Building a Slim-Jim antenna for 6 metre band, using 300 ohm ribbon.

Here I look at this article  by Geoff VK6HOG, on Hamuniverse website, building a 6 metre band Ribbon Slim-Jim 


quoting extracts from the article:

"(The 6 metre Slim Jim antenna is centered on 52 MHz as I am only allowed to Tx in this region of 6 metres !!)
The formulae will obviously work at which ever frequency you desire.

Overall length: 4155mm (163.58 inches)
Short element (from base): 1338mm  (52.67 inches)
Top element (from top): 2740mm (107.87 inches)
Leaving a gap of 77mm. (3.03 inches)
The feedpoint is 209mm up from the base. (8.22 inches)

At the design frequency (52mhz) the SWR of the Slim Jim is almost flat, with it being around 1.5 to 1 at each end of the band.

Hopefully it doesn't change anything, but as stated, it has almost flat SWR at 52.300MHz."


The article says it is centered on 52 MHz, but I think he refering to the whole band, i.e. 52-54 segment, as I cant see why
you would be trying to use a vertical near 52.000 MHz, as the FM segment is pretty much 52.500 through to 54.000, therefore
I would centre a vertical at around 53.250 (in other words, half way between 52.500 through to 54.000).
So, Geoff has used some equations to produce the dimensions above, that's fine , but he has no provided the calculations.

Next the Editor of the article (N4UJW) adds some equations into the article, with extractions quoted here:

"Editor's notes follow: These are the formulas that you can use for designing the antenna for 6 meters or any other band for that matter.

They follow: Lengths in inches 3/4 wave (longest section) = 8415/MHZ
1/2 wave section = 5610/MHz
1/4 wave section = 2805/MHz
* 1/4 wave freespace = 2953/MHz
* distance that antenna should be from mounting boom, mast, or tower. "


Unfortunately, the Editor, (N4UJW) has quoted some other calculations, in fact, just tubed based calculations from another page,
Slim-Jim Antenna Project as using these calculations do NOT yield the same dimensions provide by Geoff VK6HOG,
who has built an antenna using 300 Ohm  ribbon.

The two issues  here are:
1. the original article provided by Geoff did not provide the calculations and all the variables, required to use
those calculations, ie. VF   Now, for the laymen, that means Velocity Factor of the transmission line or material used
to do the construction, in this case, it is 300 ohm ribbon cable.

2. The Editor of the article  (N4UJW) then provides  some calculations from this webpage Slim-Jim Antenna Project without
justifying how this is applicable to building a 300 ohm ribbon based slim-jim, or even the background as to how the
calculations are derived, i.e. are they theoretical values, or are they based on a specific VF value ?  it does not say.

It's pretty obvious from the antenna being discussed, that it has a quarter wave matching section, it is a transmission line,
it happens to be a piece of 300 ohm  ribbon cable, its a known fact that 300 ohm ribbon can have quite a wide range of VF
based on ther manufacturer, typically varying from .85 to .92, so calcuating lengths will vary based on who manufactured
the piece of 300 ohm ribbon cable you are working with.

If I reverse Engineer the dimensions provided by Geoff VK6HOG, into the calculations provided by the Editor, we end up back
at a frequency of 51.466 MHz, so this is obviously not the frequency that Geoff was working on. The trouble here is that Geoff
has not provided the exact frequency that he was using to do his calculations with, obviously its somewhere between 52.000 and
54.000 Mhz.

In summary, you can use Geoff's dimensions to make a ribbon cable Slim-Jim, but the VF of the ribbon you use may not yield the
same results as Geoff obtained.  You can use the calculations provide by N4UJW, but I cannot be sure if these calculations
are based on 300 ohm ribbon or for aluminium tubing, or if it takes into account the VF.


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Building Slim for 2 metre band.

There are quite a few articles on the web and in magazines, most of these articles are just crap. They have failed to
justify their calculations, many are just lumps of wire, trimmed to give a suitable VSWR, they may not be resonant
or providing the radiation pattern expected of them. Most are just cases of luck that they work at all. I'll try here to
discuss the articles I have collected to tell which to avoid and which to use.