double stack

[livendirect]

Guys looking for diagrams or info ect on building a double stack as im coming back on air in feb and always wanted to stick a stack up so going to build one if i can find all the info

Am i right in thinking they have to be a 1/4 spaced and feed with 75ohm coax does the 75ohm coax have to be same length

[thewisepranker]

I don't like the 75 Ohm coax method. It's not very efficient or effective and is usually full of misinformation.

Anyway, the correct spacing is actually 0.9 wavelengths.



Building and tuning an effective double stack is quite an involved operation which requires mostly knowledge and a little bit of expensive RF equipment. In a lot of cases, pirates will "get out" just as well using a well-matched single as they do using a bodged up double.

[nrgkits.nz]

This is what you need: https://www.qsl.net/dk7zb/Stacking/splitter.htm

Use copper pipes on the inside if you can, its more conductive and you can also solder the center pins of the N connectors to it with relative ease using a blow torch.

[Bton-FM]

What other aerials can you stack apart from dipoles?

[Albert H]

Really just use Harry's "6dB" aerial: http://85.226.178.87/antennas/6dbvhf0.htm He's a bit ambitious when it comes to the gain, but it's relatively easy to build, costs next to nothing, and makes a twin stack look poor by comparison. My version of this aerial is in widespread use in parts of Europe - I used to sell a kit for it some years ago. I used to get around 5.2 dB over a (properly matched) dipole when these were assembled correctly. Harry's version is so simple and cheap that it's silly to try anything else!

[s2000]

I will definetely be building the Harry antenna this year sometime but there are some aspects of the double stack that I find interesting (theory wise) and would like to know peoples thoughts...

Usually when you build a single dipole, you would fit a pawsey stub balun on but is it required for each dipole element on a double stack? The double stack has two pieces of 75ohm coax (if you build it that way) which forms a simple wilkinson splitter (without the 100 ohm resistor). If you didn't fit the pawseys on the dipoles, would the isolation factor inherent with a wilkinson (between ports) prevent the feedline coax becoming part of the antenna?

Also, why is it that in the double stack example, it is not neccesary to fit a 100 ohm resistor in the wilkinson network? I understand that (depending on how you build the splitter), it may not be physically possible to fit a resistor (i.e between the far apart boom arms ). Lets say you made a splitter box with 75ohm cable in with a 100ohm resistor suitable for the transmission frequency and then used 50 ohm to feed each dipole. Would it have any benefits over the splitter without a resistor?

I did build a few Wilkinsons for 2x power amps in 2019, it seemed to work quite well as far as my experiments went.

[Albert H]

For stacked dipoles - when I had a customer who insisted on them - I used baluns inside the boom arms. (Actually, they were un-bals, since they were going from unbalanced coax feedline to balanced antennas!). The way we did it also required 75Ω lines and 150Ω lines for the impedance transformations. The optimum distance (for maximum gain) is 0.88 wavelength according to our "in the field" measurements. We assembled and matched the stacks on site, using far-field measurements to optimise them. They never worked as well as a simple single-slot colinear (like Harry's aerial linked above) - usually being at least 2.5dB down.

A better aerial is a twin-slot colinear. We built lots of these over the years. Our version is based on the 70MHz version published in the mid-80s ARRL Antenna Handbook (recommended reading for all of you!). Scaling isn't entirely straightforward - the 146MHz version of the same antenna shows that some of the dimensions don't linearly scale. Done right, the antenna has a 50Ω unbalanced feedpoint, and shows a 1:1 match over about 1MHz. The match worsens rapidly either side of the design frequency, showing that the "Q" of the aerial is very high.

Forget stacking dipoles. The rig thieves (both legal and illegal) recognise them from a long way away, and it's just so obvious what's there! For installs up blocks, I used to favour ⅝-wave or ⅞-wave verticals with groundplanes. These look like CB aerials to the idiots, show useful omni-directional gain, are easy to build and match, and don't radiate much into the building below. The field strength at the bottom of the block was so low that you'd think that you'd come to the wrong block!

[rigmo]

Really just use Harry's "6dB" aerial: http://85.226.178.87/antennas/6dbvhf0.htm He's a bit ambitious when it comes to the gain, but it's relatively easy to build, costs next to nothing, and makes a twin stack look poor by comparison. My version of this aerial is in widespread use in parts of Europe - I used to sell a kit for it some years ago. I used to get around 5.2 dB over a (properly matched) dipole when these were assembled correctly. Harry's version is so simple and cheap that it's silly to try anything else!

http://www.cqham.ru/6dbvhf0.htm

[Albert H]

http://sm0vpo.altervista.org/antennas/6dbvhf0.htm if you want it in English!