[UK-CONTEST] More about DX loops

[Andy Summers]

Hi Callum, hope you don't mind me wading in...

Even for 1-wavelength circumference loops the highest radiation resistance
(and therefore antenna efficiency) is obtained by maximising the enclosed
area. Maximum enclosed area is realised with a circular loop. Quad and delta
loops are convenient physical realisations at the expense of a very small
loss in efficiency. You can infer here that a quad loop's efficiency is
slightly better than a delta loop. Sticking with the delta, maximum
efficiency (& Z) is obtained with an equilateral triangle. This gives a Z of
about 100 Ohms. If you turn it into an isoscolese the Z will reduce as base
length is altered +/- about the ideal equilateral length. It's therefore
possible to match directly to 50R coax. You do start to notice the
efficiency loss then apparently.

Regarding 100-to-50 Ohm matching, yes you can use 75 Ohm coax for your
1/4-wave transformer. The required Zo is an easy calculation: Zo =
sqrt(Z1*Z2) = 70.7 Ohms. You can rearrange this equation to see what you get
with 75 Ohm coax: Z2 = 75^2/100 = 56.25 Ohms. Close enough.

There may be some argument about whether the antenna should be fed as a
balanced or unbalanced antenna. If it's the latter then coax is fine. The
VSWR is not very high on it so it should stay fairly low loss. If you feed
an apex-up vertical delta 1/4-wave from the apex you've really got two bent
vertical dipoles fed in-phase. You'd feed a vertical dipole with coax,
wouldn't you? It's a different kettle of fish with horizontal loops because
it's symmetric about ground physically and is therefore clearly balanced.

Hope this was of some help.
73,
Andy, G4KNO.

> -----Original Message-----
> Date: Wed, 6 Apr 2005 16:15:29 -0000
> From: "Callum" <callum at mccormick.uk.com>
> Subject: Re: [UK-CONTEST] More about DX loops
> To: <uk-contest at contesting.com>
> Message-ID: <200504061615.j36GFSxj013671 at mail.intermip.net>
> Content-Type: text/plain;	charset="us-ascii"
>
> Guys,
>
> I have 6 sets of Westflex 103 running from the shack here at the front of
> the house to the loft and then into the garden etc. If I assume that these
> loops have roughly an impedance of circa 100 ohms, what's the best way of
> transform this to 50 - because I won't be able to "ribbon" from
> the shack to
> the loops.
>
> So, there's two topics now..(!) Feeding horizontal loops at the house (I
> have two) and feeding the field day vertical loop. I'll feed the field day
> one at the corner, but ribbon? That's slightly outside my sphere because
> I've never used it. Can I not cheat and use coax like I was taught on the
> foundation course!! ?? :)
>
> BTW, I've now been lent the low band DX book, so perhaps the
> answers are in
> there - however Chris, Donald, Clive and Adrian (and Darren -
> nearly missed
> you!) all seem to have slightly more knowledge about this than me.
>
> (Apologies for waffling about antennas and not contests!)
>
> Callum
> M0MCX