A simple low cost 2M & 70cm vertical aerial.

[vk3aif]

A simple low cost 2M & 70cm vertical aerial.

I built this aerial initially for portable operations as I wanted something easily transported and robust that could be hauled up a suitable tree, mounted on a pole or just clamped to my roof rack while on my travels.

When the 2M 2 x 5/8 in phase antenna on the top of my mast was damaged, probably by destructive Galahs, I put up my portable unit on a temporary basis while I repaired the original one. It is still there after 2 or 3 years and will probably stay there as the negligible loss of performance on 2 is more than made up for with the 70cm improvement. I have 2 others mounted a bit lower in use as well.

The following pdf and picture is pretty self explanatory and there is some further information here at Andy VK3CAH's web page http://users.tpg.com.au/acwalton/. The design has been reproduced many times and in various iterations and so far everyone I know of is happy with their experience. There has also been several other ones scaled to other bands besides Andy's 3M iteration.

1776

The picture above shows 2 of the possibilities, one in the plastic pipe as in the drawing, the other minus sleeve and pipe so that it can be rolled up for transportation and hung from any convenient support. They work very well when compared to the rubber dummy load as supplied with HTs.

1777

The plot shows the 2M radiation pattern (green), 70cm with sleeve (brown) and 70cm without sleeve. These are simulated patterns in free space and modeled using MMANA-Gal software. As you can see, there is about a 3db penalty by not incorporating the sleeve on 70cm.

http://users.tpg.com.au/davepars/images/2M_70cm_pattern.png

SWR is well within accepted limits across both bands.

[N7BES]

Interesting design, I am thinking I want to try building one, seems simple enough, sounds like it works well, and seems like it will hold up pretty well.

One quick question (probably a stupid one...) since I can't tell for sure from the drawing.
The "9 turns of close wound RG58" is this a separate piece of coax from the feed coax coming in?
or is it the same piece of coax (ie come in, wind 9 times, continue up)

[AC0VH]

There are many variations of the collinear dual band configuration like Dave used for his. Good performer, robust and cheap. I think there's one in the ARRL Antenna Handbook, too. The phasing sleeve is pretty clever.

http://9amtalk.net/new2/sites/default/files/DualBand2.pdf

http://www.hamuniverse.com/w7lpnCollinear002.pdf
http://www.hamuniverse.com/2m440collinearvertical.html

It's also possible to build an OCF sleeved dipole in a PVC pipe like this.

http://mysite.verizon.net/cpthaines/id14.html

In both types, the turns of coax are an air core feedline choke. You can wind it as part of the antenna itself or build one separately. Some constructions use ferrite cores to make the choke instead of an ugly balun.

[vk3aif]

In both types, the turns of coax are an air core feedline choke. You can wind it as part of the antenna itself or build one separately. Some constructions use ferrite cores to make the choke instead of an ugly balun.

Thanks for the links I had not seen them but I was aware of the VK2ZOI flower pot antenna mentioned in one of them and at one stage there was a VK5 club making something very similar but without the sleeve.

The ugly balun was sized to have a reactance of at least 1K and is closer to 1100 ohms at 146MHz and roughly three times that at 440 by my calculations which I consider is sufficient decoupling of antenna from feedline. I have no idea what its self resonance is as it is swamped by the antenna resonances and I have never bothered to test one on its own and that would be effected by the stray C from the coax either side of it anyway.

The phasing section in the centre acts as a transmission line in the harmonic operation and has little effect on the fundamental frequency as noted in the links. I played with many different lengths of the sleeve before arriving at that one although it was not overly critical. In the first of the links the question was raised "Perhaps the 2 meter design was counting on some of the choke being part of the dipole" that is exactly what happens, each of the turns adds to the impedance like a bunch of near lossless resistors in series, this acts like a soft cut-off as well as increasing the bandwidth so it does make up some part of the antenna.


N7BES

Interesting design, I am thinking I want to try building one, seems simple enough, sounds like it works well, and seems like it will hold up pretty well.

One quick question (probably a stupid one...) since I can't tell for sure from the drawing.
The "9 turns of close wound RG58" is this a separate piece of coax from the feed coax coming in?
or is it the same piece of coax (ie come in, wind 9 times, continue up)

Certainly not a stupid question and as you are not the first to have asked I should have addressed this in the first place. It is the feedline that exits the pipe and is wrapped around the outside for the 9 turns and re-enters then out the base to the radio. I find it easiest to poke excess coax through the pre-drilled top hole so that it is accessible at the top of the pipe and connect the top section of radiator. Then feed it back down stripping and folding 10-12 mm over the end before pushing the cap on to hold it in place, then wind the coil and feed the remaining out through the bottom hole and terminate in the bottom cap or feed it through and terminate it outside, it's up to you which you prefer.

[N7BES]

Certainly not a stupid question and as you are not the first to have asked I should have addressed this in the first place. It is the feedline that exits the pipe and is wrapped around the outside for the 9 turns and re-enters then out the base to the radio. I find it easiest to poke excess coax through the pre-drilled top hole so that it is accessible at the top of the pipe and connect the top section of radiator. Then feed it back down stripping and folding 10-12 mm over the end before pushing the cap on to hold it in place, then wind the coil and feed the remaining out through the bottom hole and terminate in the bottom cap or feed it through and terminate it outside, it's up to you which you prefer.

Thanks for the clarification.

One more question since I don't see it directly addressed and we know what happens when you assume.... Is the shield (braid, whatever you want to call it) on the coax not hooked up to anything? just strip it back to the point indicated and that is that?

[vk3aif]

Thanks for the clarification.

One more question since I don't see it directly addressed and we know what happens when you assume.... Is the shield (braid, whatever you want to call it) on the coax not hooked up to anything? just strip it back to the point indicated and that is that?

Yes that is correct, antenna currents flow on the outside of the shield and transmission line ones on the inside.