Nov 2, 2008

New omnidirectional verticals for 145 and 430 Mhz

The Half Wave Sleeve Antenna
This model acts similar to the typical J-pole designs; a 0.5 wave radiator plus a 0.25 matching section. However, the coaxial pipe or sleeve section in this antenna has a much better choking action. This should keep the coax free from radiating and result in a better radiation pattern. In theory that is.
The popular J-pole is easier to construct, but the brutal feeding of the (unbalanced)coax to the (balanced)matching section is questionable. But there are new, better variations of the J-pole.
And let's not forget; both offer plenty of fun as they are very efficient antenna's.

Bad alternative; the commercial 'collinear' dual-band vertical
OK, I admit; I have used these typical white fiberglass collinear constructions for years. Everyone has (had) one right? It is by far the most succesful commercial antenna for VHF/UHF. Offers plenty of QSO's too, nothing to bash about that. I still have a 'short' Comet GP3 VHF/UHF for portable use.
But as soon as SWR acts funny you start investigating what this antenna is all about.
What I do not like:
- collinear sections, having no true stacking distance, do not offer the "theoretically calculated" gain. Funkamateur magazine did a comparison between a single rod 5/8 home made antenna and various commercial collinear antenna's back in 2005. Even the super 3x 5/8 (5mtr high) offered no more than some 2 dB gain.....
- bad connection joints "offer" a variation in SWR in due time
- capacitor failure in the bottom section
- 3 metal spokes function as 'groundplane'
When opening up such antenna you find pieces of welding copper, matching coils and small ceramic caps.

The Sperrtopf (german) or Fietspomp (dutch) antenna
Actually this sleeve antenna was designed by A.B. Baily and described in u.s. patent #2184729 in 1937.
The german word 'Sperrtopf' refers to its coax sleeve section.
The dutch word 'Fietspomp' refers to what it looks like; a cycle-pump. I always found them very ugly to look at, but this is not about cosmetics but performance & durability.
You can find some more info on how to build your own at:

The 438-470Mhz Sirtel CX400D
I bought 2 of these a few years ago on a flea market. There's already one on my roof, see my blog of feb 24th.

To get them to resonate correctly in our 70cm UHF band section, I need to lengthen the radiator by about 6cm, or was it 8cm? Darn! Why do I forget to write down such evident info? My antenna analyzer stops at 30MHz. Well, l know someone who has an HP network analyzer AND makes great coffee. I will do a separate blog on his Barrista qualities soon :)

See above a really pathetic attempt to measure and adjust the radiator length. Surely the HT batterypack was near empty. And so was the reserve pack.
Let's finish this blog quickly and go to that coffee....

The Wimo 145MHz version offers a finished version at an extremely cheap price.
I pop-riveted the short alum mounting section to a clamp. A few layers of spray paint should ensure weather durability. The horizontal tube section is meant for creating a distance from my antenna tower, in order to get the best omni pattern. Mind you; I cannot mount this antenna on the top of the tower. Local restrictions do not allow this. Long story, let's not go there.
It is all about maximizing efficiency and setting priorities.

Where was I ?
Ah, time to get me that coffee :)

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