Anyone got 2m end fed antenna experience?

[VK2DDS]

G'day,

Yesterday I had a crack at building some HF end fed antennas using the matching circuit described in this article: http://www.w7dk.org/10%20meter%20vertical%20project.pdf

It's basically your standard end fed half wave but uses an autotransformer for impedance matching and a short run of RG58 as the capacitor which tunes out the magnetising inductance.

So far I've managed to build matching units which get good results on 40m (33T on 20mm PCV pipe, 70cm of RG58 capacitance) and 10m (13T with ~15cm of RG58) and was wondering if anyone has experience scaling this design to work on the 2m band. The 15cm length of RG58 has a capacitance of about 10-15pF. With a capacitance this size the transformer inductance would need to be 120nH. This is all well and good, but I've got no idea what the physical dimensions of an inductor that size would be. If anyone's got a ballpark figure that'd be great .

The end goal is to build a 1/2 wave end fed antenna for use where ground planes are hard to come by (backpacks, push bikes etc). I'd heavily prefer not to use a J-pole simply because they're 50% longer (or the 1/2 wave 33% shorter, depending on your point of view).

[VK3AIF]

Try one of these; http://users.tpg.com.au/davepars/images/antenna.pdf I and several others have used them to good effect.

More here; http://users.tpg.com.au/acwalton/Antenna%20Photos.htm

By my calculations a coil of 9mm inside diameter 13mm in length and consisting of 5 turns should get you pretty close to 120nH. The actual inductance can be varied by squeezing or stretching the coil. The problem you might find with such a small inductance value is the stray L and Cs. Another possibility is 2 turns 5mm long by 20mm dia this might be a more workable solution?

There are many ways to achieve this impedance transformation and the best/ easiest method changes with frequency, many use a single turn tapped coil at this frequency too.

[VK2DDS]

Thanks a lot for the link, that's not a design I'd seen a before.

VK3CAH's page mentions that you can remove the sleeve for 2m only, I'm guessing that's the internal 320mm one?

What's the purpose of the 300x32mm section? Is it to supplement the choke balun?

Cheers,
Brenton

Edit: I just whipped up a proof-of-concept version out of some cheap Jaycar RG-58 that was lying around. The center conductor section was cut to your dimensions + 5mm and it achieves an SWR of <1.5:1 from 144 to 148MHz. I've made the coax section a bit shorter though, mine measures 385mm from the end of the choke balun to the end of the coax sheath (Ed: realised this was incorrect and extended it to your dimensions, I'd overlooked the fact that the coax velocity factor doesn't count for the reflected current).

Many thanks for the design. This was much simpler than building a matching circuit!

[VK3AIF]

G'day Brenton,

The 300 x 32 mm sections only function is for mounting so as not to crush the PVC pipe but Andy VK3CAH did not use it and claims to have had no trouble. All other iterations that I am aware of have used it.

The sleeve (the 20 mm dia section) forms a transmission line so as to make the antenna behave as two half waves in phase on 70 cm when presented with that frequency otherwise it just behaves as a half wave on 2 M. Without it on 70 cm most of the radiation is at a high angle and not much use for terrestrial work at least.

If you omit the 70 cm sleeve it could be made much thinner, if you were to use a thinner sleeve I don't know if it would detract from the uhf performance or not, I messed around for ages with the one diameter but different lengths and stopped there. You are welcome to try it and please let me know how you go if you do.

If you want further details etc my email is vk3aif at wia.org.au

73
Dave

[VK2DDS]

Hi Dave,

Thanks for the info .

If you omit the 70 cm sleeve it could be made much thinner, if you were to use a thinner sleeve I don't know if it would detract from the uhf performance or not, I messed around for ages with the one diameter but different lengths and stopped there. You are welcome to try it and please let me know how you go if you do.

At this stage I'm wanting to build it for use on a pushbike so being able to make it lighter and reduce the wind drag is more important than operating on 70cm. I'll certainly keep the design in mind for a future project though.