Hi again David,
I remembered that there was no advantage in a 'Slim-Jim' compared to a 'J-Pole' and the following link will provide some detail as to why :-
I can understand that those building typical VHF examples will be utilising twin-line feeder and it may seem more advantageous to simply clip out/short together sections than removing whole swathes of the feeder. As there seems to be no real advantage or disadvantage to either it's obvious the easier path will be taken with such feed line constructions.
However, if building the antenna
from other resources such as copper pipe it would be more advantageous to leave out the hassle and expense of adding the fold back of the 'Slim-Jim' sections especially if it adds nothing useful to the final construction.
There are those that will argue a Slim-Jim will somehow outperform a J-Pole despite evidence to the contrary and you have to make sure that their view isn't a vested interest. Those that sell them will obviously swear their advantages as well as those that have gone to the expense and efforts of building them. Some are in it for the profit and none of us like to feel like a fool.
I couldn't find a reference to a Practical Wireless example but can give you a link to an article from the venerable L.B. Cebik who describes one for 10m :-
If I come across anything of help I'll be sure to post it up for you.
In my own experience I didn't build a VHF Slim-Jim as the once cheap and plentiful twin line feeder now attracts awfully knocked up prices. After discovering that it added no benefit to the final antenna
I constructed a J-Pole simply from wire and sure enough it worked. It didn't however provide anything more than the simple wire dipole arrangement performance wise. The only advantage I could see was that it was an end fed arrangement but it was far more sensitive to it's environment than the dipole ever was.
My 'walk-away' experience from that was to stick with simple dipoles including on HF frequencies never being able to contemplate or justify the hassle/expense of such arrangements on a bigger scale.
Experiences and opinions may vary.
Others may have more to add, maybe questions on 'velocity factor', your pipe diameters, etc. etc. Part of the 'fun' in such ventures can be in wading through the quagmire of 'information' in the hope you get successfully to the other side. Often it's simply just 'fun' being involved in something that takes our interest and fills our time....who am I to argue with that?
I hope you get somewhere with it all, despite the frustrations that can often occur. (40MHz???)
All the best,