antennae that meet the above criteria....

A future project will be a 2m slim jim, or similar - again a very easy construction, parent friendly and relatively unobtrusive.

Thanks again guys

Tim R
VK4YEH

[VK4QB]

Hi Tim
You have some valid points . I like the dipole because its not limited to the the old ham bands but works on the new ones as well. and really is the same size as a G5RV. I have a Zmatch tuner which gives 1:1 on any frequency and the line length is not critical , in fact I don't know what the feed line length is. As long as it resonates on 80 then it'll do the rest. No baluns or special line lengths required. I'm going to pull my all band vertical down .It doesnt work as good as the dipole on most bands The only bad thing I have with the dipole is the bats run into it, and as its tied to the house we hear them hit it. . .
73
Brian