Sunday, 3 May 2015

Excess Aerial Baggage

Having turned over another page on the calendar and frightened myself by seeing the word "May", there's no avoiding the fact that I soon need to pack my bags and wend my weary way to Dayton.

I see with a mixture of surprise and amusement that I've been billed as "The Man from Laramie" (on QRP-L)...

I'll give James Stewart a run for his money!

Particularly fitting as my trip to Dayton is the first leg of a longer trip to my old hometown of - you guessed it -  Laramie, WY.

Those bags I need to pack are going to be pretty stuffed - not least because the XYL is coming along on the trip too and she has her ornithological paraphernalia - as well as the proverbial kitchen sink.

I've decided I need to take an HF antenna - partly to demonstrate the Parallel IF receiver which I'll be talking about and partly because I might be doing some operating (I'm getting a new rig - more about that when I get back). But what antenna to take?

I'm not exactly big on mobile operating - so my mobile antenna choices are limited. There's the Walkabout that I've used on marine mobile exploits and damaged on the beach. Then there's my "holiday dipole", which actually is the very first HF antenna I ever had here at the home station - so it is a rather clumsy QRO affair, hardly suitable for sticking in a suitcase...

I decided to make a new antenna, more in keeping with the "mobile" label. It would be - in the first instance, 40m only. Perhaps in the fullness of time and given a rush of blood to the head I might add traps for 20m but - for now - 40m only.

I looked in the junk boxes and found some BN-34-301 bino cores that looked as if they offer the right balance between sufficiently small size for porterage and sufficiently large size to handle a few Watts to make a viable 1:1 balun for a QRP antenna, which I set about making according to the following scheme...

The schematic above is well-configured for seeing how the balun should be wired up, but not especially helpful for developing any sense of how it actually works - specifically how it achieves the necessary 1:1 impedance ratio (as compared to the familiar and simpler 1:4 balun which can be made with only two windings).

So, here's a re-arranged version of the schematic in a form which (I think) carries a little more teaching...

Note carefully the phasing of the coils (indicated by the dots) without which it will not work!

Here's the balun, realised in a little potting box...

I've arranged for some thick monofilament nylon to take the strain at the dipole centre (rather than the conductor itself) and the feeder is connected by a BNC, as you can see. This would be useless in a permanent installation due to moisture ingress - but this isn't a permanent installation!

I set the length of the dipole elements by matching them to the original "holiday dipole" and gave the new antenna a quick check with the FT 817 - it showed a nice low SWR across the 40m band.

Some hot-melt to stop the bino core wagging around and the box top to tidy things up had the new dream catcher finished...

It is, as you see by comparing the centimetre grid on which it is standing, rather more appropriately sized for portability than the old QRO antenna it replaces.

That's one more thing I can cross off the list of things to do before setting off for the New World,

...-.- de m0xpd, aka the Man from Laramie !

Update (4 May)

The weather being a little more cooperative again, I took the new antenna out into the back garden and hoisted it into an inverted vee - flying the centre from a telescopic fishing pole. Had chance to check not only that it returned "a nice low SWR" - but also that I could radiate a decent signal from it, courtesy of spots on the Reverse Beacon Network from all over the place and a QSO with Steve, g0evj.

I was using my FT 817 and a new version of the Occam's Dagger rig, which I'll tell you about when I get chance for a longer post...

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